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

[Wolfram Sang <wsa+renesas@sang-engineering.com>]

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On Mon, May 10, 2021 at 02:07:55PM +0200, Geert Uytterhoeven 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.

Makes also sense to me to have the trigger at fifosize-1 to have one
spare byte to handle latencies.

Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>


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