Hi Philipp,

see below...

On Thu, 2019-09-19 at 16:29 +0200, Philipp Puschmann wrote:
> For some years and since many kernel versions there are reports that the
> RX UART SDMA channel stops working at some point. The workaround was to
> disable DMA for RX. This commit tries to fix the problem itself.
> 
> Due to its license i wasn't able to debug the sdma script itself but it
> somehow leads to blocking the scheduling of the channel script when a
> running sdma script does not find any free descriptor in the ring to put
> its data into.
> 
> If we detect such a potential case we manually restart the channel.
> 
> As sdmac->desc is constant we can move desc out of the loop.
> 
> Fixes: 1ec1e82f2510 ("dmaengine: Add Freescale i.MX SDMA support")
> Signed-off-by: Philipp Puschmann <philipp.puschmann@emlix.com>
> Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
> ---
> 
> Changelog v4:
>  - fixed the fixes tag
>  
> Changelog v3:
>  - use correct dma_wmb() instead of dma_wb()
>  - add fixes tag
>  
> Changelog v2:
>  - clarify comment and commit description
> 
>  drivers/dma/imx-sdma.c | 21 +++++++++++++++++----
>  1 file changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/dma/imx-sdma.c b/drivers/dma/imx-sdma.c
> index e029a2443cfc..a32b5962630e 100644
> --- a/drivers/dma/imx-sdma.c
> +++ b/drivers/dma/imx-sdma.c
> @@ -775,21 +775,23 @@ static void sdma_start_desc(struct sdma_channel *sdmac)
>  static void sdma_update_channel_loop(struct sdma_channel *sdmac)
>  {
>  	struct sdma_buffer_descriptor *bd;
> -	int error = 0;
> -	enum dma_status	old_status = sdmac->status;
> +	struct sdma_desc *desc = sdmac->desc;
> +	int error = 0, cnt = 0;
> +	enum dma_status old_status = sdmac->status;
>  
>  	/*
>  	 * loop mode. Iterate over descriptors, re-setup them and
>  	 * call callback function.
>  	 */
> -	while (sdmac->desc) {
> -		struct sdma_desc *desc = sdmac->desc;
> +	while (desc) {
>  
>  		bd = &desc->bd[desc->buf_tail];
>  
>  		if (bd->mode.status & BD_DONE)
>  			break;
>  
> +		cnt++;
> +
>  		if (bd->mode.status & BD_RROR) {
>  			bd->mode.status &= ~BD_RROR;
>  			sdmac->status = DMA_ERROR;
> @@ -822,6 +824,17 @@ static void sdma_update_channel_loop(struct sdma_channel *sdmac)
>  		if (error)
>  			sdmac->status = old_status;
>  	}
> +
> +	/* In some situations it may happen that the sdma does not found any
                                                          ^ hasn't
> +	 * usable descriptor in the ring to put data into. The channel is
> +	 * stopped then. While there is no specific error condition we can
> +	 * check for, a necessary condition is that all available buffers for
> +	 * the current channel have been written to by the sdma script. In
> +	 * this case and after we have made the buffers available again,
> +	 * we restart the channel.
> +	 */

Are you sure we can't miss cases where we only had to make some buffers
available again, but the SDMA already ran out of buffers before?

A while ago, I was debugging a similar issue triggered by receiving
data with a wrong baud rate, which leads to all descriptors being
marked with the error flag very quickly (and the SDMA stalling).
I noticed that you can check if the channel is still running by
checking the SDMA_H_STATSTOP register & BIT(sdmac->channel).

I also added a flag for the sdmac->flags field to allow stopping the
channel from the callback (otherwise it would enable the channel
again).

Attached is my current version of that patch for reference.

> +	if (cnt >= desc->num_bd)
> +		sdma_enable_channel(sdmac->sdma, sdmac->channel);
>  }
>  
>  static void mxc_sdma_handle_channel_normal(struct sdma_channel *data)