media: adv7842: enable reduced fps detection

The pixelclock detection of the adv7842 is precise enough to detect
if the framerate is 60 Hz or 59.94 Hz (aka "reduced fps").

Implement this detection.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/i2c/adv7842.c

@@ -1525,6 +1525,7 @@ static void adv7842_fill_optional_dv_timings_fields(struct v4l2_subdev *sd,
 	v4l2_find_dv_timings_cap(timings, adv7842_get_dv_timings_cap(sd),
 			is_digital_input(sd) ? 250000 : 1000000,
 			adv7842_check_dv_timings, NULL);
+	timings->bt.flags |= V4L2_DV_FL_CAN_DETECT_REDUCED_FPS;
 }
 
 static int adv7842_query_dv_timings(struct v4l2_subdev *sd,
@@ -1596,6 +1597,14 @@ static int adv7842_query_dv_timings(struct v4l2_subdev *sd,
 			bt->il_vbackporch = 0;
 		}
 		adv7842_fill_optional_dv_timings_fields(sd, timings);
+		if ((timings->bt.flags & V4L2_DV_FL_CAN_REDUCE_FPS) &&
+		    freq < bt->pixelclock) {
+			u32 reduced_freq = ((u32)bt->pixelclock / 1001) * 1000;
+			u32 delta_freq = abs(freq - reduced_freq);
+
+			if (delta_freq < ((u32)bt->pixelclock - reduced_freq) / 2)
+				timings->bt.flags |= V4L2_DV_FL_REDUCED_FPS;
+		}
 	} else {
 		/* find format
 		 * Since LCVS values are inaccurate [REF_03, p. 339-340],