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

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Message-Id: <E307FA6A-749E-450D-B200-4509848BDA69@cutebit.org>
Date:   Tue, 23 Aug 2022 13:53:00 +0200
From:   Martin Povišer <povik+lin@...ebit.org>
To:     Mark Brown <broonie@...nel.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 23. 8. 2022, at 13:51, Martin Povišer <povik+lin@...ebit.org> wrote:
> 
> 
> 
>> On 23. 8. 2022, at 13:31, Mark Brown <broonie@...nel.org> wrote:
>> 
>> 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:
> 
>>>>> +	/*
>>>>> +	 * 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.
> 
> This seems to be the way to do it. Quoting from documentation of
> dev_pm_domain_attach_by_id, by which we obtain the mca->dev and cl->pd_dev
> the link is between:
> 
> * This function should typically be invoked by a driver during the probe phase,
> * in case its device requires power management through multiple PM domains. The
> * driver may benefit from using the received device, to configure device-links
> * towards its original device. Depending on the use-case and if needed, the
> * links may be dynamically changed by the driver, which allows it to control
> * the power to the PM domains independently from each other.
> 
> --
> Martin

Pardon, just the cl->pd_dev is from dev_pm_domain_attach_by_id, mca->dev is
the original device.

Martin