linux-kernel - Re: [PATCH 13/13] ASoC: ti: davinci-i2s: Opitonally drive DX pin during capture streams

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Date: Wed, 20 Mar 2024 09:52:37 +0100
From: Bastien Curutchet <bastien.curutchet@...tlin.com>
To: Péter Ujfalusi <peter.ujfalusi@...il.com>,
 Liam Girdwood <lgirdwood@...il.com>, Mark Brown <broonie@...nel.org>,
 Rob Herring <robh+dt@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.org>, Jaroslav Kysela <perex@...ex.cz>,
 Takashi Iwai <tiwai@...e.com>
Cc: linux-sound@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, alsa-devel@...a-project.org,
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>, herve.codina@...tlin.com,
 christophercordahi@...ometrics.ca
Subject: Re: [PATCH 13/13] ASoC: ti: davinci-i2s: Opitonally drive DX pin
 during capture streams

Hi Péter,

On 3/19/24 19:29, Péter Ujfalusi wrote:
> 
> 
> On 15/03/2024 13:27, Bastien Curutchet wrote:
>> The McBSP's DX pin that outputs serial data during playback streams can
>> be used during capture streams to repeatedly output a chosen pattern.
>> For instance, this can be useful to drive an active-low signal during
>> captures (by choosing <0> as output pattern).
> 
> Are there really any other use of this than to pull down or up the DX
> pin (0 or 0xffff)
I don't know, indeed today I can only think about these two patterns.
I tried to do something in a 'generic' way so it can evolve if needed.

> If you just use the pin as GPIO then you don't need to change anything
> in the driver, The playback would not erach the pin, so no need to block it.
> 
>> Enable this behaviour when the device-tree property 'ti,drive-dx' is
>> present. DX pin is driven with the provided pattern every time a
>> capture stream is launched.
> 
> It is an interesting use of the hardware... You are controlling an
> external device (light an LED when capture is on)?

Yes I control the chip select pin of the ADC that is sending data to DR 
pin, that's why I need the DX pin to be synchronized with capture
streams.

>> This property is not compatible with classic playback stream so
>> davinci_i2s_trigger() returns an error if a playback stream is started
>> while 'ti,drive-dx' flag is present.
> 
> Propbaly add the .startup() callback and block the playback right there?
> 

Ok, TBH my mastery of the sound subsystem is not high enough to have an
opinion of where this should go so I'll trust you on this.

>>
>> This has been tested on a board designed of a DAVINCI/OMAP-L138 where
>> the DX pin is linked to the chip select pin of the converters of the
>> capture side.
> 
> Isn't the DX will be pulled down as soon as the McBSP is enabled?
> Can you just re-configure the PUPD_SEL for the pin group to make the pin
> to be pulled the other way?
> 

Well, the acquisition chain in my use case is a bit convoluted. The DX
pin's main purpose is to drive ADC chip select but it is also connected
to other components and all this needs synchronization upon captures.


I'll integrate your feedback about the code in next iteration, thank
you.


Best regards,
Bastien