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

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Message-ID: <2025052101-lunchbox-catacomb-79fe@gregkh>
Date: Wed, 21 May 2025 13:38:18 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: "Jiri Slaby (SUSE)" <jirislaby@...nel.org>
Cc: linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	Mans Rullgard <mans@...sr.com>, stable@...r.kernel.org
Subject: Re: [PATCH] tty: serial: 8250_omap: fix TX with DMA for am33xx

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@...sr.com>
> 
> Fixes: 1788cf6a91d9 ("tty: serial: switch from circ_buf to kfifo")
> Reported-by: Mans Rullgard <mans@...sr.com>
> Cc: stable@...r.kernel.org
> 
> ---

You forgot to sign off on this patch :(

Can you resend it with that?

thanks,

greg k-h