linux-kernel - Re: [PATCH 4/4] media: mt9m111: remove .s

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Message-ID: <20220819081755.wlsryhcr7xjouor5@uno.localdomain>
Date:   Fri, 19 Aug 2022 10:17:55 +0200
From:   Jacopo Mondi <jacopo@...ndi.org>
To:     Marco Felsch <m.felsch@...gutronix.de>
Cc:     mchehab@...nel.org, sakari.ailus@...ux.intel.com,
        laurent.pinchart+renesas@...asonboard.com,
        jacopo+renesas@...ndi.org, akinobu.mita@...il.com,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...gutronix.de
Subject: Re: [PATCH 4/4] media: mt9m111: remove .s_power callback

Hi Marco

On Fri, Aug 19, 2022 at 10:06:26AM +0200, Marco Felsch wrote:
> On 22-08-19, Jacopo Mondi wrote:
> > Hi Marco
> >
> > On Fri, Aug 19, 2022 at 09:18:32AM +0200, Marco Felsch wrote:
> > > Hi Jacopo,
> > >
> > > thanks for your fast feedback :)
> > >
> > > On 22-08-18, Jacopo Mondi wrote:
> > > > Hi Marco
> > > >
> > > > On Thu, Aug 18, 2022 at 04:47:12PM +0200, Marco Felsch wrote:
> > > > > This is in preparation of switching to the generic dev PM mechanism.
> > > > > Since the .s_power callback will be removed in the near future move the
> > > > > powering into the .s_stream callback. So this driver no longer depends
> > > > > on the .s_power mechanism.
> > > > >
> > > > > Signed-off-by: Marco Felsch <m.felsch@...gutronix.de>
> > > >
> > > > If you want to move to runtime_pm, I would implement it first and have
> > > > s_power call the _resume and _suspend routines, as some platform
> > > > drivers still use s_power() and some of them might depend on it.
> > >
> > > Do we really have platforms which depend on this? IMHO if that is the
> >
> > $ git grep "v4l2_subdev_call(.*, s_power" drivers/media/platform/  | cut -d : -f1 | uniq  | wc -l
> > 8
> >
> > > case than we should fix those platfoms. Since new drivers shouldn't use
> > > this callback anymore.
> >
> > Patches are clearly welcome I guess..
>
> :)
>
> > > In my case, I worked on [1] and wondered why the sensor was enabled
> > > before I told him to do so. Since I didn't implement the s_power()
> > > callback, I had no chance to get enabled before.
> > >
> >
> > I'm not sure I got this part
>
> What I mean is, that the MT9M131 sensor gets enabled and immediately
> start sending frames before I told him to do so e.g. by calling

Why does this happen ?

> s_stream(). This can confuse the downstream device. The only way to get
> enable the downstream device first is to add the s_power() callback.
>
> > > Can we please decide:
> > >  - Do we wanna get rid of the s_power() callback?
> >
> > I think that would be everyone's desire, but drivers have to be moved
> > away from it
> >
> > >  - If not, how do we handle those devices then with drivers not
> > >    implementing this callback?
> >
> > By maintaining compatibility. I suggested to move to runtime_pm() and
> > wrap _resume/_suspend in s_power().
>
> But then you're introducing new drivers with s_power() callbacks and so
> the behaviour isn't really changed.
>

I only meant in existing ones

> > My understanding is that the two (runtime_pm/s_power) are mutually
> > exclusive, but even if that was not the case, runtime_pm is reference
> > counted, hence as long as calls are balanced this should work, right ?
>
> Right but the s_power() behaviour is not changed and drivers still rely
> on it to work as right now.

As long as we have bridge drivers using it, isn't this what we want ?
>
> Regards,
>   Marco