Re: [RFC PATCH 0/6] soc-pcm: Add separate snd_pcm_runtime for BEs

[Codrin.Ciubotariu]

On 19.05.2021 17:15, Takashi Iwai wrote:
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> On Wed, 19 May 2021 12:48:36 +0200,
> Codrin Ciubotariu wrote:
>>
>> This patchset adds a different snd_pcm_runtime in the BE's substream,
>> replacing the FE's snd_pcm_runtime. With a different structure, the BE
>> HW capabilities and constraints will no longer merge with the FE ones.
>> This allows for error detection if the be_hw_params_fixup() applies HW
>> parameters not supported by the BE DAIs. Also, it calculates values
>> needed for mem-to-dev/dev-to-mem DMA transfers, such as buffer size and
>> period size, if needed.
>>
>> The first 4 patches are preparatory patches, that just group and export
>> functions used to allocate and initialize the snd_pcm_runtime. Also, the
>> functions that set and apply the HW constraints are exported.
>> The 5th patch does (almost) everything need to create the new snd_pcm_runtime
>> for BEs, which includes allocation, initializing the HW capabilities,
>> HW constraints and HW parameters. The BE HW parameters are no longer
>> copied from the FE. They are recalculated, based on HW capabilities,
>> constraints and the be_hw_params_fixup() callback.
>> The 6th and last patch basically adds support for the PCM generic
>> dmaengine to be used as a platform driver for BE DAI links. It allocates
>> a buffer, needed by the DMA transfers that do not support dev-to-dev
>> transfers between FE and BE DAIs.
>>
>> This is a superset of
>> https://mailman.alsa-project.org/pipermail/alsa-devel/2021-March/182630.html
>> which only handles the BE HW constraints. This patchset aims to be more
>> complete, defining a a snd_pcm_runtime between each FE and BE and can
>> be used between any DAI link connection. I am sure I am not handling all
>> the needed members of snd_pcm_runtime (such as handling
>> struct snd_pcm_mmap_status *status), but I would like to have your
>> feedback regarding this idea.
> 
> I'm also concerned about the handling of other fields in runtime
> object, maybe allocating a complete runtime object for each BE is an
> overkill and fragile.  Could it be rather only hw_constraints to be
> unique for each BE, instead?

I tried with only the hw constraints in the previous patchset and it's 
difficult to handle the snd_pcm_hw_rule_add() calls, without changing 
the function's declaration. This solution requires no changes to 
constraints API, nor to their 'clients'. I agree that handling all the 
runtime fields might be over-complicated. From what I see, the scary 
ones are used to describe the buffer and the status of the transfers. I 
do not think there are BEs that use these values at this moment (the FE 
ones). I think that the HW params, private section, hardware description 
and maybe DMA members (at least in my case) are mostly needed by BEs.

> Also, the last patch allows only IRAM type, which sounds already
> doubtful.  The dmaengine code should be generic.

dmaengine, when used with normal PCM, preallocates only IRAM buffers 
[1]. This BE buffer would only be needed if DMA dev-to-mem or mem-to-dev 
transfers are needed, between FE and BE. I agree that it could be 
handled differently, I added it here mostly to express my goal, which is 
to use the generic dmaengine for BEs. My DMA has no dev-to-dev DMA 
capability, so I need a buffer to move the data between FE and BE.

> 
> Last but not least, one minor nitpick: please use EXPORT_SYMBOL_GPL()
> for the newly introduced symbols.

Sure, this is an oversight on my side. I will make all of them 
EXPORT_SYMBOL_GPL.

Thank you very much for your review!

Best regards,
Codrin


[1] 
https://elixir.bootlin.com/linux/v5.13-rc2/source/sound/soc/soc-generic-dmaengine-pcm.c#L266