linux-kernel - Re: [PATCH 1/3] ASoC: soc-generic-dmaengine-pcm: add option to start DMA after DAI

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Message-ID: <61ce577d-03d3-5437-52ce-ec823a7a89ca@microchip.com>
Date:   Fri, 17 Feb 2023 10:59:37 +0000
From:   <Claudiu.Beznea@...rochip.com>
To:     <lars@...afoo.de>, <broonie@...nel.org>
CC:     <lgirdwood@...il.com>, <robh+dt@...nel.org>,
        <krzysztof.kozlowski+dt@...aro.org>, <perex@...ex.cz>,
        <tiwai@...e.com>, <Nicolas.Ferre@...rochip.com>,
        <alexandre.belloni@...tlin.com>, <alsa-devel@...a-project.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/3] ASoC: soc-generic-dmaengine-pcm: add option to start
 DMA after DAI

On 16.02.2023 15:53, Lars-Peter Clausen wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the
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> 
> On 2/16/23 01:49, Claudiu.Beznea@...rochip.com wrote:
>> On 14.02.2023 23:26, Mark Brown wrote:
>>> On Tue, Feb 14, 2023 at 10:14:28AM -0800, Lars-Peter Clausen wrote:
>>>> On 2/14/23 08:14, Claudiu Beznea wrote:
>>>>> @@ -450,6 +450,8 @@ int snd_dmaengine_pcm_register(struct device *dev,
>>>>>            else
>>>>>                    driver = &dmaengine_pcm_component;
>>>>> +  driver->start_dma_last = config->start_dma_last;
>>>> This will break if you have multiple sound cards in the system.
>>>> dmaengine_pcm_component must stay const.
>>> Right, if we need to modify it we either need to select which of
>>> multiple const structs to register or to take a copy and modify
>>> that.  I've not looked at the actual changes yet.
>> OK, I will try that and return with a new patch.
>>
>> On the other hand do you think the other solution presented in cover letter
>> would be better? From the cover letter:
>>
>> "The other solution that was identified for this was to extend the already
>> existing mechanism around struct snd_soc_dai_link::stop_dma_first. The
>> downside
>> of this was that a potential struct snd_soc_dai_link::start_dma_last
>> would have to be populated on sound card driver thus, had to be taken
>> into account in all sound card drivers. At the moment, the mchp-pdmc is
>> used only with simple-audio-card. In case of simple-audio-card a new DT
>> binding would had to be introduced to specify this action on dai-link
>> descriptions (as of my investigation)."
>>
> Can't you just set `start_dma_last` on the `mchp_pdmc_dai_component`? In
> your code you iterate over all the components of the link and if any of
> them has it set the DMA is started last.

Yes, that is also working.

In this patch I chose to have it on DMA component as the operation is
specific to DMA... It looked better to me this way. But is true that having
it on mchp_pdmc_dai_component wouldn't affect the behavior.

Thank you,
Claudiu