linux-kernel - Re: [PATCH 14/15] ARM: pxa: change SSP devices allocation

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Date:   Tue, 03 Apr 2018 17:32:58 +0200
From:   Robert Jarzmik <robert.jarzmik@...e.fr>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Daniel Mack <daniel@...que.org>,
        Haojian Zhuang <haojian.zhuang@...il.com>,
        Vinod Koul <vinod.koul@...el.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        alsa-devel@...a-project.org
Subject: Re: [PATCH 14/15] ARM: pxa: change SSP devices allocation

Arnd Bergmann <arnd@...db.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@...e.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