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

[Laurent Pinchart]

On Thu, Mar 21, 2024 at 11:50:48PM +0200, Laurent Pinchart wrote:
> On Mon, Feb 19, 2024 at 04:07:12PM +0100, Ricardo Ribalda wrote:
> > On Mon, 19 Feb 2024 at 11:40, Laurent Pinchart wrote:
> > > 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.
> 
> Could you please add
> 
> This does not change the current behaviour of the driver, as the size is
> always a power of two, but prepares for reworks that will change the
> size to a non power of two.
> 
> or something similar ?
> 
> > > > > > Nontheless, this is not the intented behaviour and should be fixed.
> > > > > >
> > > > > > Fixes: 66847ef013cc ("[media] uvcvideo: Add UVC timestamps support")
> 
> I think this should be dropped, the patch doesn't fix an issue, but
> prepares for further changes that add new features. I'd also like to
> update the commit message to avoid stating "Fix", to avoid this being
> picked for stable kernels automatically.

After reviewing the whole series, it seems that clock->size stays at its
current value of 32. Do we thus need this patch ?

> > > > > > Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
> > > > > > ---
> > > > > >  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