[PATCH v4 0/5] serial: 8250_dw: describe the Allwinner H616 UART configuration

[PATCH v4 0/5] serial: 8250_dw: describe the Allwinner H616 UART configuration

[James Hilliard]

H616 has 256-byte UART FIFOs, but lacks the DesignWare component parameter
register used to discover their capacity. Its debug-divisor and RS485
registers also overlap offsets used to probe optional DesignWare features.

Add an H616 compatible for the integration-specific setup. Keep the
existing 16550A port type and describe its fixed FIFO depth in platform
data. Use common Allwinner helpers to configure the transmit load and
receive thresholds from that depth. Configure automatic flow control,
preserve the BUSY-safe integer divisor path and keep the UART gate enabled
during termios changes. The H616 configuration uses PIO.

The series first introduces platform setup callbacks and a shared RX
trigger conversion helper, then adds the binding, H616 setup and SoC DTS
changes. Based on tty-next at 6b4b543fa187 with the posted RX
trigger callback-lifetime fix [0] and DW BUSY-safe divisor fix v4 [1].
Platform setup runs before installing the BUSY-safe divisor setter.

[0] https://lore.kernel.org/r/20261024-submit-serial-8250-rxtrig-lifetime-v1-v1-1-731b78d5003d@gmail.com
[1] https://lore.kernel.org/r/20261027-submit-serial-8250-dw-divisor-v1-v4-1-41714a75e15a@gmail.com

Assisted-by: Codex:gpt-6-astra
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
Changes in v4:
- Introduce common Allwinner setup helpers with the fixed FIFO depth
  supplied by compatible-specific platform data.
- Rebase onto tty-next at 6b4b543fa187 and the posted DW divisor v4 fix,
  installing the BUSY-safe setter after platform setup.
- Use an early return for FIFO-less DMA setup, following Andy's review.
- Default to generic setup when platform data has no setup callback.
- Select Octeon's optional-register discovery with IS_ENABLED() in its
  setup helper, preserving the existing 32-bit and 64-bit behavior.
- Extract the shared 8250 bytes-to-FCR conversion, including the
  unsupported-table check, into a preparatory patch and use it for
  H616, following Andy's review.
- Set the fixed 256-byte FIFO depth in H616 platform data and omit
  fifo-size from its binding and DTS, following Krzysztof's review.
- Precompute the Allwinner RX thresholds during setup and retain the
  sunxi FIFO-depth helper for filling the table. Use named FCR encodings
  in the helper, following Jiri's review.
- Share threshold-table initialization and callback assignment through
  a sunxi RX trigger setup helper.
- Group and comment the H616 trigger and RS485 setup.
- Link to v3: https://patch.msgid.link/20261024-submit-h616-uart-fifo-v1-v3-0-286fc1b8c3c1@gmail.com

Changes in v3:
- Introduce a platform-data setup callback in a preparatory patch,
  replacing skip_autocfg and preserving the existing Octeon setup.
- Describe each H616 UART's FIFO capacity with the standard fifo-size DT
  property and use the value already read by uart_read_port_properties().
- Derive the Allwinner RX trigger thresholds from the FIFO depth.
- Use an unsigned trigger index and conditional runtime PM guard,
  preserving error propagation when the device cannot resume.
- Rebase onto tty-next, replacing the earlier DMA prerequisites with the
  posted RX trigger callback-lifetime fix.
- Separate DMA enablement into follow-up work, retaining PIO operation
  for the H616 FIFO configuration.
- Link to v2: https://lore.kernel.org/r/20261023-submit-h616-uart-fifo-v1-v2-0-af0593504635@gmail.com

Changes in v2:
- Keep PORT_16550A and set the FIFO size, transmit load and capabilities
  during setup, following Andy's review.
- Select H616 setup through the matched platform-data pointer and remove
  the type and DMA burst-size platform-data members.
- Use get_rxtrig/set_rxtrig for the 1/64/128/254-byte receive thresholds,
  with port locking and runtime PM around register updates. Keep the
  standard half-full FCR default, selecting 128 bytes on H616.
- Advertise UART_CAP_AFE so CRTSCTS enables hardware automatic flow control.
- Rebase onto next-20261023 and declare the posted sun6i pause/lifetime
  fixes and their prerequisites for DMA enablement.
- Link to v1: https://lore.kernel.org/r/20261023-submit-h616-uart-fifo-v1-v1-0-e4a3bae2eba8@gmail.com

To: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Jiri Slaby <jirislaby@kernel.org>
To: Rob Herring <robh@kernel.org>
To: Krzysztof Kozlowski <krzk+dt@kernel.org>
To: Conor Dooley <conor+dt@kernel.org>
To: Chen-Yu Tsai <wens@kernel.org>
To: Jernej Skrabec <jernej.skrabec@gmail.com>
To: Samuel Holland <samuel@sholland.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-serial@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-sunxi@lists.linux.dev

