linux-kernel - Re: [PATCH 1/3] ASoC: ti: omap-mcbsp: Ignore errors for getting fck

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Date:   Wed, 25 Oct 2023 17:21:12 +0300
From:   Péter Ujfalusi <peter.ujfalusi@...il.com>
To:     Andreas Kemnade <andreas@...nade.info>
Cc:     Tony Lindgren <tony@...mide.com>, bcousson@...libre.com,
        robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
        conor+dt@...nel.org, lgirdwood@...il.com, broonie@...nel.org,
        perex@...ex.cz, tiwai@...e.com, jarkko.nikula@...mer.com,
        dmitry.torokhov@...il.com, linux-omap@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        alsa-devel@...a-project.org
Subject: Re: [PATCH 1/3] ASoC: ti: omap-mcbsp: Ignore errors for getting
 fck_src


On 13/10/2023 14:25, Andreas Kemnade wrote:
>> I guess it is because of the pm_runtime_put_sync() in the
>> omap2_mcbsp_set_clks_src() around the fclk re-parenting.
>> That is a bit dubious thing for sure. We need to disable the device to
>> be able to re-parent the fclk but if we disable the device it is going
>> to be powered down, right? I think we have appropriate context handling,
>> so it might work, but it is certainly not a rock solid code... If you
>> have a stream running already, you don't really want to kill the McBSP.
>>
> Ok, so if the device is powered of at omap2_mcbsp_set_clks_src() 
> we get the usage count underflow, and the counter is incremented
> immediately again in the runtime put function. So things get out of balance...
> I'll check Tony's fix here.
> 
>> The problem is that this mux is outside of the McBSP IP, so we need a
>> system level (iow, clk API) way to change it runtime.
>>
>> What is the machine driver where this happens? If you set the sysclk in
>> hw_params of the machine driver, it will be OK, but if you do that in
>> probe time then it is likely going to fail as you experienced
>>
> As you see in the other patches of this series,
> it is a simple-audio-card with a tlv320aic3x codec
> in combination with the mcbsp.

To be honest I would be happier if we can just remove the whole
omap2_mcbsp_set_clks_src() and leave the CLKS source selection outside
of the driver.
But omap3pandora is selecting external clock as parent
(OMAP_MCBSP_SYSCLK_CLKS_EXT - in hw_params, so it actually works) and I
don't know what happens if this functionality is removed. Likely going
to break Pandora.
That is fixable, but what worries me is this comment and code:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/sound/soc/ti/omap-mcbsp.c#n388

Which is added by me a long time ago:
e386615c01d37 ("ASoC: omap-mcbsp: When closing the port select PRCM
source for CLKS signal")

I'm not sure if this is possible to do in any other way.

-- 
Péter