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