Re: [PATCH v9 1/6] media: uvcvideo: Fix negative modulus calculation

[Ricardo Ribalda <ribalda@chromium.org>]

Hi Laurent

On Mon, 19 Feb 2024 at 11:40, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
>
> Hi Ricardo,
>
> On Mon, Feb 19, 2024 at 11:28:03AM +0100, Ricardo Ribalda wrote:
> > On Mon, 12 Feb 2024 at 23:59, Laurent Pinchart wrote:
> > > On Wed, Mar 15, 2023 at 02:30:12PM +0100, Ricardo Ribalda wrote:
> > > > If head is 0, last will be addressing the index 0 instead of clock->size
> > > > -1. Luckily clock->head is unsiged, otherwise it would be addressing
> > > > 0xffffffff.
> > >
> > > I'm not following you. In the expression
> > >
> > >         (clock->head - 1) % clock->size
> > >
> > > clock->head is an unsigned int, and 1 as a signed int, so the result of
> > > the subtraction is promoted to an unsigned int. When clock->head is 0, the expression evaluates to
> > >
> > >         0xffffffff % clock->size
> > >
> > > clock->size is a power of two (hardcoded to 32 at the moment), so the
> > > expression evaluates to 31, as intended.
> > >
> > > Am I missing something ?
> >
> > Take a look to: https://godbolt.org/z/xYeqTx6ba
> >
> > The expression only works because the size is a power of two. In this
> > set I am allowing sizes that are not powers of two.
>
> Could you then update the commit message to explain that ?
>
> I'll review the rest of the series this week.
Thanks

Will update with the following text after the review:

The tail of the list lives at the position before the head. This is
mathematically noted as
```
(head-1)  mod size.
```

Unfortunately C, does not have a modulus operator, but a remainder
operator (%).
The reminder operation has a different result than the modulus if
(head -1) is a negative number and size is not a power of two.

Adding size to (head-1) allows the code to run with any value of size.

>
> > > > Nontheless, this is not the intented behaviour and should be fixed.
> > > >
> > > > Fixes: 66847ef013cc ("[media] uvcvideo: Add UVC timestamps support")
> > > > Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
> > > > ---
> > > >  drivers/media/usb/uvc/uvc_video.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> > > > index d4b023d4de7c..4ff4ab4471fe 100644
> > > > --- a/drivers/media/usb/uvc/uvc_video.c
> > > > +++ b/drivers/media/usb/uvc/uvc_video.c
> > > > @@ -732,7 +732,7 @@ void uvc_video_clock_update(struct uvc_streaming *stream,
> > > >               goto done;
> > > >
> > > >       first = &clock->samples[clock->head];
> > > > -     last = &clock->samples[(clock->head - 1) % clock->size];
> > > > +     last = &clock->samples[(clock->head - 1 + clock->size) % clock->size];
> > > >
> > > >       /* First step, PTS to SOF conversion. */
> > > >       delta_stc = buf->pts - (1UL << 31);
>
> --
> Regards,
>
> Laurent Pinchart



-- 
Ricardo Ribalda