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([2401:4900:9397:bb62:5cb7:971b:ead7:8dc5]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3ab36f4737esm3426729a91.7.2026.10.09.05.27.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 09 Oct 2026 05:27:16 -0700 (PDT) From: chitranshmandhaniya@gmail.com To: hansg@kernel.org Cc: ilpo.jarvinen@linux.intel.com, platform-driver-x86@vger.kernel.org, linux-kernel@vger.kernel.org, Chitransh_Mandhaniya@dell.com Subject: [PATCH v3 2/2] platform/x86: dell-uart-backlight: fall back to PCH PWM on UART timeout Date: Fri, 9 Oct 2026 17:56:02 +0530 Message-ID: <20261009122602.56624-3-chitranshmandhaniya@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20261009122602.56624-1-chitranshmandhaniya@gmail.com> References: <20261009072401.52509-1-chitranshmandhaniya@gmail.com> <20261009122602.56624-1-chitranshmandhaniya@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Chitransh Mandhaniya 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 --- .../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 #include #include +#include #include #include +#include #include #include +#include #include #include #include @@ -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