---
James Hilliard (5):
      serial: 8250_dw: select port setup through platform data
      serial: 8250: share the RX trigger conversion helper
      dt-bindings: serial: snps-dw-apb-uart: add Allwinner H616
      serial: 8250_dw: configure the Allwinner H616 UART
      arm64: dts: allwinner: h616: describe the UART integration

 .../bindings/serial/snps-dw-apb-uart.yaml          |   1 +
 arch/arm64/boot/dts/allwinner/sun50i-h616.dtsi     |  12 +-
 drivers/tty/serial/8250/8250.h                     |  16 +++
 drivers/tty/serial/8250/8250_dw.c                  | 138 +++++++++++++++++++--
 drivers/tty/serial/8250/8250_port.c                |  12 +-
 5 files changed, 151 insertions(+), 28 deletions(-)
---
base-commit: 6b4b543fa18757297ac756a6fe6e64f506b7844a
change-id: 20261022-submit-h616-uart-fifo-v1-c5afcdfe5cb0
prerequisite-message-id: <20261027-submit-serial-8250-dw-divisor-v1-v4-1-41714a75e15a@gmail.com>
prerequisite-patch-id: d8219673210dac82d3adab41578ff2f76f5862c9
prerequisite-message-id: <20261024-submit-serial-8250-rxtrig-lifetime-v1-v1-1-731b78d5003d@gmail.com>
prerequisite-patch-id: 9a9395c715c2bf7bbcae5bdd16a339d7f2af6269

Best regards,
--  
James Hilliard <james.hilliard1@gmail.com>

[PATCH v4 1/5] serial: 8250_dw: select port setup through platform data

[James Hilliard]

Select feature discovery and DMA attachment through a platform-data setup
callback. The generic callback probes the optional registers and attaches
DMA when a FIFO size is available. Integrations with a fixed configuration
can supply their own setup before the port is registered.

Replace skip_autocfg with the Octeon setup selection. On 64-bit builds,
Octeon continues to skip optional-register probing while retaining its DMA
setup; on 32-bit builds it retains the generic setup. Keep generic setup
for devices without matched platform data as well.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
 drivers/tty/serial/8250/8250_dw.c | 44 +++++++++++++++++++++++++++++----------
 1 file changed, 33 insertions(+), 11 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 9cbe076463e0..1351523f0c47 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -64,6 +64,7 @@ struct dw8250_platform_data {
 	u8 usr_reg;
 	u32 cpr_value;
 	unsigned int quirks;
+	void (*setup)(struct uart_port *p);
 };
 
 struct dw8250_data {
@@ -76,7 +77,6 @@ struct dw8250_data {
 	struct clk		*pclk;
 	struct reset_control	*rst;
 
-	unsigned int		skip_autocfg:1;
 	unsigned int		uart_16550_compatible:1;
 	unsigned int		in_idle:1;
 
@@ -596,7 +596,6 @@ static void dw8250_quirks(struct uart_port *p, struct dw8250_data *data)
 		p->serial_out = dw8250_serial_outq;
 		p->flags = UPF_SKIP_TEST | UPF_SHARE_IRQ | UPF_FIXED_TYPE;
 		p->type = PORT_OCTEON;
-		data->skip_autocfg = true;
 	}
 #endif
 
@@ -631,6 +630,33 @@ static void dw8250_shutdown(struct uart_port *port)
 	d->no_int_count = 0;
 }
 
+static void dw8250_setup_dma(struct uart_port *p)
+{
+	struct dw8250_data *data = to_dw8250_data(p->private_data);
+	struct uart_8250_port *up = up_to_u8250p(p);
+
+	/* DMA setup requires a FIFO. */
+	if (!p->fifosize)
+		return;
+
+	data->data.dma.rxconf.src_maxburst = p->fifosize / 4;
+	data->data.dma.txconf.dst_maxburst = p->fifosize / 4;
+	up->dma = &data->data.dma;
+}
+
+static void dw8250_setup(struct uart_port *p)
+{
+	dw8250_setup_port(p);
+	dw8250_setup_dma(p);
+}
+
+static void dw8250_octeon_setup(struct uart_port *p)
+{
+	if (!IS_ENABLED(CONFIG_64BIT))
+		dw8250_setup_port(p);
+	dw8250_setup_dma(p);
+}
+
 static int dw8250_probe(struct platform_device *pdev)
 {
 	struct uart_8250_port uart = {}, *up = &uart;
@@ -765,19 +791,14 @@ static int dw8250_probe(struct platform_device *pdev)
 
 	dw8250_setup_dma_filter(p, data);
 
-	if (!data->skip_autocfg)
-		dw8250_setup_port(p);
+	if (data->pdata && data->pdata->setup)
+		data->pdata->setup(p);
+	else
+		dw8250_setup(p);
 
 	/* Use the BUSY-safe sequence for both integer and fractional divisors. */
 	p->set_divisor = dw8250_set_divisor;
 
