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

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Message-ID: <51E50D3B.2050207@metafoo.de>
Date:	Tue, 16 Jul 2013 11:07:07 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Mark Brown <broonie@...nel.org>
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 07/15/2013 09:51 PM, Mark Brown wrote:
> 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.

I would assume that most of them don't express hardware limitations but
rather are sensible lower limits which allow operation without
over-/underruns. But that's something that doesn't necessarily depend on the
DMA controller, but rather on the system as a whole, e.g. on a slower
machine you'd typically set the limit higher so the CPU has a better chance
to keep up. So this isn't something you'd want to set in the DMA controller
driver. But I'm not sure if there is a good way to calculate a sensible
minimum buffer size based on the whole system's constraints.

- Lars

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