Re: [PATCH] tty: serial: 8250_omap: fix TX with DMA for am33xx

Greg KH
2025-05-21T11:38:21Z

On Wed, May 14, 2025 at 09:20:35AM +0200, Jiri Slaby (SUSE) wrote:
> Commit 1788cf6a91d9 ("tty: serial: switch from circ_buf to kfifo")
> introduced an error in the TX DMA handling for 8250_omap.
>
> When the OMAP_DMA_TX_KICK flag is set, the "skip_byte" is pulled from
> the kfifo and emitted directly in order to start the DMA. While the
> kfifo is updated, dma->tx_size is not decreased. This leads to
> uart_xmit_advance() called in omap_8250_dma_tx_complete() advancing the
> kfifo by one too much.
>
> In practice, transmitting N bytes has been seen to result in the last
> N-1 bytes being sent repeatedly.
>
> This change fixes the problem by moving all of the dma setup after the
> OMAP_DMA_TX_KICK handling and using kfifo_len() instead of the DMA size
> for the 4-byte cutoff check. This slightly changes the behaviour at
> buffer wraparound, but it still transmits the correct bytes somehow.
>
> Now, the "skip_byte" would no longer be accounted to the stats. As
> previously, dma->tx_size included also this skip byte, up->icount.tx was
> updated by aforementioned uart_xmit_advance() in
> omap_8250_dma_tx_complete(). Fix this by using the uart_fifo_out()
> helper instead of bare kfifo_get().
>
> Based on patch by Mans Rullgard <mans@mansr.com>
>
> Fixes: 1788cf6a91d9 ("tty: serial: switch from circ_buf to kfifo")
> Reported-by: Mans Rullgard <mans@mansr.com>
> Cc: stable@vger.kernel.org
>
> ---

You forgot to sign off on this patch :(

Can you resend it with that?

thanks,

greg k-h