[PATCH v3] tty: serial: samsung_tty: loopback mode support

[Marek Szyprowski]

Hi Chanho,

On 29.06.2022 02:41, Chanho Park wrote:
> Internal loopback mode can be supported by setting
> UCON register's Loopback Mode bit. The mode & bit can be supported
> since s3c2410 and later SoCs. The prefix of LOOPBACK / BIT(5) naming
> should be also changed to S3C2410_ in order to avoid confusion.
>
> Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski at linaro.org>
> Reviewed-by: Ilpo Järvinen <ilpo.jarvinen at linux.intel.com>
> Signed-off-by: Chanho Park <chanho61.park at samsung.com>
> ---
> Changes from v2:
> - Add Ilop and Krzysztof's R-B tags
> - Drop linux-serial-test related commit messages
> - Wrap commit messages at 72 columns
>
> Changes from v1:
> - Drop TIOCM_LOOP return from s3c24xx_serial_get_mctrl as pointed out
>    by Ilpo. Documentation/driver-api/serial/driver.rst indicates the
>    bit is only for set_mctrl.
> - Change the loopback bit definition from S3C2443_UCON_LOOPBACK to
>    S3C2410_UCON_LOOPBACK because it has been supported since s3c2410.
> - Remove the unnecessary footnote and put a blank line before
>    Signed-off-by
>
>   drivers/tty/serial/samsung_tty.c | 8 ++++++++
>   include/linux/serial_s3c.h       | 2 +-
>   2 files changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
> index d5ca904def34..03ef4ff506fd 100644
> --- a/drivers/tty/serial/samsung_tty.c
> +++ b/drivers/tty/serial/samsung_tty.c
> @@ -1012,6 +1012,7 @@ static unsigned int s3c24xx_serial_get_mctrl(struct uart_port *port)
>   static void s3c24xx_serial_set_mctrl(struct uart_port *port, unsigned int mctrl)
>   {
>   	unsigned int umcon = rd_regl(port, S3C2410_UMCON);
> +	unsigned int ucon = rd_reg(port, S3C2410_UCON);
>   
>   	if (mctrl & TIOCM_RTS)
>   		umcon |= S3C2410_UMCOM_RTS_LOW;
> @@ -1019,6 +1020,13 @@ static void s3c24xx_serial_set_mctrl(struct uart_port *port, unsigned int mctrl)
>   		umcon &= ~S3C2410_UMCOM_RTS_LOW;
>   
>   	wr_regl(port, S3C2410_UMCON, umcon);
> +
> +	if (mctrl & TIOCM_LOOP)
> +		ucon |= S3C2410_UCON_LOOPBACK;
> +	else
> +		ucon &= ~S3C2410_UCON_LOOPBACK;
> +
> +	wr_regl(port, S3C2410_UCON, ucon);

S3C2410_UCON must be modified at least under the local_irq_save() or port lock, otherwise it breaks kernel console operation on older Exynos SoCs (especially when DMA mode is used).

The above read-modify-write sequence should be replaced with the following pattern:

         if (mctrl & TIOCM_LOOP)
                 s3c24xx_set_bit(port, S3C2410_UCON_LOOPBACK, S3C2410_UCON);
         else
                 s3c24xx_clear_bit(port, S3C2410_UCON_LOOPBACK, S3C2410_UCON);


>   }
>   
>   static void s3c24xx_serial_break_ctl(struct uart_port *port, int break_state)
> diff --git a/include/linux/serial_s3c.h b/include/linux/serial_s3c.h
> index dec15f5b3dec..1672cf0810ef 100644
> --- a/include/linux/serial_s3c.h
> +++ b/include/linux/serial_s3c.h
> @@ -83,7 +83,7 @@
>   #define S3C2410_UCON_RXIRQMODE	  (1<<0)
>   #define S3C2410_UCON_RXFIFO_TOI	  (1<<7)
>   #define S3C2443_UCON_RXERR_IRQEN  (1<<6)
> -#define S3C2443_UCON_LOOPBACK	  (1<<5)
> +#define S3C2410_UCON_LOOPBACK	  (1<<5)
>   
>   #define S3C2410_UCON_DEFAULT	  (S3C2410_UCON_TXILEVEL  | \
>   				   S3C2410_UCON_RXILEVEL  | \

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland