drm: lcdif: Set and enable FIFO Panic threshold

In case the LCDIFv3 is used to drive a 4k panel via i.MX8MP HDMI bridge,
the LCDIFv3 becomes susceptible to FIFO underflows, these lead to nasty
flicker of the image on the panel, or image being shifted by half frame
horizontally every second frame. The flicker can be easily triggered by
running 3D application on top of weston compositor, like neverball or
chromium. Surprisingly glmark2-es2-wayland or glmark2-es2-drm does not
trigger this effect so easily.

Configure the FIFO Panic threshold register and enable the FIFO Panic
mode, which internally boosts the NoC interconnect priority for LCDIFv3
transactions in case of possible underflow. This mitigates the flicker
effect on 4k panels as well.

Fixes: 9db35bb ("drm: lcdif: Add support for i.MX8MP LCDIF variant")
Signed-off-by: Marek Vasut <[email protected]>
Tested-by: Liu Ying <[email protected]> # i.MX8mp EVK
Reviewed-by: Liu Ying <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: Marek Vasut

drivers/gpu/drm/mxsfb/lcdif_kms.c

@@ -5,6 +5,7 @@
  * This code is based on drivers/gpu/drm/mxsfb/mxsfb*
  */
 
+#include <linux/bitfield.h>
 #include <linux/clk.h>
 #include <linux/io.h>
 #include <linux/iopoll.h>
@@ -332,6 +333,18 @@ static void lcdif_enable_controller(struct lcdif_drm_private *lcdif)
 {
 	u32 reg;
 
+	/* Set FIFO Panic watermarks, low 1/3, high 2/3 . */
+	writel(FIELD_PREP(PANIC0_THRES_LOW_MASK, 1 * PANIC0_THRES_MAX / 3) |
+	       FIELD_PREP(PANIC0_THRES_HIGH_MASK, 2 * PANIC0_THRES_MAX / 3),
+	       lcdif->base + LCDC_V8_PANIC0_THRES);
+
+	/*
+	 * Enable FIFO Panic, this does not generate interrupt, but
+	 * boosts NoC priority based on FIFO Panic watermarks.
+	 */
+	writel(INT_ENABLE_D1_PLANE_PANIC_EN,
+	       lcdif->base + LCDC_V8_INT_ENABLE_D1);
+
 	reg = readl(lcdif->base + LCDC_V8_DISP_PARA);
 	reg |= DISP_PARA_DISP_ON;
 	writel(reg, lcdif->base + LCDC_V8_DISP_PARA);
@@ -359,6 +372,9 @@ static void lcdif_disable_controller(struct lcdif_drm_private *lcdif)
 	reg = readl(lcdif->base + LCDC_V8_DISP_PARA);
 	reg &= ~DISP_PARA_DISP_ON;
 	writel(reg, lcdif->base + LCDC_V8_DISP_PARA);
+
+	/* Disable FIFO Panic NoC priority booster. */
+	writel(0, lcdif->base + LCDC_V8_INT_ENABLE_D1);
 }
 
 static void lcdif_reset_block(struct lcdif_drm_private *lcdif)

drivers/gpu/drm/mxsfb/lcdif_regs.h

@@ -255,6 +255,7 @@
 
 #define PANIC0_THRES_LOW_MASK		GENMASK(24, 16)
 #define PANIC0_THRES_HIGH_MASK		GENMASK(8, 0)
+#define PANIC0_THRES_MAX		511
 
 #define LCDIF_MIN_XRES			120
 #define LCDIF_MIN_YRES			120