[PATCH] drm/bridge: tc358767: Fix odd pixel alignment

[Marek Vasut]

Horizontal Timing Control0 Register 1/2 (HTIM01/HTIM02) Register
bitfields description state "These bits must be multiple of even
pixel". It is not possible to simply align every bitfield to the
nearest even pixel, because that would unalign the line width and
cause visible distortion. Instead, attempt to re-align the timings
such that the hardware requirement is fulfilled without changing
the line width if at all possible.

Warn the user in case a panel with odd active pixel width or full
line width is used, this is not possible to support with this one
bridge.

Signed-off-by: Marek Vasut <marex at denx.de>
---
Cc: Andrzej Hajda <andrzej.hajda at intel.com>
Cc: David Airlie <airlied at gmail.com>
Cc: Jernej Skrabec <jernej.skrabec at gmail.com>
Cc: Jonas Karlman <jonas at kwiboo.se>
Cc: Laurent Pinchart <Laurent.pinchart at ideasonboard.com>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: Maxime Ripard <mripard at kernel.org>
Cc: Neil Armstrong <neil.armstrong at linaro.org>
Cc: Robert Foss <rfoss at kernel.org>
Cc: Simona Vetter <simona at ffwll.ch>
Cc: Thomas Zimmermann <tzimmermann at suse.de>
Cc: dri-devel at lists.freedesktop.org
---
 drivers/gpu/drm/bridge/tc358767.c | 63 +++++++++++++++++++++++++++++--
 1 file changed, 60 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/bridge/tc358767.c b/drivers/gpu/drm/bridge/tc358767.c
index 0a6894498267e..7968183510e63 100644
--- a/drivers/gpu/drm/bridge/tc358767.c
+++ b/drivers/gpu/drm/bridge/tc358767.c
@@ -901,6 +901,63 @@ static int tc_set_common_video_mode(struct tc_data *tc,
 	int vsync_len = mode->vsync_end - mode->vsync_start;
 	int ret;
 
+	/*
+	 * Horizontal Timing Control0 Register 1/2 (HTIM01/HTIM02) Register
+	 * bitfields description state "These bits must be multiple of even
+	 * pixel". It is not possible to simply align every bitfield to the
+	 * nearest even pixel, because that would unalign the line width.
+	 * Instead, attempt to re-align the timings.
+	 */
+
+	/* Panels with odd active pixel count are not supported by the bridge */
+	if (mode->hdisplay & 1)
+		dev_warn(tc->dev, "Panels with odd pixel count per active line are not supported.\n");
+
+	/* HPW is odd */
+	if (hsync_len & 1) {
+		/* Make sure there is some margin left */
+		if (left_margin >= 2) {
+			/* Align HPW up */
+			hsync_len++;
+			left_margin--;
+		} else if (right_margin >= 2) {
+			/* Align HPW up */
+			hsync_len++;
+			right_margin--;
+		} else if (hsync_len > 2) {
+			/* Align HPW down as last-resort option */
+			hsync_len--;
+			left_margin++;
+		} else {
+			dev_warn(tc->dev, "HPW is odd, not enough margins to compensate.\n");
+		}
+	}
+
+	/* HBP is odd (HPW is surely even now) */
+	if (left_margin & 1) {
+		/* Make sure there is some margin left */
+		if (right_margin >= 2) {
+			/* Align HBP up */
+			left_margin++;
+			right_margin--;
+		} else if (hsync_len > 2) {
+			/* HPW is surely even and > 2, which means at least 4 */
+			hsync_len -= 2;
+			/*
+			 * Subtract 2 from sync pulse and distribute it between
+			 * margins. This aligns HBP and keeps HPW aligned.
+			 */
+			left_margin++;
+			right_margin++;
+		} else {
+			dev_warn(tc->dev, "HBP is odd, not enough pixels to compensate.\n");
+		}
+	}
+
+	/* HFP is odd (HBP and HPW is surely even now) */
+	if (right_margin & 1)
+		dev_warn(tc->dev, "HFP is odd, panels with odd pixel count per full line are not supported.\n");
+
 	dev_dbg(tc->dev, "set mode %dx%d\n",
 		mode->hdisplay, mode->vdisplay);
 	dev_dbg(tc->dev, "H margin %d,%d sync %d\n",
@@ -922,14 +979,14 @@ static int tc_set_common_video_mode(struct tc_data *tc,
 		return ret;
 
 	ret = regmap_write(tc->regmap, HTIM01,
-			   FIELD_PREP(HBPR, ALIGN(left_margin, 2)) |
-			   FIELD_PREP(HPW, ALIGN(hsync_len, 2)));
+			   FIELD_PREP(HBPR, left_margin) |
+			   FIELD_PREP(HPW, hsync_len));
 	if (ret)
 		return ret;
 
 	ret = regmap_write(tc->regmap, HTIM02,
 			   FIELD_PREP(HDISPR, ALIGN(mode->hdisplay, 2)) |
-			   FIELD_PREP(HFPR, ALIGN(right_margin, 2)));
+			   FIELD_PREP(HFPR, right_margin));
 	if (ret)
 		return ret;
 
-- 
2.45.2