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
