From:   Ricardo Ribalda <ribalda@chromium.org>
Date:   Wed, 15 Mar 2023 14:30:12 +0100
Subject: [PATCH v9 1/6] media: uvcvideo: Fix negative modulus calculation
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Message-Id: <20220920-resend-hwtimestamp-v9-1-55a89f46f6be@chromium.org>
References: <20220920-resend-hwtimestamp-v9-0-55a89f46f6be@chromium.org>
In-Reply-To: <20220920-resend-hwtimestamp-v9-0-55a89f46f6be@chromium.org>
To:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>
Cc:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Ricardo Ribalda <ribalda@chromium.org>,
        linux-kernel@vger.kernel.org, "hn.chen" <hn.chen@sunplusit.com>,
        linux-media@vger.kernel.org
Precedence: bulk

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.

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@chromium.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 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);

-- 
2.40.0.rc1.284.g88254d51c5-goog-b4-0.11.0-dev-696ae