[PATCH] platform/x86: dell-uart-backlight: fall back to PCH PWM on UART timeout

[PATCH] platform/x86: dell-uart-backlight: fall back to PCH PWM on UART timeout

[chitranshmandhaniya]

From: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>

Some Dell OptiPlex AIO 7410 (35W, board 01KP8D) ship with DELL0501 in
the ACPI DSDT (status = 0x0F) even though no UART scalar board is
physically present.  This causes acpi_video_get_backlight_type() to
return acpi_backlight_dell_uart, which makes i915 skip registering
intel_backlight.  The dell-uart-backlight driver then times out talking
to the absent board and the probe fails, leaving the machine with zero
backlight control.

A BIOS update fixes this on some models (e.g. OptiPlex 7420, board
0914RT) by setting DELL0501 _STA to 0, but Dell has not shipped a
corrected BIOS for the 7410 even in the latest release (1.37.0,
2026-05-28).

When the initial get_version command returns -ETIMEDOUT, fall back to
driving the backlight through the Intel PCH PWM registers (BLC_PWM_CTL /
BLC_PWM_FREQ / BLC_PWM_DUTY at MMIO BAR0 + 0xC8250).  The fallback
reads the BIOS-programmed PWM frequency as max_brightness and registers
a BACKLIGHT_PLATFORM device so that compositors (Weston, GNOME, KDE)
pick it up normally.

Tested on:
  - OptiPlex AIO 7410 35W (01KP8D) -- timeout, fallback activates, works
  - OptiPlex AIO 7420 35W (0914RT, old BIOS) -- timeout, fallback works
  - OptiPlex AIO 7420 35W (0914RT, new BIOS) -- DELL0501 absent, driver
    not loaded, intel_backlight works natively (no regression)

Signed-off-by: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>
---
 .../platform/x86/dell/dell-uart-backlight.c   | 116 +++++++++++++++++-
 1 file changed, 113 insertions(+), 3 deletions(-)

diff --git a/drivers/platform/x86/dell/dell-uart-backlight.c b/drivers/platform/x86/dell/dell-uart-backlight.c
index f323a667dc2d..a2045e6c49bd 100644
--- a/drivers/platform/x86/dell/dell-uart-backlight.c
+++ b/drivers/platform/x86/dell/dell-uart-backlight.c
@@ -13,8 +13,10 @@
 #include <linux/delay.h>
 #include <linux/device.h>
 #include <linux/err.h>
+#include <linux/io.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
+#include <linux/pci.h>
 #include <linux/platform_device.h>
 #include <linux/serdev.h>
 #include <linux/string.h>
@@ -50,6 +52,11 @@
 #define MIN_RESP_LEN			3
 #define MAX_RESP_LEN			80
 
+/* PCH PWM backlight registers relative to Intel GPU MMIO BAR0 */
+#define PCH_BLC_PWM_CTL		0xC8250
+#define PCH_BLC_PWM_FREQ	0xC8254
+#define PCH_BLC_PWM_DUTY	0xC8258
+
 struct dell_uart_backlight {
 	struct mutex mutex;
 	wait_queue_head_t wait_queue;
@@ -64,6 +71,11 @@ struct dell_uart_backlight {
 	int power;
 };
 
+struct dell_pwm_backlight {
+	void __iomem *mmio;
+	u32 max_brightness;
+};
+
 /* Checksum: SUM(Length and Cmd and Data) xor 0xFF */
 static u8 dell_uart_checksum(u8 *buf, int len)
 {
@@ -267,6 +279,97 @@ static const struct serdev_device_ops dell_uart_bl_serdev_ops = {
 	.write_wakeup = serdev_device_write_wakeup,
 };
 
+/*
+ * Some Dell AIO models (e.g. OptiPlex 7410) declare DELL0501 in ACPI but
+ * have no physical UART scalar board.  When the UART times out fall back
+ * to driving the Intel PCH PWM backlight registers directly.
+ */
+static int dell_pwm_update_status(struct backlight_device *bd)
+{
+	struct dell_pwm_backlight *pwm = bl_get_data(bd);
+	u32 duty;
+
+	if (bd->props.power != BACKLIGHT_POWER_ON ||
+	    bd->props.state & BL_CORE_SUSPENDED)
+		duty = 0;
+	else
+		duty = bd->props.brightness;
+
+	writel(duty, pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
+	return 0;
+}
+
+static int dell_pwm_get_brightness(struct backlight_device *bd)
+{
+	struct dell_pwm_backlight *pwm = bl_get_data(bd);
+
+	return readl(pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
+}
+
+static const struct backlight_ops dell_pwm_backlight_ops = {
+	.update_status  = dell_pwm_update_status,
+	.get_brightness = dell_pwm_get_brightness,
+};
+
+static int dell_pwm_fallback_register(struct device *dev)
+{
+	struct dell_pwm_backlight *pwm;
+	struct backlight_properties props = {};
+	struct backlight_device *bl;
+	struct pci_dev *gpu;
+	resource_size_t bar0;
+	u32 freq, duty, ctl;
+
+	gpu = pci_get_class(PCI_CLASS_DISPLAY_VGA << 8, NULL);
+	if (!gpu || gpu->vendor != PCI_VENDOR_ID_INTEL) {
+		pci_dev_put(gpu);
+		dev_warn(dev, "No Intel GPU found, cannot fall back to PCH PWM\n");
+		return -ENODEV;
+	}
+
+	bar0 = pci_resource_start(gpu, 0);
+	pci_dev_put(gpu);
+	if (!bar0) {
+		dev_warn(dev, "Intel GPU BAR0 not assigned\n");
+		return -ENODEV;
+	}
+
+	pwm = devm_kzalloc(dev, sizeof(*pwm), GFP_KERNEL);
+	if (!pwm)
+		return -ENOMEM;
+
+	pwm->mmio = devm_ioremap(dev, bar0 + PCH_BLC_PWM_CTL, 0x10);
+	if (!pwm->mmio) {
+		dev_warn(dev, "Failed to ioremap PCH PWM registers\n");
+		return -ENOMEM;
+	}
+
+	ctl  = readl(pwm->mmio);
+	freq = readl(pwm->mmio + (PCH_BLC_PWM_FREQ - PCH_BLC_PWM_CTL));
+	duty = readl(pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
+
+	if (!(ctl & BIT(31)) || freq == 0) {
+		dev_warn(dev, "PCH PWM not enabled by BIOS (ctl=0x%08x freq=%u)\n",
+			 ctl, freq);
+		return -ENODEV;
+	}
+
+	pwm->max_brightness = freq;
+
+	dev_info(dev, "Falling back to PCH PWM backlight (max=%u current=%u)\n",
+		 freq, duty);
+
+	props.type = BACKLIGHT_PLATFORM;
+	props.brightness = duty;
+	props.max_brightness = freq;
+
+	bl = devm_backlight_device_register(dev, "dell_uart_backlight",
+					    dev, pwm,
+					    &dell_pwm_backlight_ops,
+					    &props);
+	return PTR_ERR_OR_ZERO(bl);
+}
+
 static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
 {
 	u8 get_version[GET_CMD_LEN], resp[MAX_RESP_LEN];
@@ -300,18 +403,18 @@ static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
 
 	ret = dell_uart_bl_command(dell_bl, get_version, GET_CMD_LEN, resp, MAX_RESP_LEN);
 	if (ret)
-		return dev_err_probe(dev, ret, "getting firmware version\n");
+		goto uart_fail;
 
 	dev_dbg(dev, "Firmware version: %.*s\n", resp[RESP_LEN] - 3, resp + RESP_DATA);
 
