[alsa-devel] [PATCH 14/15] ARM: pxa: change SSP devices allocation

[Robert Jarzmik]

Arnd Bergmann <arnd at arndb.de> writes:

chop chop ... removed several mail recipients to leave only the ASoC / PXA
subset ...

> On Mon, Apr 2, 2018 at 4:26 PM, Robert Jarzmik <robert.jarzmik at free.fr> wrote:
>
>>
>> +static struct pxa_ssp_info pxa_ssp_infos[] = {
>> +       { .dma_chan_rx_name = "ssp1_rx", .dma_chan_tx_name = "ssp1_tx", },
>> +       { .dma_chan_rx_name = "ssp1_rx", .dma_chan_tx_name = "ssp1_tx", },
>> +       { .dma_chan_rx_name = "ssp2_rx", .dma_chan_tx_name = "ssp2_tx", },
>> +       { .dma_chan_rx_name = "ssp2_rx", .dma_chan_tx_name = "ssp2_tx", },
>> +       { .dma_chan_rx_name = "ssp3_rx", .dma_chan_tx_name = "ssp3_tx", },
>> +       { .dma_chan_rx_name = "ssp3_rx", .dma_chan_tx_name = "ssp3_tx", },
>> +       { .dma_chan_rx_name = "ssp4_rx", .dma_chan_tx_name = "ssp4_tx", },
>> +       { .dma_chan_rx_name = "ssp4_rx", .dma_chan_tx_name = "ssp4_tx", },
>> +};
>
> This part looks odd to me, you're adding an extra level of indirection to
> do two stages of lookups in some form of platform data.
That's unfortunately right.

> Why can't you just always use "rx" and "tx" as the names here?
Well I couldn't. I'll explain you why, and maybe you'll find a better solution.

That all is related to how ASoC and SSP interact.
If I remember correctly, here is how it works :
 - the DMA channel is requested in sound/arm/pxa2xx-pcm-lib.c:128
	return snd_dmaengine_pcm_open(
		substream, dma_request_slave_channel(rtd->platform->dev,
   The trick is that the device here is _not_ the SSP one, it's if memory serves
   me well the pxa-pcm-audio one.

   As a consequence, the device cannot be used to differenciate which SSP
   exactly is providing the sound samples stream. This information is
   nevertheless required to choose the correct requestor line, which is a 1-to-1
   match to the SSP port.

   The indirection in the channel name is used to choose the correct requestor
   line for a given SSP port providing the samples.

   It also must be underlined that this dma request serves both AC97 and SSP as
   sample providers.

> (also, I don't see why each line is duplicated, but I'm sure there's
> an easy answer for that).
Ahh that is an unfortunate rebase most probably :)

Cheers.

-- 
Robert