media: uvcvideo: Support timestamp lists of any size

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@chromium.org>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20240323-resend-hwtimestamp-v10-1-b08e590d97c7@chromium.org
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
index 7cbf4692bd87..659c9e9880a9 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);