-	/* If we have a valid fifosize, try hooking up DMA */
-	if (p->fifosize) {
-		data->data.dma.rxconf.src_maxburst = p->fifosize / 4;
-		data->data.dma.txconf.dst_maxburst = p->fifosize / 4;
-		up->dma = &data->data.dma;
-	}
-
 	data->data.uport = serial8250_register_8250_port(up);
 	if (IS_ERR(data->data.uport))
 		return PTR_ERR(data->data.uport);
@@ -860,6 +881,7 @@ static const struct dw8250_platform_data dw8250_dw_apb = {
 static const struct dw8250_platform_data dw8250_octeon_3860_data = {
 	.usr_reg = OCTEON_UART_USR,
 	.quirks = DW_UART_QUIRK_OCTEON,
+	.setup = dw8250_octeon_setup,
 };
 
 static const struct dw8250_platform_data dw8250_armada_38x_data = {

-- 
2.53.0

[PATCH v4 2/5] serial: 8250: share the RX trigger conversion helper

[James Hilliard]

Extract the receive-trigger threshold search into serial8250_bytes_to_fcr()
so drivers with per-port threshold tables can use the same conversion as
the generic 8250 implementation.

Include the unsupported-table check in the helper and preserve the existing
nearest-lower selection and highest-encoding fallback.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
 drivers/tty/serial/8250/8250.h      | 16 ++++++++++++++++
 drivers/tty/serial/8250/8250_port.c | 12 +-----------
 2 files changed, 17 insertions(+), 11 deletions(-)

diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index 21ed9c41b751..195ed58a2231 100644
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -8,6 +8,7 @@
  */
 
 #include <linux/bits.h>
+#include <linux/errno.h>
 #include <linux/serial_8250.h>
 #include <linux/serial_core.h>
 #include <linux/dmaengine.h>
@@ -73,6 +74,21 @@ struct serial8250_config {
 	unsigned int	flags;
 };
 
+static inline int serial8250_bytes_to_fcr(const unsigned char *rxtrig_bytes,
+					  unsigned char bytes)
+{
+	if (!rxtrig_bytes[UART_FCR_R_TRIG_BITS(UART_FCR_R_TRIG_00)])
+		return -EOPNOTSUPP;
+
+	for (unsigned int i = 1; i < UART_FCR_R_TRIG_MAX_STATE; i++) {
+		if (bytes < rxtrig_bytes[i])
+			/* Use the nearest lower value */
+			return (i - 1) << UART_FCR_R_TRIG_SHIFT;
+	}
+
+	return UART_FCR_R_TRIG_11;
+}
+
 enum uart_port_type {
 	/* Internal 8250 only */
 	UART_PORT_AIROHA		= 124,
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 36b2b0a2c05c..c332963dc2ed 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -3018,18 +3018,8 @@ static int fcr_get_rxtrig_bytes(struct uart_8250_port *up)
 static int bytes_to_fcr_rxtrig(struct uart_8250_port *up, unsigned char bytes)
 {
 	const struct serial8250_config *conf_type = &uart_config[up->port.type];
-	int i;
 
-	if (!conf_type->rxtrig_bytes[UART_FCR_R_TRIG_BITS(UART_FCR_R_TRIG_00)])
-		return -EOPNOTSUPP;
-
-	for (i = 1; i < UART_FCR_R_TRIG_MAX_STATE; i++) {
-		if (bytes < conf_type->rxtrig_bytes[i])
-			/* Use the nearest lower value */
-			return (--i) << UART_FCR_R_TRIG_SHIFT;
-	}
-
-	return UART_FCR_R_TRIG_11;
+	return serial8250_bytes_to_fcr(conf_type->rxtrig_bytes, bytes);
 }
 
 static int do_get_rxtrig(struct tty_port *port)

-- 
2.53.0

Re: [PATCH v4 2/5] serial: 8250: share the RX trigger conversion helper

[Ilpo Järvinen]

On Tue, 27 Oct 2026, James Hilliard wrote:

> Extract the receive-trigger threshold search into serial8250_bytes_to_fcr()
> so drivers with per-port threshold tables can use the same conversion as
> the generic 8250 implementation.
> 
> Include the unsupported-table check in the helper and preserve the existing
> nearest-lower selection and highest-encoding fallback.
> 
> Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
> ---
>  drivers/tty/serial/8250/8250.h      | 16 ++++++++++++++++
>  drivers/tty/serial/8250/8250_port.c | 12 +-----------
>  2 files changed, 17 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> index 21ed9c41b751..195ed58a2231 100644
> --- a/drivers/tty/serial/8250/8250.h
> +++ b/drivers/tty/serial/8250/8250.h
> @@ -8,6 +8,7 @@
>   */
>  
>  #include <linux/bits.h>
> +#include <linux/errno.h>
>  #include <linux/serial_8250.h>
>  #include <linux/serial_core.h>
>  #include <linux/dmaengine.h>
> @@ -73,6 +74,21 @@ struct serial8250_config {
>  	unsigned int	flags;
>  };
>  
> +static inline int serial8250_bytes_to_fcr(const unsigned char *rxtrig_bytes,

This looks way to complicated to be inline function so I think it should 
be kept in 8250_port and exported from there.