 	/* Initialize bl_power to a known value */
 	ret = dell_uart_set_bl_power(dell_bl, BACKLIGHT_POWER_ON);
 	if (ret)
-		return ret;
+		goto uart_fail;
 
 	ret = dell_uart_get_brightness(dell_bl);
 	if (ret < 0)
-		return ret;
+		goto uart_fail;
 
 	props.type = BACKLIGHT_PLATFORM;
 	props.brightness = ret;
@@ -323,6 +426,13 @@ static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
 						     &dell_uart_backlight_ops,
 						     &props);
 	return PTR_ERR_OR_ZERO(dell_bl->bl);
+
+uart_fail:
+	if (ret == -ETIMEDOUT) {
+		dev_info(dev, "UART backlight controller not responding, trying PCH PWM fallback\n");
+		return dell_pwm_fallback_register(dev);
+	}
+	return dev_err_probe(dev, ret, "UART backlight communication failed\n");
 }
 
 static struct serdev_device_driver dell_uart_bl_serdev_driver = {
-- 
2.43.0

Re: [PATCH] platform/x86: dell-uart-backlight: fall back to PCH PWM on UART timeout

[Ilpo Järvinen]

On Thu, 8 Oct 2026, chitranshmandhaniya@gmail.com wrote:

> From: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>
> 
> Some Dell OptiPlex AIO 7410 (35W, board 01KP8D) ship with DELL0501 in
> the ACPI DSDT (status = 0x0F) even though no UART scalar board is
> physically present.  This causes acpi_video_get_backlight_type() to
> return acpi_backlight_dell_uart, which makes i915 skip registering
> intel_backlight.  The dell-uart-backlight driver then times out talking
> to the absent board and the probe fails, leaving the machine with zero
> backlight control.
> 
> A BIOS update fixes this on some models (e.g. OptiPlex 7420, board
> 0914RT) by setting DELL0501 _STA to 0, but Dell has not shipped a
> corrected BIOS for the 7410 even in the latest release (1.37.0,
> 2026-05-28).
> 
> When the initial get_version command returns -ETIMEDOUT, fall back to
> driving the backlight through the Intel PCH PWM registers (BLC_PWM_CTL /
> BLC_PWM_FREQ / BLC_PWM_DUTY at MMIO BAR0 + 0xC8250).  The fallback
> reads the BIOS-programmed PWM frequency as max_brightness and registers
> a BACKLIGHT_PLATFORM device so that compositors (Weston, GNOME, KDE)
> pick it up normally.
> 
> Tested on:
>   - OptiPlex AIO 7410 35W (01KP8D) -- timeout, fallback activates, works
>   - OptiPlex AIO 7420 35W (0914RT, old BIOS) -- timeout, fallback works
>   - OptiPlex AIO 7420 35W (0914RT, new BIOS) -- DELL0501 absent, driver
>     not loaded, intel_backlight works natively (no regression)
> 
> Signed-off-by: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>
> ---
>  .../platform/x86/dell/dell-uart-backlight.c   | 116 +++++++++++++++++-
>  1 file changed, 113 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/platform/x86/dell/dell-uart-backlight.c b/drivers/platform/x86/dell/dell-uart-backlight.c
> index f323a667dc2d..a2045e6c49bd 100644
> --- a/drivers/platform/x86/dell/dell-uart-backlight.c
> +++ b/drivers/platform/x86/dell/dell-uart-backlight.c
> @@ -13,8 +13,10 @@
>  #include <linux/delay.h>
>  #include <linux/device.h>
>  #include <linux/err.h>
> +#include <linux/io.h>
>  #include <linux/module.h>
>  #include <linux/mutex.h>
> +#include <linux/pci.h>
>  #include <linux/platform_device.h>
>  #include <linux/serdev.h>
>  #include <linux/string.h>
> @@ -50,6 +52,11 @@
>  #define MIN_RESP_LEN			3
>  #define MAX_RESP_LEN			80
>  
> +/* PCH PWM backlight registers relative to Intel GPU MMIO BAR0 */
> +#define PCH_BLC_PWM_CTL		0xC8250
> +#define PCH_BLC_PWM_FREQ	0xC8254
> +#define PCH_BLC_PWM_DUTY	0xC8258
> +
>  struct dell_uart_backlight {
>  	struct mutex mutex;
>  	wait_queue_head_t wait_queue;
> @@ -64,6 +71,11 @@ struct dell_uart_backlight {
>  	int power;
>  };
>  
> +struct dell_pwm_backlight {
> +	void __iomem *mmio;
> +	u32 max_brightness;
> +};
> +
>  /* Checksum: SUM(Length and Cmd and Data) xor 0xFF */
>  static u8 dell_uart_checksum(u8 *buf, int len)
>  {
> @@ -267,6 +279,97 @@ static const struct serdev_device_ops dell_uart_bl_serdev_ops = {
>  	.write_wakeup = serdev_device_write_wakeup,
>  };
>  
> +/*
> + * Some Dell AIO models (e.g. OptiPlex 7410) declare DELL0501 in ACPI but
> + * have no physical UART scalar board.  When the UART times out fall back

Nit, one space is enough.

> + * to driving the Intel PCH PWM backlight registers directly.
> + */
> +static int dell_pwm_update_status(struct backlight_device *bd)
> +{
> +	struct dell_pwm_backlight *pwm = bl_get_data(bd);
> +	u32 duty;
> +
> +	if (bd->props.power != BACKLIGHT_POWER_ON ||
> +	    bd->props.state & BL_CORE_SUSPENDED)
> +		duty = 0;
> +	else
> +		duty = bd->props.brightness;
> +
> +	writel(duty, pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));

Please add linux/io.h to includes.

> +	return 0;
> +}
> +
> +static int dell_pwm_get_brightness(struct backlight_device *bd)
> +{
> +	struct dell_pwm_backlight *pwm = bl_get_data(bd);
> +
> +	return readl(pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
> +}
> +
> +static const struct backlight_ops dell_pwm_backlight_ops = {
> +	.update_status  = dell_pwm_update_status,
> +	.get_brightness = dell_pwm_get_brightness,
> +};
> +
> +static int dell_pwm_fallback_register(struct device *dev)
> +{
> +	struct dell_pwm_backlight *pwm;
> +	struct backlight_properties props = {};
> +	struct backlight_device *bl;
> +	struct pci_dev *gpu;
> +	resource_size_t bar0;
> +	u32 freq, duty, ctl;
> +
> +	gpu = pci_get_class(PCI_CLASS_DISPLAY_VGA << 8, NULL);
> +	if (!gpu || gpu->vendor != PCI_VENDOR_ID_INTEL) {
> +		pci_dev_put(gpu);
> +		dev_warn(dev, "No Intel GPU found, cannot fall back to PCH PWM\n");

Add include.

