Re: [PATCH] Revert "tty: serial: simplify qcom_geni_serial_send_chunk_fifo()"

[Johan Hovold <johan@kernel.org>]

On Mon, Mar 04, 2024 at 05:49:53PM -0800, Douglas Anderson wrote:
> This reverts commit 5c7e105cd156fc9adf5294a83623d7a40c15f9b9.
> 
> As identified by KASAN, the simplification done by the cleanup patch
> was not legal.
> 
> From tracing through the code, it can be seen that we're transmitting
> from a 4096-byte circular buffer. We copy anywhere from 1-4 bytes from
> it each time. The simplification runs into trouble when we get near
> the end of the circular buffer. For instance, we might start out with
> xmit->tail = 4094 and we want to transfer 4 bytes. With the code
> before simplification this was no problem. We'd read buf[4094],
> buf[4095], buf[0], and buf[1]. With the new code we'll do a
> memcpy(&buf[4094], 4) which reads 2 bytes past the end of the buffer
> and then skips transmitting what's at buf[0] and buf[1].

Good catch!

> Running "ls -al" on large directories also made the missing bytes
> obvious since columns didn't line up.

I had not noticed this in my limited use of the serial console on the
sc8280xp CRD, but sure enough there are garbage characters and missing
characters in the output of 'ls -al' before applying this patch.

> While the original code may not be the most elegant, we only talking
> about copying up to 4 bytes here. Let's just go back to the code that
> worked.
> 
> Fixes: 5c7e105cd156 ("tty: serial: simplify qcom_geni_serial_send_chunk_fifo()")
> Signed-off-by: Douglas Anderson <dianders@chromium.org>

Tested-by: Johan Hovold <johan+linaro@kernel.org>

and as Jiri already pointed out:

Cc: stable@vger.kernel.org      # 6.4

Johan