Re: [PATCH v5] serial: 8250_omap: clear rx_running on zero-length DMA completes

[andriy.shevchenko@xxxxxxxxxxxxxxx]

On Tue, May 26, 2026 at 07:35:09AM +0000, Matthias Feser wrote:
> On AM33xx RX DMA only triggers when the FIFO reaches the
> configured threshold (typically 48 bytes). For smaller bursts
> no DMA request is issued and the FIFO is drained by RX timeout.
> 
> In this case __dma_rx_do_complete() can legitimately see count == 0.
> 
> The current code exits early in this case and does not clear
> dma->rx_running, leaving the DMA state inconsistent. This can
> prevent RX DMA from restarting and may cause
> omap_8250_rx_dma_flush() to fail, marking DMA as broken.
> 
> Fix this by clearing dma->rx_running once the DMA transfer has
> completed or been terminated, even if no data was transferred.

...

> +	dma->rx_running = 0;
>  	if (!count)
>  		goto out;
>  	ret = tty_insert_flip_string(tty_port, dma->rx_buf, count);
>  
> -	dma->rx_running = 0;

I'm wondering if this opens a window when dma->rx_buf may be rewritten (or
unmapped or something else) before tty layer has a chance to insert bytes to
its ring buffer. I.o.w. is this change synchronous to other threads?

-- 
With Best Regards,
Andy Shevchenko