linux-kernel - Re: [RFC 1/2] ASoC: dmaengine-pcm: Add support for querying DMA capabilities

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Date:	Mon, 15 Jul 2013 20:51:45 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Lars-Peter Clausen <lars@...afoo.de>
Cc:	Vinod Koul <vinod.koul@...el.com>,
	Liam Girdwood <lgirdwood@...il.com>,
	alsa-devel@...a-project.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 1/2] ASoC: dmaengine-pcm: Add support for querying DMA
 capabilities

On Mon, Jul 15, 2013 at 08:20:28PM +0200, Lars-Peter Clausen wrote:
> On 07/15/2013 07:57 PM, Mark Brown wrote:
> >On Mon, Jul 15, 2013 at 10:27:21PM +0530, Vinod Koul wrote:

> >Right, we probably want to set an artificial floor here but it still
> >seems like we should be checking that the device actually supports this.
> >If the hardware can only support 64 bytes then the above code won't work
> >properly.

> It shouldn't be to hard to extend the dma_caps API with a
> min_sg_len. But is this something you've actually seen in existing
> hardware for that the driver would make use of the dmaengine PCM
> framework? If it is more of theoretical nature we can still easily
> add it later if it becomes necessary.

I'm not aware of anything but equally well I made zero effort to look
and note that quite a few existing drivers appear to have minimum values
quite a bit above 16 though I doubt they are all actual restrictions.

> That said it is not uncommon that the segment size needs to be a
> multiple of the burst size. Adding support for that is still on my
> TODO list but will require some changes to some of the existing
> users, since implementing this will be a lot easier if all users use
> the snd_dmaengine_dai_dma_data struct for their DAI DMA data.

Yes, burst sizes are one source of restriction I've seen - probably the
main one.

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