[PATCH v7 10/12] serial: 8250_dw: discover Allwinner A523 FIFO depths through FCC

[James Hilliard]

The A523 UART integration, also used by A527 and T527, has 64-byte FIFOs
on UART0 and 128-byte FIFOs on UART1 through UART7. Each controller reports
its TX/RX FIFO depth in bits 31:8 of the FIFO Clock Control register at
offset 0xf0. The field holds a byte count, not a logarithmic encoding.

Read this field in the sunxi setup callback when platform data does not
provide a fixed FIFO size. A fixed size takes precedence, so H616 retains
its 256-byte configuration without accessing FCC. The probe has already
enabled the clocks and deasserted reset before invoking the callback.
Leave the FIFO clock-control bits untouched.

Use the shared DesignWare trigger setup to validate the depth and build
the FIFO-relative threshold table. Invalid depths fail probe before
registration instead of producing zero or truncated thresholds. The 8250
core derives the transmit load from the discovered depth.

Select this setup for the A523 compatible. Like H616, this integration
has Allwinner-specific debug-divisor and RS485 registers overlapping the
generic DesignWare feature probes. Use the common sunxi PIO setup,
BUSY-safe integer divisor programming and software RS485 handling. Keep
the APB clock gate enabled during termios updates with SKIP_SET_RATE.

Signed-off-by: James Hilliard <james.hilliard1@xxxxxxxxx>
---
 drivers/tty/serial/8250/8250_dw.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 0e7367ba872e..cdd3a62dfa7e 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -44,6 +44,10 @@
 #define RZN1_UART_xDMACR_8_WORD_BURST	(2 << 1)
 #define RZN1_UART_xDMACR_BLK_SZ(x)	((x) << 3)
 
+/* Allwinner FIFO Clock Control register */
+#define SUNXI_UART_FCC			0xf0
+#define SUNXI_UART_FCC_FIFO_DEPTH		GENMASK(31, 8)
+
 /* Quirks */
 #define DW_UART_QUIRK_OCTEON		BIT(0)
 #define DW_UART_QUIRK_ARMADA_38X	BIT(1)
@@ -63,6 +67,7 @@
 struct dw8250_platform_data {
 	u8 usr_reg;
 	u32 cpr_value;
+	/* Fixed FIFO depth; zero selects discovery during setup */
 	unsigned int fifo_size;
 	unsigned int quirks;
 	int (*setup)(struct uart_port *p);
@@ -666,7 +671,13 @@ static int dw8250_sunxi_setup(struct uart_port *p)
 
 	p->type = PORT_16550A;
 	p->flags |= UPF_FIXED_TYPE;
-	p->fifosize = data->pdata->fifo_size;
+	if (data->pdata->fifo_size) {
+		p->fifosize = data->pdata->fifo_size;
+	} else {
+		u32 fcc = dw8250_readl_ext(p, SUNXI_UART_FCC);
+
+		p->fifosize = FIELD_GET(SUNXI_UART_FCC_FIFO_DEPTH, fcc);
+	}
 	up->capabilities = UART_CAP_FIFO | UART_CAP_NOTEMT | UART_CAP_AFE;
 
 	ret = dw8250_setup_rxtrig(p);
@@ -912,6 +923,12 @@ static const struct dw8250_platform_data dw8250_h616_data = {
 	.setup = dw8250_sunxi_setup,
 };
 
+static const struct dw8250_platform_data dw8250_a523_data = {
+	.usr_reg = DW_UART_USR,
+	.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,
@@ -964,6 +981,7 @@ 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 = "allwinner,sun55i-a523-uart", .data = &dw8250_a523_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