> +		return -ENODEV;
> +	}
> +
> +	bar0 = pci_resource_start(gpu, 0);
> +	pci_dev_put(gpu);

Don't duplicating pci_dev_put(), __free() can be used to autorelease it.

> +	if (!bar0) {
> +		dev_warn(dev, "Intel GPU BAR0 not assigned\n");
> +		return -ENODEV;
> +	}
> +
> +	pwm = devm_kzalloc(dev, sizeof(*pwm), GFP_KERNEL);
> +	if (!pwm)
> +		return -ENOMEM;
> +
> +	pwm->mmio = devm_ioremap(dev, bar0 + PCH_BLC_PWM_CTL, 0x10);

SZ_xx + add include.

> +	if (!pwm->mmio) {
> +		dev_warn(dev, "Failed to ioremap PCH PWM registers\n");
> +		return -ENOMEM;
> +	}
> +
> +	ctl  = readl(pwm->mmio);
> +	freq = readl(pwm->mmio + (PCH_BLC_PWM_FREQ - PCH_BLC_PWM_CTL));
> +	duty = readl(pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
> +
> +	if (!(ctl & BIT(31)) || freq == 0) {

Please name BIT(31) with a define.

> +		dev_warn(dev, "PCH PWM not enabled by BIOS (ctl=0x%08x freq=%u)\n",
> +			 ctl, freq);
> +		return -ENODEV;
> +	}
> +
> +	pwm->max_brightness = freq;
> +
> +	dev_info(dev, "Falling back to PCH PWM backlight (max=%u current=%u)\n",
> +		 freq, duty);
> +
> +	props.type = BACKLIGHT_PLATFORM;
> +	props.brightness = duty;
> +	props.max_brightness = freq;
> +
> +	bl = devm_backlight_device_register(dev, "dell_uart_backlight",
> +					    dev, pwm,
> +					    &dell_pwm_backlight_ops,
> +					    &props);

Fits to less lines.

You may also use up to 100 chars for code where it helps.

> +	return PTR_ERR_OR_ZERO(bl);
> +}
> +
>  static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
>  {
>  	u8 get_version[GET_CMD_LEN], resp[MAX_RESP_LEN];
> @@ -300,18 +403,18 @@ static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
>  
>  	ret = dell_uart_bl_command(dell_bl, get_version, GET_CMD_LEN, resp, MAX_RESP_LEN);
>  	if (ret)
> -		return dev_err_probe(dev, ret, "getting firmware version\n");
> +		goto uart_fail;
>  
>  	dev_dbg(dev, "Firmware version: %.*s\n", resp[RESP_LEN] - 3, resp + RESP_DATA);
>  
>  	/* Initialize bl_power to a known value */
>  	ret = dell_uart_set_bl_power(dell_bl, BACKLIGHT_POWER_ON);
>  	if (ret)
> -		return ret;
> +		goto uart_fail;
>  
>  	ret = dell_uart_get_brightness(dell_bl);
>  	if (ret < 0)
> -		return ret;
> +		goto uart_fail;
>  
>  	props.type = BACKLIGHT_PLATFORM;
>  	props.brightness = ret;
> @@ -323,6 +426,13 @@ static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
>  						     &dell_uart_backlight_ops,
>  						     &props);
>  	return PTR_ERR_OR_ZERO(dell_bl->bl);
> +
> +uart_fail:
> +	if (ret == -ETIMEDOUT) {
> +		dev_info(dev, "UART backlight controller not responding, trying PCH PWM fallback\n");
> +		return dell_pwm_fallback_register(dev);

This looks like rollback path but isn't.

How about adding a function for the "normal" BL init so you can structure 
the code more logically.

> +	}
> +	return dev_err_probe(dev, ret, "UART backlight communication failed\n");
>  }
>  
>  static struct serdev_device_driver dell_uart_bl_serdev_driver = {
> 

-- 
 i.

[PATCH v2] platform/x86: dell-uart-backlight: fall back to PCH PWM on UART timeout

[chitranshmandhaniya]

From: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>

Some Dell OptiPlex AIO 7410 (35W, board 01KP8D) ship with DELL0501 in
the ACPI DSDT (status = 0x0F) even though no UART scalar board is
physically present.  This causes acpi_video_get_backlight_type() to
return acpi_backlight_dell_uart, which makes i915 skip registering
intel_backlight.  The dell-uart-backlight driver then times out talking
to the absent board and the probe fails, leaving the machine with zero
backlight control.

A BIOS update fixes this on some models (e.g. OptiPlex 7420, board
0914RT) by setting DELL0501 _STA to 0, but Dell has not shipped a
corrected BIOS for the 7410 even in the latest release (1.37.0,
2026-05-28).

When the initial get_version command returns -ETIMEDOUT, fall back to
driving the backlight through the Intel PCH PWM registers (BLC_PWM_CTL /
BLC_PWM_FREQ / BLC_PWM_DUTY at MMIO BAR0 + 0xC8250).  The fallback
reads the BIOS-programmed PWM frequency as max_brightness and registers
a BACKLIGHT_PLATFORM device so that compositors (Weston, GNOME, KDE)
pick it up normally.

Tested on:
  - OptiPlex AIO 7410 35W (01KP8D) -- timeout, fallback activates, works
  - OptiPlex AIO 7420 35W (0914RT, old BIOS) -- timeout, fallback works
  - OptiPlex AIO 7420 35W (0914RT, new BIOS) -- DELL0501 absent, driver
    not loaded, intel_backlight works natively (no regression)

Signed-off-by: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>
---
v1 -> v2:
  - Use __free(pci_dev_put) for automatic PCI device cleanup (Ilpo)
  - Replace 0x10 with SZ_16, add linux/sizes.h (Ilpo)
  - Name BIT(31) as PCH_BLC_PWM_ENABLE (Ilpo)
  - Fix double-space in block comment (Ilpo)
  - Reduce devm_backlight_device_register() to fewer lines (Ilpo)
  - Extract dell_uart_bl_init() for the normal UART backlight path
    so the probe function reads more clearly (Ilpo)

 .../platform/x86/dell/dell-uart-backlight.c   | 157 +++++++++++++++---
 1 file changed, 134 insertions(+), 23 deletions(-)

diff --git a/drivers/platform/x86/dell/dell-uart-backlight.c b/drivers/platform/x86/dell/dell-uart-backlight.c
index f323a667dc2d..edc7cfe38069 100644
--- a/drivers/platform/x86/dell/dell-uart-backlight.c
+++ b/drivers/platform/x86/dell/dell-uart-backlight.c
@@ -13,10 +13,13 @@
 #include <linux/delay.h>
 #include <linux/device.h>
 #include <linux/err.h>
+#include <linux/io.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
+#include <linux/pci.h>
 #include <linux/platform_device.h>
 #include <linux/serdev.h>
+#include <linux/sizes.h>
 #include <linux/string.h>
 #include <linux/types.h>
 #include <linux/wait.h>
@@ -50,6 +53,12 @@
 #define MIN_RESP_LEN			3
 #define MAX_RESP_LEN			80
 
