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

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

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

> From: Long Zhao <longzhao@ambarella.com>
> 
> 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 <longzhao@ambarella.com>
> ---
>  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.

> +	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 },
> 
>