Hi Laurent On Mon, 19 Feb 2024 at 11:40, Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> 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@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 -- Ricardo Ribalda