+/* PCH PWM backlight registers relative to Intel GPU MMIO BAR0 */
+#define PCH_BLC_PWM_CTL		0xC8250
+#define PCH_BLC_PWM_FREQ	0xC8254
+#define PCH_BLC_PWM_DUTY	0xC8258
+#define PCH_BLC_PWM_ENABLE	BIT(31)
+
 struct dell_uart_backlight {
 	struct mutex mutex;
 	wait_queue_head_t wait_queue;
@@ -64,6 +73,11 @@ struct dell_uart_backlight {
 	int power;
 };
 
+struct dell_pwm_backlight {
+	void __iomem *mmio;
+	u32 max_brightness;
+};
+
 /* Checksum: SUM(Length and Cmd and Data) xor 0xFF */
 static u8 dell_uart_checksum(u8 *buf, int len)
 {
@@ -267,32 +281,97 @@ static const struct serdev_device_ops dell_uart_bl_serdev_ops = {
 	.write_wakeup = serdev_device_write_wakeup,
 };
 
-static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
+/*
+ * Some Dell AIO models (e.g. OptiPlex 7410) declare DELL0501 in ACPI but
+ * have no physical UART scalar board. When the UART times out fall back
+ * to driving the Intel PCH PWM backlight registers directly.
+ */
+static int dell_pwm_update_status(struct backlight_device *bd)
 {
-	u8 get_version[GET_CMD_LEN], resp[MAX_RESP_LEN];
+	struct dell_pwm_backlight *pwm = bl_get_data(bd);
+	u32 duty;
+
+	if (bd->props.power != BACKLIGHT_POWER_ON ||
+	    bd->props.state & BL_CORE_SUSPENDED)
+		duty = 0;
+	else
+		duty = bd->props.brightness;
+
+	writel(duty, pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
+	return 0;
+}
+
+static int dell_pwm_get_brightness(struct backlight_device *bd)
+{
+	struct dell_pwm_backlight *pwm = bl_get_data(bd);
+
+	return readl(pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
+}
+
+static const struct backlight_ops dell_pwm_backlight_ops = {
+	.update_status  = dell_pwm_update_status,
+	.get_brightness = dell_pwm_get_brightness,
+};
+
+static int dell_pwm_fallback_register(struct device *dev)
+{
+	struct pci_dev *gpu __free(pci_dev_put) = NULL;
 	struct backlight_properties props = {};
-	struct dell_uart_backlight *dell_bl;
-	struct device *dev = &serdev->dev;
-	int ret;
+	struct dell_pwm_backlight *pwm;
+	struct backlight_device *bl;
+	resource_size_t bar0;
+	u32 freq, duty, ctl;
 
-	dell_bl = devm_kzalloc(dev, sizeof(*dell_bl), GFP_KERNEL);
-	if (!dell_bl)
+	gpu = pci_get_class(PCI_CLASS_DISPLAY_VGA << 8, NULL);
+	if (!gpu || gpu->vendor != PCI_VENDOR_ID_INTEL) {
+		dev_warn(dev, "No Intel GPU found, cannot fall back to PCH PWM\n");
+		return -ENODEV;
+	}
+
+	bar0 = pci_resource_start(gpu, 0);
+	if (!bar0) {
+		dev_warn(dev, "Intel GPU BAR0 not assigned\n");
+		return -ENODEV;
+	}
+
+	pwm = devm_kzalloc(dev, sizeof(*pwm), GFP_KERNEL);
+	if (!pwm)
 		return -ENOMEM;
 
-	mutex_init(&dell_bl->mutex);
-	init_waitqueue_head(&dell_bl->wait_queue);
-	dell_bl->dev = dev;
+	pwm->mmio = devm_ioremap(dev, bar0 + PCH_BLC_PWM_CTL, SZ_16);
+	if (!pwm->mmio) {
+		dev_warn(dev, "Failed to ioremap PCH PWM registers\n");
+		return -ENOMEM;
+	}
 
-	serdev_device_set_drvdata(serdev, dell_bl);
-	serdev_device_set_client_ops(serdev, &dell_uart_bl_serdev_ops);
+	ctl  = readl(pwm->mmio);
+	freq = readl(pwm->mmio + (PCH_BLC_PWM_FREQ - PCH_BLC_PWM_CTL));
+	duty = readl(pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
 
-	ret = devm_serdev_device_open(dev, serdev);
-	if (ret)
-		return dev_err_probe(dev, ret, "opening UART device\n");
+	if (!(ctl & PCH_BLC_PWM_ENABLE) || freq == 0) {
+		dev_warn(dev, "PCH PWM not enabled by BIOS (ctl=0x%08x freq=%u)\n", ctl, freq);
+		return -ENODEV;
+	}
 
-	/* 9600 bps, no flow control, these are the default but set them to be sure */
-	serdev_device_set_baudrate(serdev, 9600);
-	serdev_device_set_flow_control(serdev, false);
+	pwm->max_brightness = freq;
+
+	dev_info(dev, "Falling back to PCH PWM backlight (max=%u current=%u)\n", freq, duty);
+
+	props.type = BACKLIGHT_PLATFORM;
+	props.brightness = duty;
+	props.max_brightness = freq;
+
+	bl = devm_backlight_device_register(dev, "dell_uart_backlight", dev, pwm,
+					    &dell_pwm_backlight_ops, &props);
+	return PTR_ERR_OR_ZERO(bl);
+}
+
+static int dell_uart_bl_init(struct device *dev, struct serdev_device *serdev,
+			     struct dell_uart_backlight *dell_bl)
+{
+	u8 get_version[GET_CMD_LEN], resp[MAX_RESP_LEN];
+	struct backlight_properties props = {};
+	int ret;
 
 	get_version[0] = DELL_SOF(GET_CMD_LEN);
 	get_version[1] = CMD_GET_VERSION;
@@ -300,7 +379,7 @@ static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
 
 	ret = dell_uart_bl_command(dell_bl, get_version, GET_CMD_LEN, resp, MAX_RESP_LEN);
 	if (ret)
-		return dev_err_probe(dev, ret, "getting firmware version\n");
+		return ret;
 
 	dev_dbg(dev, "Firmware version: %.*s\n", resp[RESP_LEN] - 3, resp + RESP_DATA);
 
@@ -318,13 +397,45 @@ static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
 	props.max_brightness = DELL_BL_MAX_BRIGHTNESS;
 	props.power = dell_bl->power;
 
-	dell_bl->bl = devm_backlight_device_register(dev, "dell_uart_backlight",
-						     dev, dell_bl,
-						     &dell_uart_backlight_ops,
-						     &props);
+	dell_bl->bl = devm_backlight_device_register(dev, "dell_uart_backlight", dev, dell_bl,
+						     &dell_uart_backlight_ops, &props);
 	return PTR_ERR_OR_ZERO(dell_bl->bl);
 }
 
