linux-kernel - Re: [PATCH 0/4] media: Add devm-managed helper functions for media and V4L2 subsystems

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Message-ID:
 <PN3P287MB18294EA63E7247DAF00642FC8B5AA@PN3P287MB1829.INDP287.PROD.OUTLOOK.COM>
Date: Mon, 28 Jul 2025 05:45:34 +0000
From: Tarang Raval <tarang.raval@...iconsignals.io>
To: "sakari.ailus@...ux.intel.com" <sakari.ailus@...ux.intel.com>
CC: Laurent Pinchart <laurent.pinchart@...asonboard.com>, "hverkuil@...all.nl"
	<hverkuil@...all.nl>, Mauro Carvalho Chehab <mchehab@...nel.org>, Ricardo
 Ribalda <ribalda@...omium.org>, Hans de Goede <hansg@...nel.org>, James
 Cowgill <james.cowgill@...ize.com>, Yunke Cao <yunkec@...gle.com>, Tomi
 Valkeinen <tomi.valkeinen@...asonboard.com>, Lad Prabhakar
	<prabhakar.mahadev-lad.rj@...renesas.com>, Tommaso Merciai
	<tomm.merciai@...il.com>, "linux-media@...r.kernel.org"
	<linux-media@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] media: Add devm-managed helper functions for media
 and V4L2 subsystems

> On Wed, Jul 23, 2025 at 02:08:03PM +0000, Tarang Raval wrote:
> > Hi Laurent,
> >
> > > On Wed, Jul 23, 2025 at 03:55:04PM +0530, Tarang Raval wrote:
> > > > This patch series introduces devm-managed versions of several commonly used
> > > > media and V4L2 initialization functions. These helpers simplify resource
> > > > management by leveraging the devres infrastructure, ensuring automatic
> > > > cleanup when the associated device is detached or the driver is unloaded.
> > > 
> > > I'll let Sakari review this, but overall, I don't think we want to take
> > > this direction. Objects need to be refcounted instead of freed at remove
> > > time.
> >
> > I agree that refcounting could provide more robust lifetime management,
> > especially for shared resources. I will think in this direction, explore
> > implementing refcounting for these objects, and share an RFC for your
> > feedback.
> 
> I agree with Laurent here when it comes to the current state of the
> patches.
> 
> Tearing down things automatically, however, is what we should look into.
> But I believe it will require refcounting sub-devices first, then we could
> bind such resources to them. But before this, we'll need to merge the media
> device refcounting series
> <URL:https://git.linuxtv.org/sailus/media_tree.git/log/?h=media-ref>.

Okay, I will wait for this series to be merged.

Thanks for the reference, I will go through these patches and follow the 
progress.

> > > This patch series doesn't necessarily cause a regression as such,
> > > but it will make it more difficult to fix life time management issues in
> > > V4L2.
> >
> > Could you clarify how devm-managed helpers might complicate lifetime
> > management in V4L2? Understanding specific issues will help me design
> > a solution that aligns with the subsystem’s needs while keeping cleanup
> > simple.
> 
> The problem with devm_*() and UAPI is that once the device disappears, so
> disappear all its resources, including those being used by whatever
> programs accessing the UAPI, leading to memory safety issues. IOW devm_()
> when used on memory resources in V4L2 is often simply wrong but right now
> we don't have yet any better options in a lot of cases.

Understood. Thanks for the clarification.

Best Regards,
Tarang