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SubjectRe: [PATCH 14/15] ARM: pxa: change SSP devices allocation
Date
Arnd Bergmann <arnd@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@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

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