> +					  unsigned char bytes)
> +{
> +	if (!rxtrig_bytes[UART_FCR_R_TRIG_BITS(UART_FCR_R_TRIG_00)])
> +		return -EOPNOTSUPP;
> +
> +	for (unsigned int i = 1; i < UART_FCR_R_TRIG_MAX_STATE; i++) {
> +		if (bytes < rxtrig_bytes[i])
> +			/* Use the nearest lower value */
> +			return (i - 1) << UART_FCR_R_TRIG_SHIFT;
> +	}
> +
> +	return UART_FCR_R_TRIG_11;
> +}
> +
>  enum uart_port_type {
>  	/* Internal 8250 only */
>  	UART_PORT_AIROHA		= 124,
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 36b2b0a2c05c..c332963dc2ed 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -3018,18 +3018,8 @@ static int fcr_get_rxtrig_bytes(struct uart_8250_port *up)
>  static int bytes_to_fcr_rxtrig(struct uart_8250_port *up, unsigned char bytes)
>  {
>  	const struct serial8250_config *conf_type = &uart_config[up->port.type];
> -	int i;
>  
> -	if (!conf_type->rxtrig_bytes[UART_FCR_R_TRIG_BITS(UART_FCR_R_TRIG_00)])
> -		return -EOPNOTSUPP;
> -
> -	for (i = 1; i < UART_FCR_R_TRIG_MAX_STATE; i++) {
> -		if (bytes < conf_type->rxtrig_bytes[i])
> -			/* Use the nearest lower value */
> -			return (--i) << UART_FCR_R_TRIG_SHIFT;
> -	}
> -
> -	return UART_FCR_R_TRIG_11;
> +	return serial8250_bytes_to_fcr(conf_type->rxtrig_bytes, bytes);
>  }
>  
>  static int do_get_rxtrig(struct tty_port *port)
> 
> 

-- 
 i.

[PATCH v4 3/5] dt-bindings: serial: snps-dw-apb-uart: add Allwinner H616

[James Hilliard]

The H616 UART has 256-byte transmit and receive FIFOs, but its DesignWare
component parameter register does not report the FIFO depth. Its optional
register layout also differs from the generic DesignWare integration.

Add a specific compatible to identify this fixed configuration, retaining
snps,dw-apb-uart as a fallback.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
 Documentation/devicetree/bindings/serial/snps-dw-apb-uart.yaml | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Documentation/devicetree/bindings/serial/snps-dw-apb-uart.yaml b/Documentation/devicetree/bindings/serial/snps-dw-apb-uart.yaml
index e2ca126d58ba..fc1289073241 100644
--- a/Documentation/devicetree/bindings/serial/snps-dw-apb-uart.yaml
+++ b/Documentation/devicetree/bindings/serial/snps-dw-apb-uart.yaml
@@ -60,6 +60,7 @@ properties:
           - const: renesas,rzn1-uart
       - items:
           - enum:
+              - allwinner,sun50i-h616-uart
               - anlogic,dr1v90-uart
               - brcm,bcm11351-dw-apb-uart
               - brcm,bcm21664-dw-apb-uart

-- 
2.53.0

Re: [PATCH v4 3/5] dt-bindings: serial: snps-dw-apb-uart: add Allwinner H616

[Krzysztof Kozlowski]

On Tue, Oct 27, 2026 at 05:05:31PM -0600, James Hilliard wrote:
> The H616 UART has 256-byte transmit and receive FIFOs, but its DesignWare
> component parameter register does not report the FIFO depth. Its optional
> register layout also differs from the generic DesignWare integration.
> 
> Add a specific compatible to identify this fixed configuration, retaining
> snps,dw-apb-uart as a fallback.
> 
> Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
> ---
>  Documentation/devicetree/bindings/serial/snps-dw-apb-uart.yaml | 1 +
>  1 file changed, 1 insertion(+)

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>

Best regards,
Krzysztof

[PATCH v4 4/5] serial: 8250_dw: configure the Allwinner H616 UART

[James Hilliard]

H616 has 256-byte transmit and receive FIFOs, but its optional DesignWare
component parameter register reads as zero. Generic 16550 probing leaves
the port using the 16-byte FIFO configuration and its trigger-level table.

Keep PORT_16550A and describe H616's fixed 256-byte FIFO depth in platform
data. Use common Allwinner setup helpers to configure the transmit load
and receive trigger table from that depth.

Derive the receive thresholds from the documented 1-byte, quarter-full,
half-full and FIFO-depth-minus-two mapping. On H616 these are 1, 64, 128
and 254 bytes; the normal 16550A half-full FCR setting selects 128 bytes.

