linux-kernel - Re: [PATCH 04/12] ASoC: cs42l42: Don't reconfigure the PLL while it is running

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Message-ID: <20210811144143.GE4167@sirena.org.uk>
Date:   Wed, 11 Aug 2021 15:41:43 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Richard Fitzgerald <rf@...nsource.cirrus.com>
Cc:     alsa-devel@...a-project.org, patches@...nsource.cirrus.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/12] ASoC: cs42l42: Don't reconfigure the PLL while it
 is running

On Wed, Aug 11, 2021 at 01:21:24PM +0100, Richard Fitzgerald wrote:
> On 11/08/2021 12:56, Mark Brown wrote:
> > On Tue, Aug 10, 2021 at 05:27:45PM +0100, Richard Fitzgerald wrote:

> > > cs42l42_pll_config() could check whether it is already running and skip
> > > configuration in that case, but that seems to me a rather opaque
> > > implementation. In my opinion this doesn't really fall into the case of
> > > ignoring-bad-stuff-to-be-helpful (like free() accepting a NULL).

> > This doesn't treat the situation as an error though, it just ignores it,
> > and there's nothing to stop _pll_config() generating a warning if that
> > makes sense.

> It isn't an error. hw_params() will be called for both substreams
> (PLAYBACK and CAPTURE) and if one is already running we mustn't
> reconfigure the things we already configured. The DAI is marked
> symmetric so both substreams will always produce the same I2C BCLK.

If it's a noop reconfiguration then there's a case for saying that
_pll_config() should just silently do nothing anyway regardless of
issues with reconfiguring, though you might also want to warn dpeending
on other expectations.  If it's not a noop reconfiguration then
presumably the new configuration not taking effect might mean that other
things aren't going to see the clocks they expect.  Either way if a
reconfiguration gets introduced via a path other than hw_params(),
either now or later, having the check in the _pll_config() would catch
it.

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