Re: [PATCH] serial: sh-sci: Fix off-by-one error in FIFO threshold register setting

[Ulrich Hecht <uli@fpond.eu>]

> On 05/10/2021 2:07 PM Geert Uytterhoeven <geert+renesas@glider.be> wrote:
> 
>  
> The Receive FIFO Data Count Trigger field (RTRG[6:0]) in the Receive
> FIFO Data Count Trigger Register (HSRTRGR) of HSCIF can only hold values
> ranging from 0-127.  As the FIFO size is equal to 128 on HSCIF, the user
> can write an out-of-range value, touching reserved bits.
> 
> Fix this by limiting the trigger value to the FIFO size minus one.
> Reverse the order of the checks, to avoid rx_trig becoming zero if the
> FIFO size is one.
> 
> Note that this change has no impact on other SCIF variants, as their
> maximum supported trigger value is lower than the FIFO size anyway, and
> the code below takes care of enforcing these limits.
> 
> Reported-by: Linh Phung <linh.phung.jy@renesas.com>
> Fixes: a380ed461f66d1b8 ("serial: sh-sci: implement FIFO threshold register setting")
> Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
> ---
> Compile-tested only.
> 
> The BSP contains a different patch[1], which masks the value to write by
> 0x7f.  This is IMHO incorrect, as it would set the trigger value to zero
> when 128 is requested.
> 
> [1] "serial: sh-sci: Using mask when writing to HSRTRGR"
>     https://github.com/renesas-rcar/linux-bsp/commit/9915223f41c7d680aaaed12971601dc038ce76a3
> ---
>  drivers/tty/serial/sh-sci.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
> index ef37fdf37612f82f..4baf1316ea729931 100644
> --- a/drivers/tty/serial/sh-sci.c
> +++ b/drivers/tty/serial/sh-sci.c
> @@ -1023,10 +1023,10 @@ static int scif_set_rtrg(struct uart_port *port, int rx_trig)
>  {
>  	unsigned int bits;
>  
> +	if (rx_trig >= port->fifosize)
> +		rx_trig = port->fifosize - 1;
>  	if (rx_trig < 1)
>  		rx_trig = 1;
> -	if (rx_trig >= port->fifosize)
> -		rx_trig = port->fifosize;
>  
>  	/* HSCIF can be set to an arbitrary level. */
>  	if (sci_getreg(port, HSRTRGR)->size) {
> -- 
> 2.25.1

Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>

CU
Uli