linux-kernel - Re: [PATCH] media: uvcvideo: avoid variable shadowing in uvc_ctrl_cleanup

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Message-ID: <20250701192928.GB16835@pendragon.ideasonboard.com>
Date: Tue, 1 Jul 2025 22:29:28 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Ricardo Ribalda <ribalda@...omium.org>
Cc: Desnes Nunes <desnesn@...hat.com>, hansg@...nel.org,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: uvcvideo: avoid variable shadowing in
 uvc_ctrl_cleanup_fh

On Tue, Jul 01, 2025 at 09:25:59PM +0200, Ricardo Ribalda wrote:
> On Tue, 1 Jul 2025 at 20:42, Laurent Pinchart
> <laurent.pinchart@...asonboard.com> wrote:
> >
> > On Tue, Jul 01, 2025 at 02:20:53PM -0300, Desnes Nunes wrote:
> > > On Tue, Jul 1, 2025 at 1:48 PM Ricardo Ribalda <ribalda@...omium.org> wrote:
> > > > On Tue, 1 Jul 2025 at 16:59, Desnes Nunes <desnesn@...hat.com> wrote:
> > > > >
> > > > > This avoids a variable loop shadowing occurring between the local loop
> > > > > iterating through the uvc_entity's controls and the global one going
> > > > > through the pending async controls of the file handle
> > > > >
> > > > > Fixes: 10acb9101355 ("media: uvcvideo: Increase/decrease the PM counter per IOCTL")
> > > > If you add a fixes you need to add
> > > > Cc: stable@...nel.org
> > >
> > > Thanks for letting me know
> > >
> > > > Reviewed-by: Ricardo Ribalda <ribalda@...omium.org>
> > > > > Signed-off-by: Desnes Nunes <desnesn@...hat.com>
> > > > > ---
> > > > >  drivers/media/usb/uvc/uvc_ctrl.c | 3 +--
> > > > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/drivers/media/usb/uvc/uvc_ctrl.c b/drivers/media/usb/uvc/uvc_ctrl.c
> > > > > index 44b6513c5264..91cc874da798 100644
> > > > > --- a/drivers/media/usb/uvc/uvc_ctrl.c
> > > > > +++ b/drivers/media/usb/uvc/uvc_ctrl.c
> > > > > @@ -3260,7 +3260,6 @@ int uvc_ctrl_init_device(struct uvc_device *dev)
> > > > >  void uvc_ctrl_cleanup_fh(struct uvc_fh *handle)
> > > > >  {
> > > > >         struct uvc_entity *entity;
> > > > > -       int i;
> > > > >
> > > > >         guard(mutex)(&handle->chain->ctrl_mutex);
> > > > >
> > > > > @@ -3278,7 +3277,7 @@ void uvc_ctrl_cleanup_fh(struct uvc_fh *handle)
> > > > >         if (!WARN_ON(handle->pending_async_ctrls))
> > > > >                 return;
> > > > >
> > > > > -       for (i = 0; i < handle->pending_async_ctrls; i++)
> > > >
> > > > nitpick: I would have called the variable i, not j.  For me j usually
> > > > means nested loop. But up to you
> > >
> > > Noted - I used a different variable name because I wanted to
> > > differentiate the loops.
> >
> > Variable declaration in the loop statement is relatively new in the
> > kernel, so there's no consensus yet (to my knowledge) on clear coding
> > styles, but I would have simply used the same variable name in both
> > loops, with two separate declarations:
> >
> > diff --git a/drivers/media/usb/uvc/uvc_ctrl.c b/drivers/media/usb/uvc/uvc_ctrl.c
> > index 303b7509ec47..6b9486749c3f 100644
> > --- a/drivers/media/usb/uvc/uvc_ctrl.c
> > +++ b/drivers/media/usb/uvc/uvc_ctrl.c
> > @@ -3299,7 +3299,6 @@ int uvc_ctrl_init_device(struct uvc_device *dev)
> >  void uvc_ctrl_cleanup_fh(struct uvc_fh *handle)
> >  {
> >         struct uvc_entity *entity;
> > -       int i;
> >
> >         guard(mutex)(&handle->chain->ctrl_mutex);
> >
> > @@ -3317,7 +3316,7 @@ void uvc_ctrl_cleanup_fh(struct uvc_fh *handle)
> >         if (!WARN_ON(handle->pending_async_ctrls))
> >                 return;
> >
> > -       for (i = 0; i < handle->pending_async_ctrls; i++)
> > +       for (unsigned int i = 0; i < handle->pending_async_ctrls; i++)
> >                 uvc_pm_put(handle->stream->dev);
> >  }
> >
> > Is there a downside to this ?
> 
> The toolchain that they are using does not seem to like it,

Well, this is mainline :-)

> and there
> is no benefit for having two initialization vs one.
> 
> (also makes the lines shorter... but that is just a matter of taste).
> 
> I don't really have a preference.

Declaring the variable in the loop statement removes the possibility of
using it outside of the loop. I think that's an improvement in general,
it makes reading the code simpler, and avoids accidental misuse.

> > > > I am also not against your first version with a different commit message.
> > >
> > > Third time's a charm then!
> > >
> > > Will send a v2 with the first version having this commit message.
> > >
> > > Thanks for the review Ricardo,

-- 
Regards,

Laurent Pinchart