[RFC] drm: bridge: samsung-dsim: Recalculate timings when rounding HFP up

[Adam Ford]

When using video sync pulses, the HFP, HBP, and HSA are divided between
the available lanes if there is more than one lane.  For certain
timings and lane configurations, the HFP may not be evenly divisible
and it gets rounded down which can cause certain resolutions and
refresh rates to not sync.

ie. 720p60 on some monitors show hss of 1390 and hdisplay of 1280.  This
yields an HFP of 110. When taking the HFP of 110 divides along 4 lanes,
the result is 27.5 which rounds down to 27 and causes some monitors not
to sync.

The solultion is to round HFP up by finding the remainder of HFP /
the number of lanes and increasing the hsync_start, hsync_end, and
htotal to compensate when there is a remainder.

Signed-off-by: Adam Ford <aford173 at gmail.com>
---
This adds support for 720p60 in the i.MX8M Plus.

NXP uses a look-up table in their downstream code to accomplish this.
Using this calculation, the driver can adjust without the need for a
complicated table and should be flexible for different timings and
resolutions depending on the monitor.

I don't have a DSI analyzer, and this appears to only work on
i.MX8M Plus but not Mini and Nano for some reason, despite their
having a similar DSI bridge.

diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
index be5914caa17d..5564e85f6b63 100644
--- a/drivers/gpu/drm/bridge/samsung-dsim.c
+++ b/drivers/gpu/drm/bridge/samsung-dsim.c
@@ -1628,6 +1628,26 @@ static int samsung_dsim_atomic_check(struct drm_bridge *bridge,
 		adjusted_mode->flags |= (DRM_MODE_FLAG_PHSYNC | DRM_MODE_FLAG_PVSYNC);
 	}
 
+	/*
+	 * When using video sync pulses, the HFP, HBP, and HSA are divided between
+	 * the available lanes if there is more than one lane.  For certain
+	 * timings and lane configurations, the HFP may not be evenly divisible.
+	 * This can cause HFP to round down, and it ends up being too small which can
+	 * cause some monitors to not sync properly. In these instances, round HFP up
+	 * and adjust htotal to compensate. Through trial and error, it appears that
+	 * the HBP and HSA do not appear to need the same correction that HFP does.
+	 */
+	if (dsi->mode_flags & MIPI_DSI_MODE_VIDEO_SYNC_PULSE && dsi->lanes > 1) {
+		int hfp = adjusted_mode->hsync_start - adjusted_mode->hdisplay;
+		int remainder = hfp % dsi->lanes;
+
+		if (remainder) {
+			adjusted_mode->hsync_start += remainder;
+			adjusted_mode->hsync_end   += remainder;
+			adjusted_mode->htotal      += remainder;
+		}
+	}
+
 	return 0;
 }
 
-- 
2.40.1