+static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
+{
+	struct dell_uart_backlight *dell_bl;
+	struct device *dev = &serdev->dev;
+	int ret;
+
+	dell_bl = devm_kzalloc(dev, sizeof(*dell_bl), GFP_KERNEL);
+	if (!dell_bl)
+		return -ENOMEM;
+
+	mutex_init(&dell_bl->mutex);
+	init_waitqueue_head(&dell_bl->wait_queue);
+	dell_bl->dev = dev;
+
+	serdev_device_set_drvdata(serdev, dell_bl);
+	serdev_device_set_client_ops(serdev, &dell_uart_bl_serdev_ops);
+
+	ret = devm_serdev_device_open(dev, serdev);
+	if (ret)
+		return dev_err_probe(dev, ret, "opening UART device\n");
+
+	/* 9600 bps, no flow control, these are the default but set them to be sure */
+	serdev_device_set_baudrate(serdev, 9600);
+	serdev_device_set_flow_control(serdev, false);
+
+	ret = dell_uart_bl_init(dev, serdev, dell_bl);
+	if (ret == -ETIMEDOUT) {
+		dev_info(dev, "UART backlight controller not responding, trying PCH PWM fallback\n");
+		return dell_pwm_fallback_register(dev);
+	}
+
+	return ret;
+}
+
 static struct serdev_device_driver dell_uart_bl_serdev_driver = {
 	.probe = dell_uart_bl_serdev_probe,
 	.driver = {
-- 
2.43.0

Re: [PATCH v2] platform/x86: dell-uart-backlight: fall back to PCH PWM on UART timeout

[Ilpo Järvinen]

On Fri, 9 Oct 2026, chitranshmandhaniya@gmail.com wrote:

> From: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>
> 
> Some Dell OptiPlex AIO 7410 (35W, board 01KP8D) ship with DELL0501 in
> the ACPI DSDT (status = 0x0F) even though no UART scalar board is
> physically present.  This causes acpi_video_get_backlight_type() to
> return acpi_backlight_dell_uart, which makes i915 skip registering
> intel_backlight.  The dell-uart-backlight driver then times out talking
> to the absent board and the probe fails, leaving the machine with zero
> backlight control.
> 
> A BIOS update fixes this on some models (e.g. OptiPlex 7420, board
> 0914RT) by setting DELL0501 _STA to 0, but Dell has not shipped a
> corrected BIOS for the 7410 even in the latest release (1.37.0,
> 2026-05-28).
> 
> When the initial get_version command returns -ETIMEDOUT, fall back to
> driving the backlight through the Intel PCH PWM registers (BLC_PWM_CTL /
> BLC_PWM_FREQ / BLC_PWM_DUTY at MMIO BAR0 + 0xC8250).  The fallback
> reads the BIOS-programmed PWM frequency as max_brightness and registers
> a BACKLIGHT_PLATFORM device so that compositors (Weston, GNOME, KDE)
> pick it up normally.
> 
> Tested on:
>   - OptiPlex AIO 7410 35W (01KP8D) -- timeout, fallback activates, works
>   - OptiPlex AIO 7420 35W (0914RT, old BIOS) -- timeout, fallback works
>   - OptiPlex AIO 7420 35W (0914RT, new BIOS) -- DELL0501 absent, driver
>     not loaded, intel_backlight works natively (no regression)
> 
> Signed-off-by: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>
> ---
> v1 -> v2:
>   - Use __free(pci_dev_put) for automatic PCI device cleanup (Ilpo)
>   - Replace 0x10 with SZ_16, add linux/sizes.h (Ilpo)
>   - Name BIT(31) as PCH_BLC_PWM_ENABLE (Ilpo)
>   - Fix double-space in block comment (Ilpo)
>   - Reduce devm_backlight_device_register() to fewer lines (Ilpo)

>   - Extract dell_uart_bl_init() for the normal UART backlight path
>     so the probe function reads more clearly (Ilpo)

I suggest you do dell_uart_bl_init() refactor in a preparetory patch and 
do a two patch series. I believe it would make the patches easier to 
review.

-- 
 i.


