linux-kernel - Re: [PATCH v2 3/4] ASoC: apple: mca: Start new platform driver

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Date:   Tue, 23 Aug 2022 12:31:32 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Martin Povišer <povik+lin@...ebit.org>
Cc:     Liam Girdwood <lgirdwood@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Hector Martin <marcan@...can.st>,
        Sven Peter <sven@...npeter.dev>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        asahi@...ts.linux.dev, alsa-devel@...a-project.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/4] ASoC: apple: mca: Start new platform driver

On Tue, Aug 23, 2022 at 09:33:36AM +0200, Martin Povišer wrote:
> > On 22. 8. 2022, at 19:39, Mark Brown <broonie@...nel.org> wrote:
> > On Fri, Aug 19, 2022 at 02:54:29PM +0200, Martin Povišer wrote:

> >> +// SPDX-License-Identifier: GPL-2.0-only
> >> +/*
> >> + * Apple SoCs MCA driver
> >> + *
> >> + * Copyright (C) The Asahi Linux Contributors
> >> + *
> >> + * The MCA peripheral is made up of a number of identical units called clusters.

> > Please make the entire comment block a C++ one so things look more
> > intentional.

> Is this so that it does not look like the SPDX header was added
> mechanically? I will do it, just curious what the reasoning is.

Yes, broadly.

> >> +	/*
> >> +	 * We can't power up the device earlier than this because
> >> +	 * the power state driver would error out on seeing the device
> >> +	 * as clock-gated.
> >> +	 */
> >> +	cl->pd_link = device_link_add(mca->dev, cl->pd_dev,
> >> +				      DL_FLAG_STATELESS | DL_FLAG_PM_RUNTIME |
> >> +					      DL_FLAG_RPM_ACTIVE);

> > I'm not clear on this dynamically adding and removing device links stuff
> > - it looks like the main (only?) purpose is to take a runtime PM
> > reference to the target device which is fine but it's not clear why
> > device links are involved given that the links are created and destroyed
> > every time the DAI is used, AFAICT always in the same fixed
> > relationship.  It's not a problem, it's just unclear.

> Indeed the only purpose is powering up the cluster’s power domain (there’s
> one domain for each cluster). Adding the links is the only way I know to
> do it. They need to be added dynamically (as opposed to statically linking,
> say, the DAI’s ->dev to the cluster’s ->pd_dev, which I guess may do
> something similar), because we need to sequence the power-up/power-down
> with the enablement of the clocks.

You could also just do the underlying runtime power management
operations directly couldn't you?  It's not clear what the device link
stuff is adding.

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