Re: [PATCH v1] serial: qcom_geni: fix kfifo underflow when flush precedes DMA completion IRQ

[Bartosz Golaszewski]

On Wed, 6 May 2026 06:45:21 +0200, Viken Dadhaniya
<viken.dadhaniya@xxxxxxxxxxxxxxxx> said:
> When uart_flush_buffer() runs before the DMA completion IRQ is delivered,
> the following race can occur (all steps serialized by uart_port_lock):
>
>   1. DMA starts: tx_remaining = N, kfifo contains N bytes
>   2. DMA completes in hardware; IRQ is pending but not yet delivered
>   3. uart_flush_buffer() acquires the port lock and calls kfifo_reset(),
>      making kfifo_len() = 0 while tx_remaining remains N
>   4. uart_flush_buffer() releases the port lock
>   5. DMA IRQ fires; handle_tx_dma() acquires the port lock and calls
>      uart_xmit_advance(uport, tx_remaining) on an empty kfifo
>
> uart_xmit_advance() increments kfifo->out by tx_remaining. Since
> kfifo_reset() already set both in and out to 0, out wraps past in,
> causing kfifo_len() to return UART_XMIT_SIZE - tx_remaining. The next
> start_tx_dma() call then submits a DMA transfer of stale buffer data.
>
> Fix this by snapshotting kfifo_len() at the start of handle_tx_dma()
> and skipping uart_xmit_advance() when fifo_len < tx_remaining, which
> indicates the kfifo was reset by a preceding flush.
>
> Fixes: 2aaa43c70778 ("tty: serial: qcom-geni-serial: add support for serial engine DMA")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Viken Dadhaniya <viken.dadhaniya@xxxxxxxxxxxxxxxx>
> ---

Make sense, thanks.

Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxxxxxxxx>