linux-kernel - Re: [PATCH 38/78] media: i2c: mt9m001: use pm_runtime_resume_and

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Message-ID: <20210428132853.65b162a0@coco.lan>
Date:   Wed, 28 Apr 2021 13:28:53 +0200
From:   Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     Jacopo Mondi <jacopo@...ndi.org>, linuxarm@...wei.com,
        mauro.chehab@...wei.com, Hans Verkuil <hverkuil-cisco@...all.nl>,
        Jacopo Mondi <jacopo+renesas@...ndi.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org
Subject: Re: [PATCH 38/78] media: i2c: mt9m001: use
 pm_runtime_resume_and_get()

Em Wed, 28 Apr 2021 12:05:26 +0200
Johan Hovold <johan@...nel.org> escreveu:

> On Wed, Apr 28, 2021 at 10:31:48AM +0200, Mauro Carvalho Chehab wrote:
> > Em Tue, 27 Apr 2021 14:23:20 +0200
> > Johan Hovold <johan@...nel.org> escreveu:  
> 

> > > You're call, but converting functioning drivers where the authors knew
> > > what they were doing just because you're not used to the API and risk
> > > introducing new bugs in the process isn't necessarily a good idea.  
> > 
> > The problem is that the above assumption is not necessarily true:
> > based on the number of drivers that pm_runtime_get_sync() weren't
> > decrementing usage_count on errors, several driver authors were not 
> > familiar enough with the PM runtime behavior by the time the drivers
> > were written or converted to use the PM runtime, instead of the
> > media .s_power()/.s_stream() callbacks.  
> 
> That may very well be the case. My point is just that this work needs to
> be done carefully and by people familiar with the code (and runtime pm)
> or you risk introducing new issues.

Yeah, that's for sure.

> I really don't want the bot-warning-suppression crew to start with this
> for example.
> 
> > > Especially since the pm_runtime_get_sync() will continue to be used
> > > elsewhere, and possibly even in media in cases where you don't need to
> > > check for errors (e.g. remove()).  
> > 
> > Talking about the remove(), I'm not sure if just ignoring errors
> > there would do the right thing. I mean, if pm_runtime_get_sync()
> > fails, probably any attempts to disable clocks and other things
> > that depend on PM runtime will also (silently) fail.
> > 
> > This may put the device on an unknown PM and keep clock lines enabled
> > after its removal.  
> 
> Right, a resume failure is a pretty big issue and it's not really clear
> how to to even handle that generally. But at remove() time you don't
> have much choice but to go on and release resource anyway. 
> 
> So unless actually implementing some error handling too, using
> pm_runtime_sync_get() without checking for errors is still preferred
> over pm_runtime_resume_and_get(). That is 
> 
> 	pm_runtime_get_sync();
> 	/* cleanup */
> 	pm_runtime_disable()
> 	pm_runtime_put_noidle();
> 
> is better than:
> 
> 	ret = pm_runtime_resume_and_get();
> 	/* cleanup */
> 	pm_runtime_disable();
> 	if (ret == 0)
> 		pm_runtime_put_noidle();
> 
> unless you also start doing something ret.

Perhaps the best would be to use, instead:

	pm_runtime_get_noresume();
 	/* cleanup */
 	pm_runtime_disable()
 	pm_runtime_put_noidle();
	pm_runtime_set_suspended();

I mean, at least for my eyes, it doesn't make sense to do a PM
resume during driver's removal/unbind time.

Regards,
Mauro


> 
> Johan



Thanks,
Mauro