>  .../platform/x86/dell/dell-uart-backlight.c   | 157 +++++++++++++++---
>  1 file changed, 134 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/platform/x86/dell/dell-uart-backlight.c b/drivers/platform/x86/dell/dell-uart-backlight.c
> index f323a667dc2d..edc7cfe38069 100644
> --- a/drivers/platform/x86/dell/dell-uart-backlight.c
> +++ b/drivers/platform/x86/dell/dell-uart-backlight.c
> @@ -13,10 +13,13 @@
>  #include <linux/delay.h>
>  #include <linux/device.h>
>  #include <linux/err.h>
> +#include <linux/io.h>
>  #include <linux/module.h>
>  #include <linux/mutex.h>
> +#include <linux/pci.h>
>  #include <linux/platform_device.h>
>  #include <linux/serdev.h>
> +#include <linux/sizes.h>
>  #include <linux/string.h>
>  #include <linux/types.h>
>  #include <linux/wait.h>
> @@ -50,6 +53,12 @@
>  #define MIN_RESP_LEN			3
>  #define MAX_RESP_LEN			80
>  
> +/* PCH PWM backlight registers relative to Intel GPU MMIO BAR0 */
> +#define PCH_BLC_PWM_CTL		0xC8250
> +#define PCH_BLC_PWM_FREQ	0xC8254
> +#define PCH_BLC_PWM_DUTY	0xC8258
> +#define PCH_BLC_PWM_ENABLE	BIT(31)
> +
>  struct dell_uart_backlight {
>  	struct mutex mutex;
>  	wait_queue_head_t wait_queue;
> @@ -64,6 +73,11 @@ struct dell_uart_backlight {
>  	int power;
>  };
>  
> +struct dell_pwm_backlight {
> +	void __iomem *mmio;
> +	u32 max_brightness;
> +};
> +
>  /* Checksum: SUM(Length and Cmd and Data) xor 0xFF */
>  static u8 dell_uart_checksum(u8 *buf, int len)
>  {
> @@ -267,32 +281,97 @@ static const struct serdev_device_ops dell_uart_bl_serdev_ops = {
>  	.write_wakeup = serdev_device_write_wakeup,
>  };
>  
> -static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
> +/*
> + * Some Dell AIO models (e.g. OptiPlex 7410) declare DELL0501 in ACPI but
> + * have no physical UART scalar board. When the UART times out fall back
> + * to driving the Intel PCH PWM backlight registers directly.
> + */
> +static int dell_pwm_update_status(struct backlight_device *bd)
>  {
> -	u8 get_version[GET_CMD_LEN], resp[MAX_RESP_LEN];
> +	struct dell_pwm_backlight *pwm = bl_get_data(bd);
> +	u32 duty;
> +
> +	if (bd->props.power != BACKLIGHT_POWER_ON ||
> +	    bd->props.state & BL_CORE_SUSPENDED)
> +		duty = 0;
> +	else
> +		duty = bd->props.brightness;
> +
> +	writel(duty, pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
> +	return 0;
> +}
> +
> +static int dell_pwm_get_brightness(struct backlight_device *bd)
> +{
> +	struct dell_pwm_backlight *pwm = bl_get_data(bd);
> +
> +	return readl(pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
> +}
> +
> +static const struct backlight_ops dell_pwm_backlight_ops = {
> +	.update_status  = dell_pwm_update_status,
> +	.get_brightness = dell_pwm_get_brightness,
> +};
> +
> +static int dell_pwm_fallback_register(struct device *dev)
> +{
> +	struct pci_dev *gpu __free(pci_dev_put) = NULL;
>  	struct backlight_properties props = {};
> -	struct dell_uart_backlight *dell_bl;
> -	struct device *dev = &serdev->dev;
> -	int ret;
> +	struct dell_pwm_backlight *pwm;
> +	struct backlight_device *bl;
> +	resource_size_t bar0;
> +	u32 freq, duty, ctl;
>  
> -	dell_bl = devm_kzalloc(dev, sizeof(*dell_bl), GFP_KERNEL);
> -	if (!dell_bl)
> +	gpu = pci_get_class(PCI_CLASS_DISPLAY_VGA << 8, NULL);
> +	if (!gpu || gpu->vendor != PCI_VENDOR_ID_INTEL) {
> +		dev_warn(dev, "No Intel GPU found, cannot fall back to PCH PWM\n");
> +		return -ENODEV;
> +	}
> +
> +	bar0 = pci_resource_start(gpu, 0);
> +	if (!bar0) {
> +		dev_warn(dev, "Intel GPU BAR0 not assigned\n");
> +		return -ENODEV;
> +	}
> +
> +	pwm = devm_kzalloc(dev, sizeof(*pwm), GFP_KERNEL);
> +	if (!pwm)
>  		return -ENOMEM;
>  
> -	mutex_init(&dell_bl->mutex);
> -	init_waitqueue_head(&dell_bl->wait_queue);
> -	dell_bl->dev = dev;
> +	pwm->mmio = devm_ioremap(dev, bar0 + PCH_BLC_PWM_CTL, SZ_16);
> +	if (!pwm->mmio) {
> +		dev_warn(dev, "Failed to ioremap PCH PWM registers\n");
> +		return -ENOMEM;
> +	}
>  
> -	serdev_device_set_drvdata(serdev, dell_bl);
> -	serdev_device_set_client_ops(serdev, &dell_uart_bl_serdev_ops);
> +	ctl  = readl(pwm->mmio);
> +	freq = readl(pwm->mmio + (PCH_BLC_PWM_FREQ - PCH_BLC_PWM_CTL));
> +	duty = readl(pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
>  
> -	ret = devm_serdev_device_open(dev, serdev);
> -	if (ret)
> -		return dev_err_probe(dev, ret, "opening UART device\n");
> +	if (!(ctl & PCH_BLC_PWM_ENABLE) || freq == 0) {
> +		dev_warn(dev, "PCH PWM not enabled by BIOS (ctl=0x%08x freq=%u)\n", ctl, freq);
> +		return -ENODEV;
> +	}
>  
> -	/* 9600 bps, no flow control, these are the default but set them to be sure */
> -	serdev_device_set_baudrate(serdev, 9600);
> -	serdev_device_set_flow_control(serdev, false);
> +	pwm->max_brightness = freq;
> +
> +	dev_info(dev, "Falling back to PCH PWM backlight (max=%u current=%u)\n", freq, duty);
> +
> +	props.type = BACKLIGHT_PLATFORM;
> +	props.brightness = duty;
> +	props.max_brightness = freq;
> +
> +	bl = devm_backlight_device_register(dev, "dell_uart_backlight", dev, pwm,
> +					    &dell_pwm_backlight_ops, &props);
> +	return PTR_ERR_OR_ZERO(bl);
> +}
> +
> +static int dell_uart_bl_init(struct device *dev, struct serdev_device *serdev,
> +			     struct dell_uart_backlight *dell_bl)
> +{
> +	u8 get_version[GET_CMD_LEN], resp[MAX_RESP_LEN];
> +	struct backlight_properties props = {};
> +	int ret;
>  
>  	get_version[0] = DELL_SOF(GET_CMD_LEN);
>  	get_version[1] = CMD_GET_VERSION;
> @@ -300,7 +379,7 @@ static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
>  
>  	ret = dell_uart_bl_command(dell_bl, get_version, GET_CMD_LEN, resp, MAX_RESP_LEN);
>  	if (ret)
> -		return dev_err_probe(dev, ret, "getting firmware version\n");
> +		return ret;
>  
>  	dev_dbg(dev, "Firmware version: %.*s\n", resp[RESP_LEN] - 3, resp + RESP_DATA);
>  
> @@ -318,13 +397,45 @@ static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
>  	props.max_brightness = DELL_BL_MAX_BRIGHTNESS;
>  	props.power = dell_bl->power;
>  
> -	dell_bl->bl = devm_backlight_device_register(dev, "dell_uart_backlight",
> -						     dev, dell_bl,
> -						     &dell_uart_backlight_ops,
> -						     &props);
> +	dell_bl->bl = devm_backlight_device_register(dev, "dell_uart_backlight", dev, dell_bl,
> +						     &dell_uart_backlight_ops, &props);
>  	return PTR_ERR_OR_ZERO(dell_bl->bl);
>  }
>  
> +static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
> +{
> +	struct dell_uart_backlight *dell_bl;
> +	struct device *dev = &serdev->dev;
> +	int ret;
> +
> +	dell_bl = devm_kzalloc(dev, sizeof(*dell_bl), GFP_KERNEL);
> +	if (!dell_bl)
> +		return -ENOMEM;
> +
> +	mutex_init(&dell_bl->mutex);
> +	init_waitqueue_head(&dell_bl->wait_queue);
> +	dell_bl->dev = dev;
> +
> +	serdev_device_set_drvdata(serdev, dell_bl);
> +	serdev_device_set_client_ops(serdev, &dell_uart_bl_serdev_ops);
> +
> +	ret = devm_serdev_device_open(dev, serdev);
> +	if (ret)
> +		return dev_err_probe(dev, ret, "opening UART device\n");
> +
> +	/* 9600 bps, no flow control, these are the default but set them to be sure */
> +	serdev_device_set_baudrate(serdev, 9600);
> +	serdev_device_set_flow_control(serdev, false);
> +
> +	ret = dell_uart_bl_init(dev, serdev, dell_bl);
> +	if (ret == -ETIMEDOUT) {
> +		dev_info(dev, "UART backlight controller not responding, trying PCH PWM fallback\n");
> +		return dell_pwm_fallback_register(dev);
> +	}
> +
> +	return ret;
> +}
> +
>  static struct serdev_device_driver dell_uart_bl_serdev_driver = {
>  	.probe = dell_uart_bl_serdev_probe,
>  	.driver = {
> 

[PATCH v3 0/2] platform/x86: dell-uart-backlight: fall back to PCH PWM on UART timeout

[chitranshmandhaniya]

From: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>

Some Dell OptiPlex AIO 7410 (35W) ship with DELL0501 in ACPI but no
physical UART scalar board, leaving the machine with zero backlight
control after the UART probe times out.

Patch 1 is a preparatory refactor that extracts the UART initialization
sequence into dell_uart_bl_init() so the probe function can inspect its
return value.

