linux-kernel - [PATCH v10 1/6] media: uvcvideo: Support timestamp lists of any size

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Message-Id: <20240323-resend-hwtimestamp-v10-1-b08e590d97c7@chromium.org>
Date: Sat, 23 Mar 2024 10:48:02 +0000
From: Ricardo Ribalda <ribalda@...omium.org>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>, 
 Mauro Carvalho Chehab <mchehab@...nel.org>
Cc: linux-media@...r.kernel.org, linux-kernel@...r.kernel.org, 
 "hn.chen" <hn.chen@...plusit.com>, Hans Verkuil <hverkuil@...all.nl>, 
 Ricardo Ribalda <ribalda@...omium.org>, 
 Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: [PATCH v10 1/6] 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@...omium.org>
Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
---
 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