linux-kernel - Re: [alsa-devel] [PATCH v2 3/3] ASoC: TDA7802: Add turn-on diagnostic routine

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Date:   Wed, 31 Jul 2019 09:03:31 +0100
From:   Charles Keepax <ckeepax@...nsource.cirrus.com>
To:     Thomas Preston <thomas.preston@...ethink.co.uk>
CC:     Mark Brown <broonie@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        <devicetree@...r.kernel.org>, <alsa-devel@...a-project.org>,
        Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>,
        Kirill Marinushkin <kmarinushkin@...dec.tech>,
        Liam Girdwood <lgirdwood@...il.com>,
        Marco Felsch <m.felsch@...gutronix.de>,
        Annaliese McDermond <nh6z@...z.net>,
        Takashi Iwai <tiwai@...e.com>,
        Paul Cercueil <paul@...pouillou.net>,
        Vinod Koul <vkoul@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Jerome Brunet <jbrunet@...libre.com>,
        <linux-kernel@...r.kernel.org>,
        Cheng-Yi Chiang <cychiang@...omium.org>
Subject: Re: [alsa-devel] [PATCH v2 3/3] ASoC: TDA7802: Add turn-on
 diagnostic routine

On Tue, Jul 30, 2019 at 05:28:11PM +0100, Thomas Preston wrote:
> On 30/07/2019 16:50, Mark Brown wrote:
> > On Tue, Jul 30, 2019 at 04:25:56PM +0100, Thomas Preston wrote:
> >> On 30/07/2019 15:19, Mark Brown wrote:
> > 
> >>> It is unclear what this mutex usefully protects, it only gets taken when
> >>> writing to the debugfs file to trigger this diagnostic mode but doesn't
> >>> do anything to control interactions with any other code path in the
> >>> driver.
> > 
> >> If another process reads the debugfs node "diagnostic" while the turn-on 
> >> diagnostic mode is running, this mutex prevents the second process
> >> restarting the diagnostics.
> > 
> >> This is redundant if debugfs reads are atomic, but I don't think they are.
> > 
> > Like I say it's not just debugfs though, there's the standard driver
> > interface too.
> > 
> 
> Ah right, I understand. So if we run the turn-on diagnostics routine, there's
> nothing stopping anyone from interacting with the device in other ways.
> 
> I guess there's no way to share that mutex with ALSA? In that case, it doesn't
> matter if this mutex is there or not - this feature is incompatible. How
> compatible do debugfs interfaces have to be? I was under the impression anything
> goes. I would argue that the debugfs is better off for having the mutex so
> that no one re-reads "diagnostic" within the 5s poll timeout.
> 
> Alternatively, this diagnostic feature could be handled with an external-handler
> kcontrol SOC_SINGLE_EXT? I'm not sure if this is an atomic interface either.
> 
> What would be acceptable?

You could take the DAPM mutex in your debugfs handler that would
prevent any changes to the cards power state whilst your debug
stuff is running.

Thanks,
Charles