Patch 2 adds a PCH PWM fallback path: when dell_uart_bl_init() returns
-ETIMEDOUT, the driver reads the Intel PCH BLC_PWM registers and
registers a BACKLIGHT_PLATFORM device.

v2 -> v3:
  - Split into two patches: preparatory refactor + fallback (Ilpo)

v1 -> v2:
  - Use __free(pci_dev_put) for automatic PCI device cleanup (Ilpo)
  - Replace 0x10 with SZ_16, add linux/sizes.h (Ilpo)
  - Name BIT(31) as PCH_BLC_PWM_ENABLE (Ilpo)
  - Fix double-space in block comment (Ilpo)
  - Reduce devm_backlight_device_register() to fewer lines (Ilpo)
  - Extract dell_uart_bl_init() for the normal UART backlight path
    so the probe function reads more clearly (Ilpo)

Chitransh Mandhaniya (2):
  platform/x86: dell-uart-backlight: extract dell_uart_bl_init() from
    probe
  platform/x86: dell-uart-backlight: fall back to PCH PWM on UART
    timeout

 .../platform/x86/dell/dell-uart-backlight.c   | 156 +++++++++++++++---
 1 file changed, 133 insertions(+), 23 deletions(-)

-- 
2.43.0

[PATCH v3 1/2] platform/x86: dell-uart-backlight: extract dell_uart_bl_init() from probe

[chitranshmandhaniya]

From: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>

Move the UART backlight initialization sequence (get_version,
set_bl_power, get_brightness and backlight device registration) out of
dell_uart_bl_serdev_probe() into a new dell_uart_bl_init() helper.

No functional change. This is a preparatory refactor so that a
follow-up patch can inspect the return value of dell_uart_bl_init()
and take an alternative code path on failure.

Tested on OptiPlex AIO 7410 35W (01KP8D) and OptiPlex AIO 7420 35W
(0914RT) -- driver behaviour unchanged, backlight probes identically.

Signed-off-by: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>
---
 .../platform/x86/dell/dell-uart-backlight.c   | 59 ++++++++++---------
 1 file changed, 32 insertions(+), 27 deletions(-)

diff --git a/drivers/platform/x86/dell/dell-uart-backlight.c b/drivers/platform/x86/dell/dell-uart-backlight.c
index f323a667dc2d..e3b7f1beda5b 100644
--- a/drivers/platform/x86/dell/dell-uart-backlight.c
+++ b/drivers/platform/x86/dell/dell-uart-backlight.c
@@ -267,40 +267,19 @@ static const struct serdev_device_ops dell_uart_bl_serdev_ops = {
 	.write_wakeup = serdev_device_write_wakeup,
 };
 
-static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
+static int dell_uart_bl_init(struct device *dev, struct dell_uart_backlight *dell_bl)
 {
 	u8 get_version[GET_CMD_LEN], resp[MAX_RESP_LEN];
 	struct backlight_properties props = {};
-	struct dell_uart_backlight *dell_bl;
-	struct device *dev = &serdev->dev;
 	int ret;
 
-	dell_bl = devm_kzalloc(dev, sizeof(*dell_bl), GFP_KERNEL);
-	if (!dell_bl)
-		return -ENOMEM;
-
-	mutex_init(&dell_bl->mutex);
-	init_waitqueue_head(&dell_bl->wait_queue);
-	dell_bl->dev = dev;
-
-	serdev_device_set_drvdata(serdev, dell_bl);
-	serdev_device_set_client_ops(serdev, &dell_uart_bl_serdev_ops);
-
-	ret = devm_serdev_device_open(dev, serdev);
-	if (ret)
-		return dev_err_probe(dev, ret, "opening UART device\n");
-
-	/* 9600 bps, no flow control, these are the default but set them to be sure */
-	serdev_device_set_baudrate(serdev, 9600);
-	serdev_device_set_flow_control(serdev, false);
-
 	get_version[0] = DELL_SOF(GET_CMD_LEN);
 	get_version[1] = CMD_GET_VERSION;
 	get_version[2] = dell_uart_checksum(get_version, 2);
 
 	ret = dell_uart_bl_command(dell_bl, get_version, GET_CMD_LEN, resp, MAX_RESP_LEN);
 	if (ret)
-		return dev_err_probe(dev, ret, "getting firmware version\n");
+		return ret;
 
 	dev_dbg(dev, "Firmware version: %.*s\n", resp[RESP_LEN] - 3, resp + RESP_DATA);
 
@@ -318,13 +297,39 @@ static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
 	props.max_brightness = DELL_BL_MAX_BRIGHTNESS;
 	props.power = dell_bl->power;
 
-	dell_bl->bl = devm_backlight_device_register(dev, "dell_uart_backlight",
-						     dev, dell_bl,
-						     &dell_uart_backlight_ops,
-						     &props);
+	dell_bl->bl = devm_backlight_device_register(dev, "dell_uart_backlight", dev, dell_bl,
+						     &dell_uart_backlight_ops, &props);
 	return PTR_ERR_OR_ZERO(dell_bl->bl);
 }
 
+static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
+{
+	struct dell_uart_backlight *dell_bl;
+	struct device *dev = &serdev->dev;
+	int ret;
+
+	dell_bl = devm_kzalloc(dev, sizeof(*dell_bl), GFP_KERNEL);
+	if (!dell_bl)
+		return -ENOMEM;
+
+	mutex_init(&dell_bl->mutex);
+	init_waitqueue_head(&dell_bl->wait_queue);
+	dell_bl->dev = dev;
+
+	serdev_device_set_drvdata(serdev, dell_bl);
+	serdev_device_set_client_ops(serdev, &dell_uart_bl_serdev_ops);
+
+	ret = devm_serdev_device_open(dev, serdev);
+	if (ret)
+		return dev_err_probe(dev, ret, "opening UART device\n");
+
+	/* 9600 bps, no flow control, these are the default but set them to be sure */
+	serdev_device_set_baudrate(serdev, 9600);
+	serdev_device_set_flow_control(serdev, false);
+
+	return dell_uart_bl_init(dev, dell_bl);
+}
+
 static struct serdev_device_driver dell_uart_bl_serdev_driver = {
 	.probe = dell_uart_bl_serdev_probe,
 	.driver = {
-- 
2.43.0

[PATCH v3 2/2] platform/x86: dell-uart-backlight: fall back to PCH PWM on UART timeout

[chitranshmandhaniya]

From: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>

Some Dell OptiPlex AIO 7410 (35W, board 01KP8D) ship with DELL0501 in
the ACPI DSDT (status = 0x0F) even though no UART scalar board is
physically present.  This causes acpi_video_get_backlight_type() to
return acpi_backlight_dell_uart, which makes i915 skip registering
intel_backlight.  The dell-uart-backlight driver then times out talking
to the absent board and the probe fails, leaving the machine with zero
backlight control.

A BIOS update fixes this on some models (e.g. OptiPlex 7420, board
0914RT) by setting DELL0501 _STA to 0, but Dell has not shipped a
corrected BIOS for the 7410 even in the latest release (1.37.0,
2026-05-28).

