Re: [PATCH 3/3] ASoC: bcm2835: Register also as PCM device

[Lars-Peter Clausen <lars@metafoo.de>]

On 04/26/2016 03:09 PM, Matthias Reichl wrote:
> Hi Lars,
> 
> first of all thanks a lot for your detailled feedback!
> 
> On Tue, Apr 26, 2016 at 10:47:05AM +0200, Lars-Peter Clausen wrote:
>>>>> +	.periods_min		= 2,
>>>>> +	.periods_max		= 255,
>>>>> +	.buffer_bytes_max	= 128 * PAGE_SIZE,
>>>>> +};
>>>>> +
>>>>> +static const struct snd_dmaengine_pcm_config bcm2835_dmaengine_pcm_config = {
>>>>> +	.prepare_slave_config = snd_dmaengine_pcm_prepare_slave_config,
>>>>> +	.pcm_hardware = &bcm2835_pcm_hardware,
>>>>> +	.prealloc_buffer_size = 256 * PAGE_SIZE,
>>>>> +};
>>>>
>>>> The generic dmaengine PCM driver auto-discovers these things, no need to
>>>> provide them. The code is OK as it is.
>>>
>>> With the auto-discover code we loose the S16_LE format.
>>>
>>> If I understood the code in dmaengine_pcm_set_runtime_hwparams
>>> correctly, this is because the DMA controller doesn't support
>>> 16bit transfers (only multiples of 32bit are allowed).
>>>
>>> But since the I2S driver needs exactly 2 channels S16_LE actually
>>> works fine (one 32bit transfer per frame).
>>>
>>> Do you know of a better way to get S16_LE support? It could well
>>> be that I missed something important...
>>
>> With your patch we should just get an error when trying to configure a
>> stream with 16-bit samples since when setting up the DMA controller the
>> generic code will still choose a 16-bit device port size and this will be
>> rejected by the DMA controller.
> 
> We had that code in downstream RPi kernel for ages (IIRC since
> 3.10) and so far it worked fine.
> 
> I traced the code for the S16_LE case to find out why:
> 
> snd_hwparams_to_dma_slave_config() sets src/dst_addr_width to 16bit.
> 
> But immediately afterwards snd_dmaengine_pcm_set_config_from_dai_data()
> overrides addr_width to 32bit.
> 

Ok, makes sense.

> bcm2835-i2s only supports 32bit access to the FIFO/data register
> and dma_data.addr_width is set to 32bit in the I2S driver - that
> code is also in mainline since the initial bcm2835-i2s commit.
> 
> Of course all this only works because the number of channels
> is always 2.
> 
>> When you look at peripherals that have DMA support there are basically two
>> types.
>>
>> Type A expect that each write (same applies for read) to the memory mapped
>> FIFO corresponds to exactly one sample. So if the sample width is 16-bit a
>> 16-bit write is done, if the sample width is 32-bit a 32-bit write is done.
>> In this case it is up to the DMA controller to fragment the byte stream into
>> individual samples.
>>
>> Type B on the other hand has a fixed port width (usually the bus size) and
>> expects that each write to the memory mapped FIFO uses the port width. It
>> then internally unpacks the data into the sample data. E.g. if the FIFO is
>> 32-bit wide and the sample width is 16-bit it will unpack the 32-bit entry
>> into two samples.
>>
>> Currently the generic code only supports type A. It would be great if you
>> could add support for type B to support the BCM2835 I2S controller properly.
> 
> Do you have a particular solution in mind?
> 
> Introducing a flag to just auto-add some packed formats would be
> easy, but a generic, robust solution might be tricky. We'd have
> to make sure that unsupported configurations (like an odd number
> of 16bit channels on 32bit-only setups) get rejected or we might
> be in trouble.

I think in this case the DMA shouldn't limit the supported sample types.
Since the unpacking is done by the peripheral the peripheral is the one
component that limits what is supported and what is not and the DMA itself
has no influence on this. In the peripheral driver you have all the
information available there to specify the proper constraints and can handle
all the corner cases. The overall constraints are the intersection of the
DMA and peripheral constraints, so by having no DMA constraints the
peripheral is the one providing all the constraints.

We could either say that we assume that when the addr_width is fixed the DMA
shouldn't supply any constraints, or we could introduce a new flag in
snd_dmaengine_dai_dma_data that the peripheral has unpack support and the
DMA constraints don't matter.

- Lars