linux-kernel - Re: [RFC v2] dmaengine: omap-dma: Start DMA without delay for cyclic channels

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Date:	Wed, 3 Apr 2013 16:54:42 +0530
From:	Santosh Shilimkar <santosh.shilimkar@...com>
To:	Peter Ujfalusi <peter.ujfalusi@...com>
CC:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Vinod Koul <vinod.koul@...el.com>, Dan Williams <djbw@...com>,
	Tony Lindgren <tony@...mide.com>,
	Jarkko Nikula <jarkko.nikula@...mer.com>,
	Felipe Balbi <balbi@...com>, <linux-kernel@...r.kernel.org>,
	<linux-omap@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	Peter Meerwald <pmeerw@...erw.net>
Subject: Re: [RFC v2] dmaengine: omap-dma: Start DMA without delay for cyclic
 channels

On Wednesday 03 April 2013 04:47 PM, Peter Ujfalusi wrote:
> cyclic DMA is only used by audio which needs DMA to be started without a
> delay.
> If the DMA for audio is started using the tasklet we experience random
> channel switch (to be more precise: channel shift).
> 
> Reported-by: Peter Meerwald <pmeerw@...erw.net>
> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@...com>
> ---
> Hi Russell,
> 
> Instead of removing the tasklet we can identify the DMA channel used by audio
> based on the cyclic flag of the channel.
> I think this can be used as a short term solution to fix the audio channel shift
> issue and later when we have the dynamic DMA channel allocation we can adjust
> the code.
> 
> Regards,
> Peter
> 
>  drivers/dma/omap-dma.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/dma/omap-dma.c b/drivers/dma/omap-dma.c
> index 2ea3d7e..ec3fc4f 100644
> --- a/drivers/dma/omap-dma.c
> +++ b/drivers/dma/omap-dma.c
> @@ -282,12 +282,20 @@ static void omap_dma_issue_pending(struct dma_chan *chan)
>  
>  	spin_lock_irqsave(&c->vc.lock, flags);
>  	if (vchan_issue_pending(&c->vc) && !c->desc) {
If you add "!c->cyclic" in above if then you can avoid
indentation change and just have else for cyclic case.

> -		struct omap_dmadev *d = to_omap_dma_dev(chan->device);
> -		spin_lock(&d->lock);
> -		if (list_empty(&c->node))
> -			list_add_tail(&c->node, &d->pending);
> -		spin_unlock(&d->lock);
> -		tasklet_schedule(&d->task);
> +		/*
> +		 * c->cyclic is used only by audio and in this case the DMA need
> +		 * to be started without delay.
> +		 */
> +		if (!c->cyclic) {
> +			struct omap_dmadev *d = to_omap_dma_dev(chan->device);
> +			spin_lock(&d->lock);
> +			if (list_empty(&c->node))
> +				list_add_tail(&c->node, &d->pending);
> +			spin_unlock(&d->lock);
> +			tasklet_schedule(&d->task);
> +		} else {
> +			omap_dma_start_desc(c);
> +		}
>  	}
>  	spin_unlock_irqrestore(&c->vc.lock, flags);
>  }
> 

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