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

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Date: Thu, 21 Mar 2024 16:14:04 +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 3/20/24 21:30, Péter Ujfalusi wrote:
> 
> 
> On 20/03/2024 17:42, 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.
>>
>> I think the definition of the 'ti,drive-dx' is somehow odd. It allows
>> you to set it to 0x1234 and the DX pin will show 0x1234 when you capture
>> 32bit. If you capture 16bit then it will transmit 0x12 (or 0x34?), no?
>> If you have 4 channel capture then I won't speculate what will be on the
>> DX pin ;)
>>
>> Would not be better to say that the DX pin will be driven low or high
>> during capture _and_ disable the playback support?
> 
> After some thinking, it might be still better to use the DX pin as GPIO
> and either have a custom machine driver which would handle it (set low
> when a capture trigger happens) or connect it in DAPM as a supply, bias
> or something and ASoC would handle it automagically.
> 
> I think that would be cleaner in many ways. What do you think?
> 
I agree, that would be cleaner. I ran a few tests to see if that would
work on my hardware. It doesn't ... So I looked back to the schematics
and found two reasons :
  * the DX pin needs to be in sync with the clock.
  * the DX pin needs to be in a high-impedance state between two frames
    so a pull-up can drive it back up. Actually, the DX pin is also
    linked to the FSR pin so it provides the frame clock to the capture
    stream.

Bast regards,
Bastien