Build the threshold table during setup and use the shared 8250 conversion
helper to select the nearest lower threshold for sysfs writes.

Serialize trigger changes with termios and hold a runtime PM reference
while programming FCR, preserving the FIFO contents. Propagate runtime
resume errors before changing either the FCR shadow or the hardware.

Advertise automatic flow control so CRTSCTS enables MCR.AFCE. The
DesignWare termios path selects hardware CTS handling, so the capability
must be present for the corresponding hardware control to be enabled.

Select the fixed configuration through the platform setup callback.
H616 has debug-divisor and RS485 control registers at the offsets used to
detect DesignWare hardware RS485 and fractional-divisor support. Avoid
those probes and retain the BUSY-safe integer divisor hook and software
RS485 handling.

Use the existing SKIP_SET_RATE quirk to keep the UART clock enabled during
termios changes. Its clock is a gate on the shared APB2 source, rather than
an independent baud-rate generator.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
 drivers/tty/serial/8250/8250_dw.c | 94 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 94 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 1351523f0c47..2f156c01c5a1 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -19,6 +19,7 @@
 #include <linux/io.h>
 #include <linux/lockdep.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
 #include <linux/platform_device.h>
 #include <linux/pm_runtime.h>
 #include <linux/property.h>
@@ -63,6 +64,7 @@
 struct dw8250_platform_data {
 	u8 usr_reg;
 	u32 cpr_value;
+	unsigned int fifo_size;
 	unsigned int quirks;
 	void (*setup)(struct uart_port *p);
 };
@@ -81,6 +83,7 @@ struct dw8250_data {
 	unsigned int		in_idle:1;
 
 	u8			no_int_count;
+	u8			rxtrig_bytes[UART_FCR_R_TRIG_MAX_STATE];
 };
 
 static inline struct dw8250_data *to_dw8250_data(struct dw8250_port_data *data)
@@ -657,6 +660,89 @@ static void dw8250_octeon_setup(struct uart_port *p)
 	dw8250_setup_dma(p);
 }
 
+/* The trigger is the masked, unshifted FCR receive trigger field. */
+static unsigned int dw8250_sunxi_rxtrig_bytes(struct uart_port *p, unsigned int trigger)
+{
+	switch (trigger) {
+	case UART_FCR_TRIGGER_1:
+		return 1;
+	case UART_FCR_TRIGGER_4:
+		return p->fifosize / 4;
+	case UART_FCR_TRIGGER_8:
+		return p->fifosize / 2;
+	default:
+		return p->fifosize - 2;
+	}
+}
+
+static int dw8250_sunxi_get_rxtrig(struct uart_port *p)
+{
+	struct dw8250_data *data = to_dw8250_data(p->private_data);
+	struct uart_8250_port *up = up_to_u8250p(p);
+
+	guard(uart_port_lock_irqsave)(p);
+
+	return data->rxtrig_bytes[UART_FCR_R_TRIG_BITS(up->fcr)];
+}
+
+static int dw8250_sunxi_set_rxtrig(struct uart_port *p, unsigned char bytes)
+{
+	struct dw8250_data *data = to_dw8250_data(p->private_data);
+	struct uart_8250_port *up = up_to_u8250p(p);
+	int fcr, ret;
+
+	fcr = serial8250_bytes_to_fcr(data->rxtrig_bytes, bytes);
+	if (fcr < 0)
+		return fcr;
+
+	guard(mutex)(&p->state->port.mutex);
+
+	PM_RUNTIME_ACQUIRE_IF_ENABLED(p->dev, pm);
+	ret = PM_RUNTIME_ACQUIRE_ERR(&pm);
+	if (ret)
+		return ret;
+
+	guard(uart_port_lock_irqsave)(p);
+
+	up->fcr &= ~UART_FCR_TRIGGER_MASK;
+	up->fcr |= fcr;
+	/* Changing the trigger alone preserves the FIFO contents. */
+	serial_port_out(p, UART_FCR, up->fcr);
+
+	return 0;
+}
+
+static void dw8250_sunxi_setup_rxtrig(struct uart_port *p)
+{
+	struct dw8250_data *data = to_dw8250_data(p->private_data);
+
+	/* Allwinner RX thresholds differ from the standard 16550A table. */
+	for (unsigned int i = 0; i < ARRAY_SIZE(data->rxtrig_bytes); i++)
+		data->rxtrig_bytes[i] = dw8250_sunxi_rxtrig_bytes(p, i << UART_FCR_R_TRIG_SHIFT);
+	p->get_rxtrig = dw8250_sunxi_get_rxtrig;
+	p->set_rxtrig = dw8250_sunxi_set_rxtrig;
+}
+
+static void dw8250_sunxi_setup(struct uart_port *p)
+{
+	struct dw8250_data *data = to_dw8250_data(p->private_data);
+	struct uart_8250_port *up = up_to_u8250p(p);
+
+	p->type = PORT_16550A;
+	p->flags |= UPF_FIXED_TYPE;
+	p->fifosize = data->pdata->fifo_size;
+	up->tx_loadsz = p->fifosize;
+	up->capabilities = UART_CAP_FIFO | UART_CAP_NOTEMT | UART_CAP_AFE;
+
+	dw8250_sunxi_setup_rxtrig(p);
+
+	/* Use software RS485 with the Allwinner register layout. */
+	p->rs485_config = serial8250_em485_config;
+	p->rs485_supported = serial8250_em485_supported;
+	up->rs485_start_tx = serial8250_em485_start_tx;
+	up->rs485_stop_tx = serial8250_em485_stop_tx;
+}
+
 static int dw8250_probe(struct platform_device *pdev)
 {
 	struct uart_8250_port uart = {}, *up = &uart;
@@ -878,6 +964,13 @@ static const struct dw8250_platform_data dw8250_dw_apb = {
 	.usr_reg = DW_UART_USR,
 };
 
