linux-kernel - Re: [PATCH v1 1/2] media: hi846: fix usage of pm_runtime_get_if_in

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Message-ID: <20230407043227.GI31272@pendragon.ideasonboard.com>
Date:   Fri, 7 Apr 2023 07:32:27 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Sakari Ailus <sakari.ailus@....fi>
Cc:     Martin Kepplinger <martin.kepplinger@...i.sm>, mchehab@...nel.org,
        kernel@...i.sm, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/2] media: hi846: fix usage of
 pm_runtime_get_if_in_use()

Hi Sakari,

On Thu, Apr 06, 2023 at 04:36:25PM +0300, Sakari Ailus wrote:
> On Thu, Apr 06, 2023 at 04:33:38AM +0300, Laurent Pinchart wrote:
> > On Wed, Apr 05, 2023 at 03:52:52PM +0300, Sakari Ailus wrote:
> > > On Wed, Apr 05, 2023 at 11:29:03AM +0200, Martin Kepplinger wrote:
> > > > pm_runtime_get_if_in_use() does not only return nonzero values when
> > > > the device is in use, it can return a negative errno too.
> > > > 
> > > > And especially during resuming from system suspend, when runtime pm
> > > > is not yet up again, this can very well happen. And in such a case
> > > > the subsequent pm_runtime_put() call would result in a refcount underflow!
> > > 
> > > I think this issue should have a more generic solution, it's very difficult
> > > to address this in drivers only with the current APIs.
> > > 
> > > pm_runtime_get_if_in_use() will also return an error if runtime PM is
> > > disabled, so this patch will break the driver for that configuration.
> > 
> > I'm increasingly inclined to depend on CONFIG_PM for all camera sensor
> > drivers.
> 
> For what reason? This is the smallest of all problems related to power
> management. Also runtime PM has no-op functions for just this purpose.

Because it creates a myriad of sleep (or bigger) issues like this one,
and more seem to be popping up (or coming to my attention at least) over
time.

> (Frankly it'd be great if we could make CONFIG_PM go away. So perhaps
> requiring it everywhere is one feasible approach to do that.)

I'm all for it :-) For camera sensor drivers, are you aware of use
cases where !CONFIG_PM would be desired ?

-- 
Regards,

Laurent Pinchart