Re: [PATCH 4/5] tty: serial: msm: fix TX kfifo underflow on flush

[sashiko-bot@kernel.org]

> uart_flush_buffer() resets the transmit kfifo under the port lock before
> calling the driver's flush_buffer() hook. Since msm_serial does not
> implement this hook, an outstanding TX DMA transfer may complete after
> the flush and call uart_xmit_advance() using the flushed transfer's byte
> count. The existing check for a stopped transfer relies on tx_sg, which
> the flush does not clear, so it does not catch this case. The kfifo tail
> then advances past its head, causing kfifo_len() to wrap and the driver
> to transmit stale buffer contents. The same issue was fixed in 8250 by
> commit 9e512eaaf8f4 ("serial: 8250: Fix fifo underflow on flush").
> 
> Since the TX buffer is mapped separately for each transfer and the flush
> hook runs under the port lock, the driver cannot synchronously terminate
> DMA and safely unmap the buffer from that context. Instead, leave the
> outstanding transfer running and mark it as flushed, allowing the
> completion callback to unmap the buffer without advancing the kfifo.
> Clear the flag when preparing the next transfer. This avoids the kfifo
> underflow, although the data already queued for DMA before the flush may
> still be transmitted.
> 
> Fixes: 3a878c430fd6 ("tty: serial: msm: Add TX DMA support")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Samarth Narang <samarth.colleges@gmail.com>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261031123911.3784690-1-samarth.colleges@gmail.com?part=4