Re:Re: [PATCH v6 11/13] serial: 8250_dw: add Ambarella CV75 quirks

Hi Ilpo,

Thanks for the review.

I'll drop the serial_port_out() here and only keep:
	up->ier |= d->pdata->ier_mask;
so ETOI stays in the shadow and serial8250_set_ier() programs the
hardware later.

Best regards,
Long Zhao

At 2026-09-10 16:44:35, "Ilpo Järvinen" [off-list ref] wrote:

On Fri, 4 Sep 2026, Long Zhao via B4 Relay wrote:

quoted

From: Long Zhao <redacted>

The Ambarella CV75 UART is Ambarella IP, not a Synopsys DesignWare
licensed block. At the register level it is close enough to DW APB UART
that a full custom UART driver is unnecessary: reuse 8250_dw with a
small platform quirk.

Quirks:
- keep IER ETOI (bit 5) set for RX timeout behaviour
- skip set_rate; baud clock is managed outside the UART block

Signed-off-by: Long Zhao <redacted>
---
 drivers/tty/serial/8250/8250_dw.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 5fba913f3301..b15de40d1e2a 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c

@@ -43,6 +43,9 @@
 #define RZN1_UART_xDMACR_8_WORD_BURST	(2 << 1)
 #define RZN1_UART_xDMACR_BLK_SZ(x)	((x) << 3)
 
+/* Ambarella IER: receive timeout interrupt enable */
+#define AMBARELLA_UART_IER_ETOI		BIT(5)
+
 /* Quirks */
 #define DW_UART_QUIRK_OCTEON		BIT(0)
 #define DW_UART_QUIRK_ARMADA_38X	BIT(1)

@@ -60,6 +63,7 @@
 
 struct dw8250_platform_data {
 	u8 usr_reg;
+	u8 ier_mask;
 	u32 cpr_value;
 	unsigned int quirks;
 };

@@ -572,6 +576,27 @@ static void dw8250_prepare_rx_dma(struct uart_8250_port *p)
 	dw8250_writel_ext(up, RZN1_UART_RDMACR, val);
 }
 
+static int dw8250_ier_mask_startup(struct uart_port *p)
+{
+	struct dw8250_data *d = to_dw8250_data(p->private_data);
+	struct uart_8250_port *up = up_to_u8250p(p);
+	int ret;
+
+	ret = serial8250_do_startup(p);
+	if (ret)
+		return ret;
+
+	/*
+	 * Ambarella keeps variant IER bits (e.g. ETOI) set for correct RX
+	 * timeout behaviour. Force them into the 8250 IER shadow so later
+	 * generic IER updates do not clear them.
+	 */
+	up->ier |= d->pdata->ier_mask;
+	serial_port_out(p, UART_IER, up->ier);

serial8250_do_startup() seems to defer IER update on purpose. Is it fine 
to override that behavior here?

-- 
i.

quoted

+	return 0;
+}
+
 static void dw8250_quirks(struct uart_port *p, struct dw8250_data *data)
 {
 	unsigned int quirks = data->pdata->quirks;

@@ -606,6 +631,8 @@ static void dw8250_quirks(struct uart_port *p, struct dw8250_data *data)
 		p->serial_in = dw8250_serial_in32;
 		data->uart_16550_compatible = true;
 	}
+	if (data->pdata->ier_mask)
+		p->startup = dw8250_ier_mask_startup;
 }
 
 static void dw8250_reset_control_assert(void *data)

@@ -879,6 +906,12 @@ static const struct dw8250_platform_data dw8250_intc10ee = {
 	.quirks = DW_UART_QUIRK_IER_KICK,
 };
 
+static const struct dw8250_platform_data dw8250_ambarella_cv75_data = {
+	.usr_reg = DW_UART_USR,
+	.ier_mask = AMBARELLA_UART_IER_ETOI,
+	.quirks = DW_UART_QUIRK_SKIP_SET_RATE,
+};
+
 static const struct dw8250_platform_data dw8250_ultrarisc_dp1000_data = {
 	.usr_reg = DW_UART_USR,
 	.cpr_value = FIELD_PREP_CONST(DW_UART_CPR_ABP_DATA_WIDTH, 2) |

@@ -889,6 +922,7 @@ static const struct dw8250_platform_data dw8250_ultrarisc_dp1000_data = {
 };
 
 static const struct of_device_id dw8250_of_match[] = {
+	{ .compatible = "ambarella,cv75-uart", .data = &dw8250_ambarella_cv75_data },
 	{ .compatible = "snps,dw-apb-uart", .data = &dw8250_dw_apb },
 	{ .compatible = "cavium,octeon-3860-uart", .data = &dw8250_octeon_3860_data },
 	{ .compatible = "marvell,armada-38x-uart", .data = &dw8250_armada_38x_data },