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

Peter Ujfalusi
2013-04-03T11:52:47Z

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@pmeerw.net>
>> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.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).

>
>> - 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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