+static const struct dw8250_platform_data dw8250_h616_data = {
+	.usr_reg = DW_UART_USR,
+	.fifo_size = 256,
+	.quirks = DW_UART_QUIRK_SKIP_SET_RATE,
+	.setup = dw8250_sunxi_setup,
+};
+
 static const struct dw8250_platform_data dw8250_octeon_3860_data = {
 	.usr_reg = OCTEON_UART_USR,
 	.quirks = DW_UART_QUIRK_OCTEON,
@@ -929,6 +1022,7 @@ static const struct dw8250_platform_data dw8250_tda54 = {
 
 static const struct of_device_id dw8250_of_match[] = {
 	{ .compatible = "snps,dw-apb-uart", .data = &dw8250_dw_apb },
+	{ .compatible = "allwinner,sun50i-h616-uart", .data = &dw8250_h616_data },
 	{ .compatible = "cavium,octeon-3860-uart", .data = &dw8250_octeon_3860_data },
 	{ .compatible = "marvell,armada-38x-uart", .data = &dw8250_armada_38x_data },
 	{ .compatible = "renesas,rzn1-uart", .data = &dw8250_renesas_rzn1_data },

-- 
2.53.0

Re: [PATCH v4 4/5] serial: 8250_dw: configure the Allwinner H616 UART

[Ilpo Järvinen]

On Tue, 27 Oct 2026, James Hilliard wrote:

> H616 has 256-byte transmit and receive FIFOs, but its optional DesignWare
> component parameter register reads as zero. Generic 16550 probing leaves
> the port using the 16-byte FIFO configuration and its trigger-level table.
> 
> Keep PORT_16550A and describe H616's fixed 256-byte FIFO depth in platform
> data. Use common Allwinner setup helpers to configure the transmit load
> and receive trigger table from that depth.
> 
> Derive the receive thresholds from the documented 1-byte, quarter-full,
> half-full and FIFO-depth-minus-two mapping. On H616 these are 1, 64, 128
> and 254 bytes; the normal 16550A half-full FCR setting selects 128 bytes.
> 
> Build the threshold table during setup and use the shared 8250 conversion
> helper to select the nearest lower threshold for sysfs writes.

You do that, I can see from the patch, but I came to read this changelog 
to understand "why"?? You provide zero reasoning for this design decision.

Since you fifosize is constant as well, so what benefit building it on the 
fly servers?

