On Sat, Mar 23, 2024 at 10:48:02AM +0000, Ricardo Ribalda wrote: > 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. > > This does not change the current behaviour of the driver, as the size is > always a power of two, but avoid tedious debugging if we ever change its > size. > > Reviewed-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx> > 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 7cbf4692bd875..659c9e9880a99 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); > > -- > 2.44.0.396.g6e790dbe36-goog > > It could be worth mentioning that basically head == 0 is the problematic case here, but otherwise good catch. Reviewed-by: Tomasz Figa <tfiga@xxxxxxxxxxxx> Best regards, Tomasz
