linux-kernel - Re: [alsa-devel] [PATCH 14/18] ASoC: davinci: Add edma dmaengine platform driver

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Date:	Thu, 13 Mar 2014 13:09:30 +0100
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Peter Ujfalusi <peter.ujfalusi@...com>
CC:	davinci-linux-open-source@...ux.davincidsp.com,
	alsa-devel@...a-project.org, joelf@...com, vinod.koul@...el.com,
	nsekhar@...com, linux-kernel@...r.kernel.org,
	Jyri Sarha <jsarha@...com>,
	Liam Girdwood <lgirdwood@...il.com>,
	Tony Lindgren <tony@...mide.com>,
	Mark Brown <broonie@...nel.org>, mporter@...aro.org,
	dmaengine@...r.kernel.org, dan.j.williams@...el.com,
	linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [alsa-devel] [PATCH 14/18] ASoC: davinci: Add edma dmaengine
 platform driver

On 03/13/2014 12:56 PM, Peter Ujfalusi wrote:
> On 03/13/2014 12:28 PM, Lars-Peter Clausen wrote:
>> On 03/13/2014 10:18 AM, Peter Ujfalusi wrote: [...]
>>> +static const struct snd_pcm_hardware edma_pcm_hardware = { +    .info
>>> = SNDRV_PCM_INFO_MMAP | +                  SNDRV_PCM_INFO_MMAP_VALID | +
>>> SNDRV_PCM_INFO_BATCH | +                  SNDRV_PCM_INFO_PAUSE |
>>> SNDRV_PCM_INFO_RESUME | +                  SNDRV_PCM_INFO_INTERLEAVED, +
>>> .buffer_bytes_max    = 128 * 1024, +    .period_bytes_min    = 32, +
>>> .period_bytes_max    = 64 * 1024, +    .periods_min        = 2, +
>>> .periods_max        = 19, /* Limit by edma dmaengine driver */ +};
>>
>> The idea is that we can auto-discover all the things using the
>> dma_slave_caps API. Too bad we removed the possibility to specify the
>> maximum number of segments from the API. Maybe we need to add it back. Is
>> the 19 a hard-limit or could it be worked around by software in the
>> dmaengine driver?
>
> The edma dmaengine driver will refuse to configure more than 20 paRAM slots (1
> for the channel + 19 for the periods). Depending on the SoC we might have
> different number of paRAM entries available. The intention with the limit was
> to prevent cases when we run out of paRAM entires for other devices because
> audio took most of them.

The reason why we removed the max_segments field from the slave_caps struct 
was because we though it should be possible to, even when the hardware only 
supports a fixed amount of segments, emulate support for more in software. 
If this is not the case with the edma, we need to bring this field back.

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