drm: lcdif: change burst size to 256B

If a axi bus master with a higher priority do a lot of memory access
FIFO underruns can be inspected. Increase the burst size to 256B to
avoid such underruns and to improve the memory access efficiency.

Fixes: 9db35bb3 ("drm: lcdif: Add support for i.MX8MP LCDIF variant")
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221101164615.778299-1-m.felsch@pengutronix.de

drivers/gpu/drm/mxsfb/lcdif_kms.c

@@ -314,8 +314,18 @@ static void lcdif_set_mode(struct lcdif_drm_private *lcdif, u32 bus_flags)
 	       CTRLDESCL0_1_WIDTH(m->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 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.
+	 */
+	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)

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)