[PATCH] drm: lcdif: change burst size to 256B

[Marco Felsch]

FIFO underruns are seen if a AXI bus master with a higher priority do a
lot of memory access. Increase the burst size to 256B to avoid such
underruns and to improve the memory access efficiency.

Signed-off-by: Marco Felsch <m.felsch at pengutronix.de>
---
 drivers/gpu/drm/mxsfb/lcdif_kms.c  | 16 ++++++++++++++--
 drivers/gpu/drm/mxsfb/lcdif_regs.h |  4 ++++
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/mxsfb/lcdif_kms.c b/drivers/gpu/drm/mxsfb/lcdif_kms.c
index 1bec1279c8b5..1f22ea5896d5 100644
--- a/drivers/gpu/drm/mxsfb/lcdif_kms.c
+++ b/drivers/gpu/drm/mxsfb/lcdif_kms.c
@@ -143,8 +143,20 @@ static void lcdif_set_mode(struct lcdif_drm_private *lcdif, u32 bus_flags)
 	       CTRLDESCL0_1_WIDTH(m->crtc_hdisplay),
 	       lcdif->base + LCDC_V8_CTRLDESCL0_1);
 
-	writel(CTRLDESCL0_3_PITCH(lcdif->crtc.primary->state->fb->pitches[0]),
-	       lcdif->base + LCDC_V8_CTRLDESCL0_3);
+	/*
+	 * Undocumented P_SIZE and T_SIZE bit fields but according the
+	 * downstream kernel they control the AXI burst size. As of now there
+	 * are two known values:
+	 *  1 - 128Byte
+	 *  2 - 256Byte
+	 *
+	 * Downstream has set the burst size to 256Byte to improve the memory
+	 * efficiency so set it here too. This also reduces the FIFO underrun
+	 * possibility.
+	 */
+	ctrl = CTRLDESCL0_3_P_SIZE(2) | CTRLDESCL0_3_T_SIZE(2) |
+	       CTRLDESCL0_3_PITCH(lcdif->crtc.primary->state->fb->pitches[0]);
+	writel(ctrl, lcdif->base + LCDC_V8_CTRLDESCL0_3);
 }
 
 static void lcdif_enable_controller(struct lcdif_drm_private *lcdif)
diff --git a/drivers/gpu/drm/mxsfb/lcdif_regs.h b/drivers/gpu/drm/mxsfb/lcdif_regs.h
index c70220651e3a..8e8bef175bf2 100644
--- a/drivers/gpu/drm/mxsfb/lcdif_regs.h
+++ b/drivers/gpu/drm/mxsfb/lcdif_regs.h
@@ -190,6 +190,10 @@
 #define CTRLDESCL0_1_WIDTH(n)		((n) & 0xffff)
 #define CTRLDESCL0_1_WIDTH_MASK		GENMASK(15, 0)
 
+#define CTRLDESCL0_3_P_SIZE(n)		(((n) << 20) & CTRLDESCL0_3_P_SIZE_MASK)
+#define CTRLDESCL0_3_P_SIZE_MASK	GENMASK(22, 20)
+#define CTRLDESCL0_3_T_SIZE(n)		(((n) << 16) & CTRLDESCL0_3_T_SIZE_MASK)
+#define CTRLDESCL0_3_T_SIZE_MASK	GENMASK(17, 16)
 #define CTRLDESCL0_3_PITCH(n)		((n) & 0xffff)
 #define CTRLDESCL0_3_PITCH_MASK		GENMASK(15, 0)
 
-- 
2.30.2