[PATCH 3/4] media: uvcvideo: Relax the constrains for interpolating the hw clock

[Ricardo Ribalda]

In the initial version we set the min value to 250msec. Looks like
100msec can also provide a good value.

Now that we are at it, refactor a bit the code to make it cleaner.

Fixes: 6243c83be6ee8 ("media: uvcvideo: Allow hw clock updates with buffers not full")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
---
 drivers/media/usb/uvc/uvc_video.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
index c7ebedb3450f..dcbc0941ffe6 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -494,6 +494,13 @@ static int uvc_commit_video(struct uvc_streaming *stream,
  * Clocks and timestamps
  */
 
+/*
+ * The accuracy of the hardware timestamping depends on having enough data to
+ * interpolate between the different clock domains. This value is sof cycles,
+ * this is, milliseconds.
+ */
+#define MIN_HW_TIMESTAMP_DIFF 100
+
 static inline ktime_t uvc_video_get_time(void)
 {
 	if (uvc_clock_param == CLOCK_MONOTONIC)
@@ -834,15 +841,12 @@ void uvc_video_clock_update(struct uvc_streaming *stream,
 		y2 += 2048 << 16;
 
 	/*
-	 * Have at least 1/4 of a second of timestamps before we
-	 * try to do any calculation. Otherwise we do not have enough
-	 * precision. This value was determined by running Android CTS
-	 * on different devices.
+	 * Check that we have enough data to do the interpolation.
 	 *
-	 * dev_sof runs at 1KHz, and we have a fixed point precision of
-	 * 16 bits.
+	 * y1 and y2 are dev_sof with a fixed point precision of 16 bits.
 	 */
-	if (clock->size != clock->count && (y2 - y1) < ((1000 / 4) << 16))
+	if (clock->size != clock->count &&
+	    (y2 - y1) < (MIN_HW_TIMESTAMP_DIFF << 16))
 		goto done;
 
 	y = (u64)(y2 - y1) * (1ULL << 31) + (u64)y1 * (u64)x2

-- 
2.53.0.959.g497ff81fa9-goog