Re: [PATCHv2 1/4] dmaengine: rcar-dmac: use result of updated get_residue in tx_status

[Dirk Behme <dirk.behme@de.bosch.com>]

On 30.06.2016 17:15, Niklas Söderlund wrote:
> From: Muhammad Hamza Farooq <mfarooq@visteon.com>
> 
> The hardware might have complete the transfer but the interrupt handler
> might not have had a chance to run. If rcar_dmac_chan_get_residue()
> which reads HW registers finds that there is no residue return
> DMA_COMPLETE.
> 
> Signed-off-by: Muhammad Hamza Farooq <mfarooq@visteon.com>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
> [Niklas: add explanation in commit message]
> Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
> Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> ---
>   drivers/dma/sh/rcar-dmac.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/dma/sh/rcar-dmac.c b/drivers/dma/sh/rcar-dmac.c
> index dfb1792..791a064 100644
> --- a/drivers/dma/sh/rcar-dmac.c
> +++ b/drivers/dma/sh/rcar-dmac.c
> @@ -1202,6 +1202,10 @@ static enum dma_status rcar_dmac_tx_status(struct dma_chan *chan,
>   	residue = rcar_dmac_chan_get_residue(rchan, cookie);
>   	spin_unlock_irqrestore(&rchan->lock, flags);
>   
> +	/* if there's no residue, the cookie is complete */
> +	if (!residue)
> +		return DMA_COMPLETE;
> +
>   	dma_set_residue(txstate, residue);
>   
>   	return status;


We have some doubts about this change (mainline commit [1]). Not being a 
DMA expert, let me try to explain:

We are configuring a cyclic, never ending DMA 
(dmaengine_prep_dma_cyclic()). For this cyclic DMA we continuously poll 
the residue (txstate->residue) via rcar_dmac_tx_status(). Having a 
cyclic, never ending DMA we think that residue == 0 is a valid value. 
However, with above change, a residue value of 0 is 'dropped' and never 
written via dma_set_residue() to txstate. So in case we continuously 
poll, this value is lost, resulting in wrong behavior of the caller.

In our case with cyclic, never ending DMA, changing this to

--- a/drivers/dma/sh/rcar-dmac.c
+++ b/drivers/dma/sh/rcar-dmac.c
@@ -1388,7 +1388,7 @@ static enum dma_status rcar_dmac_tx_status(struct 
dma_chan *chan,

         /* if there's no residue, the cookie is complete */
         if (!residue)
-               return DMA_COMPLETE;
+               status = DMA_COMPLETE;

         dma_set_residue(txstate, residue);

seems to help. If we ignore the still wrong DMA_COMPLETE status (which 
in case of cyclic DMA doesn't make any sense?) the caller get the 
correct txstate->residue values (not dropping the 0).

So maybe we need anything like

--- a/drivers/dma/sh/rcar-dmac.c
+++ b/drivers/dma/sh/rcar-dmac.c
@@ -1387,7 +1387,7 @@ static enum dma_status rcar_dmac_tx_status(struct 
dma_chan *chan,
         spin_unlock_irqrestore(&rchan->lock, flags);

         /* if there's no residue, the cookie is complete */
-       if (!residue)
+       if (!residue && !chan->desc.running->cyclic)
                 return DMA_COMPLETE;

         dma_set_residue(txstate, residue);

?

Opinions?

Best regards

Dirk

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/dma/sh/rcar-dmac.c?h=v5.0-rc2&id=3544d2878817bd139dda238cdd86a15e1c03d037