When the initial get_version command returns -ETIMEDOUT, fall back to
driving the backlight through the Intel PCH PWM registers (BLC_PWM_CTL /
BLC_PWM_FREQ / BLC_PWM_DUTY at MMIO BAR0 + 0xC8250).  The fallback
reads the BIOS-programmed PWM frequency as max_brightness and registers
a BACKLIGHT_PLATFORM device so that compositors (Weston, GNOME, KDE)
pick it up normally.

Tested on:
  - OptiPlex AIO 7410 35W (01KP8D) -- UART times out, PCH PWM fallback
    activates, backlight fully functional via sysfs and compositor.
  - OptiPlex AIO 7420 35W (0914RT, old BIOS) -- same timeout and
    fallback, backlight works.
  - OptiPlex AIO 7420 35W (0914RT, new BIOS) -- DELL0501 absent, driver
    not loaded, intel_backlight works natively (no regression).

Signed-off-by: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>
---
 .../platform/x86/dell/dell-uart-backlight.c   | 107 +++++++++++++++++-
 1 file changed, 106 insertions(+), 1 deletion(-)

diff --git a/drivers/platform/x86/dell/dell-uart-backlight.c b/drivers/platform/x86/dell/dell-uart-backlight.c
index e3b7f1beda5b..3179f3a3f094 100644
--- a/drivers/platform/x86/dell/dell-uart-backlight.c
+++ b/drivers/platform/x86/dell/dell-uart-backlight.c
@@ -13,10 +13,13 @@
 #include <linux/delay.h>
 #include <linux/device.h>
 #include <linux/err.h>
+#include <linux/io.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
+#include <linux/pci.h>
 #include <linux/platform_device.h>
 #include <linux/serdev.h>
+#include <linux/sizes.h>
 #include <linux/string.h>
 #include <linux/types.h>
 #include <linux/wait.h>
@@ -50,6 +53,12 @@
 #define MIN_RESP_LEN			3
 #define MAX_RESP_LEN			80
 
+/* PCH PWM backlight registers relative to Intel GPU MMIO BAR0 */
+#define PCH_BLC_PWM_CTL		0xC8250
+#define PCH_BLC_PWM_FREQ	0xC8254
+#define PCH_BLC_PWM_DUTY	0xC8258
+#define PCH_BLC_PWM_ENABLE	BIT(31)
+
 struct dell_uart_backlight {
 	struct mutex mutex;
 	wait_queue_head_t wait_queue;
@@ -64,6 +73,11 @@ struct dell_uart_backlight {
 	int power;
 };
 
+struct dell_pwm_backlight {
+	void __iomem *mmio;
+	u32 max_brightness;
+};
+
 /* Checksum: SUM(Length and Cmd and Data) xor 0xFF */
 static u8 dell_uart_checksum(u8 *buf, int len)
 {
@@ -267,6 +281,91 @@ static const struct serdev_device_ops dell_uart_bl_serdev_ops = {
 	.write_wakeup = serdev_device_write_wakeup,
 };
 
+/*
+ * Some Dell AIO models (e.g. OptiPlex 7410) declare DELL0501 in ACPI but
+ * have no physical UART scalar board. When the UART times out fall back
+ * to driving the Intel PCH PWM backlight registers directly.
+ */
+static int dell_pwm_update_status(struct backlight_device *bd)
+{
+	struct dell_pwm_backlight *pwm = bl_get_data(bd);
+	u32 duty;
+
+	if (bd->props.power != BACKLIGHT_POWER_ON ||
+	    bd->props.state & BL_CORE_SUSPENDED)
+		duty = 0;
+	else
+		duty = bd->props.brightness;
+
+	writel(duty, pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
+	return 0;
+}
+
+static int dell_pwm_get_brightness(struct backlight_device *bd)
+{
+	struct dell_pwm_backlight *pwm = bl_get_data(bd);
+
+	return readl(pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
+}
+
+static const struct backlight_ops dell_pwm_backlight_ops = {
+	.update_status  = dell_pwm_update_status,
+	.get_brightness = dell_pwm_get_brightness,
+};
+
+static int dell_pwm_fallback_register(struct device *dev)
+{
+	struct pci_dev *gpu __free(pci_dev_put) = NULL;
+	struct backlight_properties props = {};
+	struct dell_pwm_backlight *pwm;
+	struct backlight_device *bl;
+	resource_size_t bar0;
+	u32 freq, duty, ctl;
+
+	gpu = pci_get_class(PCI_CLASS_DISPLAY_VGA << 8, NULL);
+	if (!gpu || gpu->vendor != PCI_VENDOR_ID_INTEL) {
+		dev_warn(dev, "No Intel GPU found, cannot fall back to PCH PWM\n");
+		return -ENODEV;
+	}
+
+	bar0 = pci_resource_start(gpu, 0);
+	if (!bar0) {
+		dev_warn(dev, "Intel GPU BAR0 not assigned\n");
+		return -ENODEV;
+	}
+
+	pwm = devm_kzalloc(dev, sizeof(*pwm), GFP_KERNEL);
+	if (!pwm)
+		return -ENOMEM;
+
+	pwm->mmio = devm_ioremap(dev, bar0 + PCH_BLC_PWM_CTL, SZ_16);
+	if (!pwm->mmio) {
+		dev_warn(dev, "Failed to ioremap PCH PWM registers\n");
+		return -ENOMEM;
+	}
+
+	ctl  = readl(pwm->mmio);
+	freq = readl(pwm->mmio + (PCH_BLC_PWM_FREQ - PCH_BLC_PWM_CTL));
+	duty = readl(pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
+
+	if (!(ctl & PCH_BLC_PWM_ENABLE) || freq == 0) {
+		dev_warn(dev, "PCH PWM not enabled by BIOS (ctl=0x%08x freq=%u)\n", ctl, freq);
+		return -ENODEV;
+	}
+
+	pwm->max_brightness = freq;
+
+	dev_info(dev, "Falling back to PCH PWM backlight (max=%u current=%u)\n", freq, duty);
+
+	props.type = BACKLIGHT_PLATFORM;
+	props.brightness = duty;
+	props.max_brightness = freq;
+
+	bl = devm_backlight_device_register(dev, "dell_uart_backlight", dev, pwm,
+					    &dell_pwm_backlight_ops, &props);
+	return PTR_ERR_OR_ZERO(bl);
+}
+
 static int dell_uart_bl_init(struct device *dev, struct dell_uart_backlight *dell_bl)
 {
 	u8 get_version[GET_CMD_LEN], resp[MAX_RESP_LEN];
@@ -327,7 +426,13 @@ static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
 	serdev_device_set_baudrate(serdev, 9600);
 	serdev_device_set_flow_control(serdev, false);
 
-	return dell_uart_bl_init(dev, dell_bl);
+	ret = dell_uart_bl_init(dev, dell_bl);
+	if (ret == -ETIMEDOUT) {
+		dev_info(dev, "UART backlight controller not responding, trying PCH PWM fallback\n");
+		return dell_pwm_fallback_register(dev);
+	}
+
+	return ret;
 }
 
 static struct serdev_device_driver dell_uart_bl_serdev_driver = {
-- 
2.43.0