linux-kernel - [PATCH V2 2/2] drm/bridge: samsung-dsim: Fix porch calcalcuation rounding

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Date: Sun, 11 Feb 2024 17:09:30 -0600
From: Adam Ford <aford173@...il.com>
To: dri-devel@...ts.freedesktop.org
Cc: marex@...x.de,
	aford@...conembedded.com,
	Adam Ford <aford173@...il.com>,
	Frieder Schrempf <frieder.schrempf@...tron.de>,
	Inki Dae <inki.dae@...sung.com>,
	Jagan Teki <jagan@...rulasolutions.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Andrzej Hajda <andrzej.hajda@...el.com>,
	Neil Armstrong <neil.armstrong@...aro.org>,
	Robert Foss <rfoss@...nel.org>,
	Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
	Jonas Karlman <jonas@...boo.se>,
	Jernej Skrabec <jernej.skrabec@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>,
	Daniel Vetter <daniel@...ll.ch>,
	Marco Felsch <m.felsch@...gutronix.de>,
	Michael Tretter <m.tretter@...gutronix.de>,
	linux-kernel@...r.kernel.org
Subject: [PATCH V2 2/2] drm/bridge: samsung-dsim: Fix porch calcalcuation rounding

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.
If the HFP is rounded down, it ends up being too small which can cause
some monitors to not sync properly. In these instances, adjust htotal
and hsync to round the HFP up, and recalculate the htotal.

Tested-by: Frieder Schrempf <frieder.schrempf@...tron.de> # Kontron BL i.MX8MM with HDMI monitor
Signed-off-by: Adam Ford <aford173@...il.com>
---
V2:  No changes

diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
index 8476650c477c..52939211fe93 100644
--- a/drivers/gpu/drm/bridge/samsung-dsim.c
+++ b/drivers/gpu/drm/bridge/samsung-dsim.c
@@ -1606,6 +1606,27 @@ 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.
+	 * If the HFP is rounded down, it ends up being too small which can cause
+	 * some monitors to not sync properly. In these instances, adjust htotal
+	 * and hsync to round the HFP up, and recalculate the htotal. Through trial
+	 * and error, it appears that the HBP and HSA do not appearto 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;
 }
 
-- 
2.43.0