> Serialize trigger changes with termios and hold a runtime PM reference
> while programming FCR, preserving the FIFO contents. Propagate runtime
> resume errors before changing either the FCR shadow or the hardware.
> 
> Advertise automatic flow control so CRTSCTS enables MCR.AFCE. The
> DesignWare termios path selects hardware CTS handling, so the capability
> must be present for the corresponding hardware control to be enabled.
> 
> Select the fixed configuration through the platform setup callback.
> H616 has debug-divisor and RS485 control registers at the offsets used to
> detect DesignWare hardware RS485 and fractional-divisor support. Avoid
> those probes and retain the BUSY-safe integer divisor hook and software
> RS485 handling.
> 
> Use the existing SKIP_SET_RATE quirk to keep the UART clock enabled during
> termios changes. Its clock is a gate on the shared APB2 source, rather than
> an independent baud-rate generator.
> 
> Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
> ---
>  drivers/tty/serial/8250/8250_dw.c | 94 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 94 insertions(+)
> 
> diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
> index 1351523f0c47..2f156c01c5a1 100644
> --- a/drivers/tty/serial/8250/8250_dw.c
> +++ b/drivers/tty/serial/8250/8250_dw.c
> @@ -19,6 +19,7 @@
>  #include <linux/io.h>
>  #include <linux/lockdep.h>
>  #include <linux/module.h>
> +#include <linux/mutex.h>
>  #include <linux/platform_device.h>
>  #include <linux/pm_runtime.h>
>  #include <linux/property.h>
> @@ -63,6 +64,7 @@
>  struct dw8250_platform_data {
>  	u8 usr_reg;
>  	u32 cpr_value;
> +	unsigned int fifo_size;
>  	unsigned int quirks;
>  	void (*setup)(struct uart_port *p);
>  };
> @@ -81,6 +83,7 @@ struct dw8250_data {
>  	unsigned int		in_idle:1;
>  
>  	u8			no_int_count;
> +	u8			rxtrig_bytes[UART_FCR_R_TRIG_MAX_STATE];
>  };
>  
>  static inline struct dw8250_data *to_dw8250_data(struct dw8250_port_data *data)
> @@ -657,6 +660,89 @@ static void dw8250_octeon_setup(struct uart_port *p)
>  	dw8250_setup_dma(p);
>  }
>  
> +/* The trigger is the masked, unshifted FCR receive trigger field. */
> +static unsigned int dw8250_sunxi_rxtrig_bytes(struct uart_port *p, unsigned int trigger)
> +{
> +	switch (trigger) {
> +	case UART_FCR_TRIGGER_1:
> +		return 1;
> +	case UART_FCR_TRIGGER_4:
> +		return p->fifosize / 4;
> +	case UART_FCR_TRIGGER_8:
> +		return p->fifosize / 2;
> +	default:
> +		return p->fifosize - 2;
> +	}
> +}
> +
> +static int dw8250_sunxi_get_rxtrig(struct uart_port *p)
> +{
> +	struct dw8250_data *data = to_dw8250_data(p->private_data);
> +	struct uart_8250_port *up = up_to_u8250p(p);
> +
> +	guard(uart_port_lock_irqsave)(p);
> +
> +	return data->rxtrig_bytes[UART_FCR_R_TRIG_BITS(up->fcr)];
> +}
> +
> +static int dw8250_sunxi_set_rxtrig(struct uart_port *p, unsigned char bytes)
> +{
> +	struct dw8250_data *data = to_dw8250_data(p->private_data);
> +	struct uart_8250_port *up = up_to_u8250p(p);
> +	int fcr, ret;
> +
> +	fcr = serial8250_bytes_to_fcr(data->rxtrig_bytes, bytes);
> +	if (fcr < 0)
> +		return fcr;
> +
> +	guard(mutex)(&p->state->port.mutex);
> +
> +	PM_RUNTIME_ACQUIRE_IF_ENABLED(p->dev, pm);
> +	ret = PM_RUNTIME_ACQUIRE_ERR(&pm);
> +	if (ret)
> +		return ret;
> +
> +	guard(uart_port_lock_irqsave)(p);
> +
> +	up->fcr &= ~UART_FCR_TRIGGER_MASK;
> +	up->fcr |= fcr;
> +	/* Changing the trigger alone preserves the FIFO contents. */
> +	serial_port_out(p, UART_FCR, up->fcr);
> +
> +	return 0;
> +}
> +
> +static void dw8250_sunxi_setup_rxtrig(struct uart_port *p)
> +{
> +	struct dw8250_data *data = to_dw8250_data(p->private_data);
> +
> +	/* Allwinner RX thresholds differ from the standard 16550A table. */
> +	for (unsigned int i = 0; i < ARRAY_SIZE(data->rxtrig_bytes); i++)
> +		data->rxtrig_bytes[i] = dw8250_sunxi_rxtrig_bytes(p, i << UART_FCR_R_TRIG_SHIFT);

FIELD_PREP(UART_FCR_TRIGGER_MASK, i)

For clarity, it might be useful to split it into two lines using an 
intermediate variable.

> +	p->get_rxtrig = dw8250_sunxi_get_rxtrig;
> +	p->set_rxtrig = dw8250_sunxi_set_rxtrig;
> +}
> +
> +static void dw8250_sunxi_setup(struct uart_port *p)
> +{
> +	struct dw8250_data *data = to_dw8250_data(p->private_data);
> +	struct uart_8250_port *up = up_to_u8250p(p);
> +
> +	p->type = PORT_16550A;
> +	p->flags |= UPF_FIXED_TYPE;
> +	p->fifosize = data->pdata->fifo_size;
> +	up->tx_loadsz = p->fifosize;
> +	up->capabilities = UART_CAP_FIFO | UART_CAP_NOTEMT | UART_CAP_AFE;
> +
> +	dw8250_sunxi_setup_rxtrig(p);
> +
> +	/* Use software RS485 with the Allwinner register layout. */
> +	p->rs485_config = serial8250_em485_config;
> +	p->rs485_supported = serial8250_em485_supported;
> +	up->rs485_start_tx = serial8250_em485_start_tx;
> +	up->rs485_stop_tx = serial8250_em485_stop_tx;
> +}
> +
>  static int dw8250_probe(struct platform_device *pdev)
>  {
>  	struct uart_8250_port uart = {}, *up = &uart;
> @@ -878,6 +964,13 @@ static const struct dw8250_platform_data dw8250_dw_apb = {
>  	.usr_reg = DW_UART_USR,
>  };
>  
> +static const struct dw8250_platform_data dw8250_h616_data = {
> +	.usr_reg = DW_UART_USR,
> +	.fifo_size = 256,
> +	.quirks = DW_UART_QUIRK_SKIP_SET_RATE,
> +	.setup = dw8250_sunxi_setup,
> +};
> +
>  static const struct dw8250_platform_data dw8250_octeon_3860_data = {
>  	.usr_reg = OCTEON_UART_USR,
>  	.quirks = DW_UART_QUIRK_OCTEON,
> @@ -929,6 +1022,7 @@ static const struct dw8250_platform_data dw8250_tda54 = {
>  
>  static const struct of_device_id dw8250_of_match[] = {
>  	{ .compatible = "snps,dw-apb-uart", .data = &dw8250_dw_apb },
> +	{ .compatible = "allwinner,sun50i-h616-uart", .data = &dw8250_h616_data },
>  	{ .compatible = "cavium,octeon-3860-uart", .data = &dw8250_octeon_3860_data },
>  	{ .compatible = "marvell,armada-38x-uart", .data = &dw8250_armada_38x_data },
>  	{ .compatible = "renesas,rzn1-uart", .data = &dw8250_renesas_rzn1_data },
> 
> 

