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

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Date:	Wed, 3 Apr 2013 17:30:13 +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 05:22 PM, Peter Ujfalusi wrote:
> On 04/03/2013 01:24 PM, Santosh Shilimkar wrote:
>> 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.
> 
> It can not be embedded there because of the existing tests. How would we
> handle the case when c->desc is _not_ NULL and c->cyclic is false? We would
> need to test again in else, but we can not do this for the
> vchan_issue_pending(&c->vc).
> 
right. Thanks for clarifying it.

Regards,
Santosh

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