[PATCH v2 06/11] video: lcdif: use 128B AXI bursts to avoid right-edge gap

[Johannes Schneider]

From: Thomas Haemmerle <thomas.haemmerle at leica-geosystems.com>

P_SIZE/T_SIZE are undocumented AXI-burst-size selectors: 1 = 128 byte,
2 = 256 byte.  The driver inherited 256B from NXP downstream, which
works for 1080p (stride 7680 B = 30 * 256 B) but breaks on narrower
32 bpp panels whose row byte count isn't a multiple of 256.

On an 800-pixel row at XRGB8888, stride = 3200 B = 12.5 * 256 B.  The
trailing partial burst is dropped and the panel shows a ~32-pixel-wide
black strip at the right edge.  128B is a divisor of every common
32 bpp stride (3200, 3840, 5120, ...), so the gap disappears.

Signed-off-by: Thomas Haemmerle <thomas.haemmerle at leica-geosystems.com>
---
 drivers/video/lcdif_kms.c | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/drivers/video/lcdif_kms.c b/drivers/video/lcdif_kms.c
index 3a31633818..a113f3e681 100644
--- a/drivers/video/lcdif_kms.c
+++ b/drivers/video/lcdif_kms.c
@@ -216,18 +216,14 @@ static void lcdif_set_mode(struct lcdif_drm_private *lcdif,
 	       lcdif->base + LCDC_V8_CTRLDESCL0_1);
 
 	/*
-	 * Undocumented P_SIZE and T_SIZE register but those written in the
-	 * downstream kernel those registers control the AXI burst size. As of
-	 * now there are two known values:
-	 *  1 - 128Byte
-	 *  2 - 256Byte
-	 * Downstream set it to 256B burst size to improve the memory
-	 * efficiency so set it here too.
+	 * P_SIZE/T_SIZE are undocumented AXI-burst-size selectors:
+	 * 1 = 128 byte, 2 = 256 byte.  Use 128B so any 32 bpp row divides
+	 * into whole bursts; 256B on an 800-pixel row produces a partial
+	 * trailing burst and a ~32-pixel black strip at the right edge.
+	 *
+	 * Stride is fixed to hdisplay * 4 (DRM_FORMAT_XRGB8888).
 	 */
-	/* NOTE: Since this driver is currently fixed to DRM_FORMAT_XRGB8888
-	 * we asume a stride of vdisplay * 4
-	 */
-	ctrl = CTRLDESCL0_3_P_SIZE(2) | CTRLDESCL0_3_T_SIZE(2) |
+	ctrl = CTRLDESCL0_3_P_SIZE(1) | CTRLDESCL0_3_T_SIZE(1) |
 	       CTRLDESCL0_3_PITCH(m->hdisplay * 4);
 	writel(ctrl, lcdif->base + LCDC_V8_CTRLDESCL0_3);
 }
-- 
2.43.0