-- 
 i.

[PATCH v4 5/5] arm64: dts: allwinner: h616: describe the UART integration

[James Hilliard]

Identify the H616 UART integration on each of the six controllers. The
specific compatible describes the fixed FIFO capacity, register layout
and receive-trigger mapping.

Retain snps,dw-apb-uart as the generic fallback. H618 includes this DTSI
and uses the same UART integration.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
 arch/arm64/boot/dts/allwinner/sun50i-h616.dtsi | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/boot/dts/allwinner/sun50i-h616.dtsi b/arch/arm64/boot/dts/allwinner/sun50i-h616.dtsi
index c6b3a3f8359a..b4b2f85726e0 100644
--- a/arch/arm64/boot/dts/allwinner/sun50i-h616.dtsi
+++ b/arch/arm64/boot/dts/allwinner/sun50i-h616.dtsi
@@ -550,7 +550,7 @@ mmc2: mmc@4022000 {
 		};
 
 		uart0: serial@5000000 {
-			compatible = "snps,dw-apb-uart";
+			compatible = "allwinner,sun50i-h616-uart", "snps,dw-apb-uart";
 			reg = <0x05000000 0x400>;
 			interrupts = <GIC_SPI 0 IRQ_TYPE_LEVEL_HIGH>;
 			reg-shift = <2>;
@@ -563,7 +563,7 @@ uart0: serial@5000000 {
 		};
 
 		uart1: serial@5000400 {
-			compatible = "snps,dw-apb-uart";
+			compatible = "allwinner,sun50i-h616-uart", "snps,dw-apb-uart";
 			reg = <0x05000400 0x400>;
 			interrupts = <GIC_SPI 1 IRQ_TYPE_LEVEL_HIGH>;
 			reg-shift = <2>;
@@ -576,7 +576,7 @@ uart1: serial@5000400 {
 		};
 
 		uart2: serial@5000800 {
-			compatible = "snps,dw-apb-uart";
+			compatible = "allwinner,sun50i-h616-uart", "snps,dw-apb-uart";
 			reg = <0x05000800 0x400>;
 			interrupts = <GIC_SPI 2 IRQ_TYPE_LEVEL_HIGH>;
 			reg-shift = <2>;
@@ -589,7 +589,7 @@ uart2: serial@5000800 {
 		};
 
 		uart3: serial@5000c00 {
-			compatible = "snps,dw-apb-uart";
+			compatible = "allwinner,sun50i-h616-uart", "snps,dw-apb-uart";
 			reg = <0x05000c00 0x400>;
 			interrupts = <GIC_SPI 3 IRQ_TYPE_LEVEL_HIGH>;
 			reg-shift = <2>;
@@ -602,7 +602,7 @@ uart3: serial@5000c00 {
 		};
 
 		uart4: serial@5001000 {
-			compatible = "snps,dw-apb-uart";
+			compatible = "allwinner,sun50i-h616-uart", "snps,dw-apb-uart";
 			reg = <0x05001000 0x400>;
 			interrupts = <GIC_SPI 4 IRQ_TYPE_LEVEL_HIGH>;
 			reg-shift = <2>;
@@ -615,7 +615,7 @@ uart4: serial@5001000 {
 		};
 
 		uart5: serial@5001400 {
-			compatible = "snps,dw-apb-uart";
+			compatible = "allwinner,sun50i-h616-uart", "snps,dw-apb-uart";
 			reg = <0x05001400 0x400>;
 			interrupts = <GIC_SPI 5 IRQ_TYPE_LEVEL_HIGH>;
 			reg-shift = <2>;

-- 
2.53.0