linux-kernel - Re: [PATCH] ALSA: hda/tegra: enable clock during probe

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Date:   Fri, 25 Jan 2019 14:58:07 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Jon Hunter <jonathanh@...dia.com>
Cc:     Sameer Pujar <spujar@...dia.com>,
        <pierre-louis.bossart@...ux.intel.com>, <perex@...ex.cz>,
        <alsa-devel@...a-project.org>, <thierry.reding@...il.com>,
        <rlokhande@...dia.com>, <linux-kernel@...r.kernel.org>,
        <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH] ALSA: hda/tegra: enable clock during probe

On Fri, 25 Jan 2019 14:26:27 +0100,
Jon Hunter wrote:
> 
> 
> On 25/01/2019 12:40, Takashi Iwai wrote:
> > On Fri, 25 Jan 2019 12:36:00 +0100,
> > Jon Hunter wrote:
> >>
> >>
> >> On 24/01/2019 19:08, Takashi Iwai wrote:
> >>> On Thu, 24 Jan 2019 18:36:43 +0100,
> >>> Sameer Pujar wrote:
> >>>>
> >>>> If CONFIG_PM is disabled or runtime PM calls are forbidden, the clocks
> >>>> will not be ON. This could cause issue during probe, where hda init
> >>>> setup is done. This patch checks whether runtime PM is enabled or not.
> >>>> If disabled, clocks are enabled in probe() and disabled in remove()
> >>>>
> >>>> This patch does following minor changes as cleanup,
> >>>>   * return code check for pm_runtime_get_sync() to take care of failure
> >>>>     and exit gracefully.
> >>>>   * In remove path runtime PM is disabled before calling snd_card_free().
> >>>>   * hda_tegra_disable_clocks() is moved out of CONFIG_PM_SLEEP check.
> >>>>   * runtime PM callbacks moved out of CONFIG_PM check
> >>>>
> >>>> Signed-off-by: Sameer Pujar <spujar@...dia.com>
> >>>> Reviewed-by: Ravindra Lokhande <rlokhande@...dia.com>
> >>>> Reviewed-by: Jon Hunter <jonathanh@...dia.com>
> >>> (snip)
> >>>> @@ -555,6 +553,13 @@ static int hda_tegra_probe(struct platform_device *pdev)
> >>>>  	if (!azx_has_pm_runtime(chip))
> >>>>  		pm_runtime_forbid(hda->dev);
> >>>>  
> >>>> +	/* explicit resume if runtime PM is disabled */
> >>>> +	if (!pm_runtime_enabled(hda->dev)) {
> >>>> +		err = hda_tegra_runtime_resume(hda->dev);
> >>>> +		if (err)
> >>>> +			goto out_free;
> >>>> +	}
> >>>> +
> >>>>  	schedule_work(&hda->probe_work);
> >>>
> >>> Calling runtime_resume here is really confusing...
> >>
> >> Why? IMO it is better to have a single handler for resuming the device
> >> and so if RPM is not enabled we call the handler directly. This is what
> >> we have been advised to do in the past and do in other drivers. See ...
> > 
> > The point is that we're not "resuming" anything there.  It's in the
> > early probe stage, and the device state is uninitialized, not really
> > suspended.  It'd end up with just calling the same helper
> > (hda_tegra_enable_clocks()), though.
> 
> Yes and you can make the same argument for every driver that calls
> pm_runtime_get_sync() during probe to turn on clocks, handle resets,
> etc, because at the end of the day the very first call to
> pm_runtime_get_sync() invokes the runtime_resume callback, when we have
> never been suspended.

Although there are some magical pm_runtime_*() in some places, most of
such pm_runtime_get_sync() is for the actual runtime PM management (to
prevent the runtime suspend), while the code above is for explicitly
setting up something for non-PM cases.

And if pm_runtime_get_sync() is obviously superfluous, we should
remove such calls.  Really.

> Yes at the end of the day it is the same and given that we have done
> this elsewhere I think it is good to be consistent if/where we can.

The code becomes less readable, and that's a good reason against it :)


thanks,

Takashi