[PATCH] drm: lcdif: Set and enable FIFO Panic threshold

[Marek Vasut]

In case the LCDIFv3 is used to drive a 4k panel via i.MX8MP HDMI bridge,
the LCDIFv3 becomes susceptible to FIFO underflows, which 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: 9db35bb349a0 ("drm: lcdif: Add support for i.MX8MP LCDIF variant")
Signed-off-by: Marek Vasut <marex at denx.de>
---
Cc: Kieran Bingham <kieran.bingham at ideasonboard.com>
Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
Cc: Liu Ying <victor.liu at nxp.com>
Cc: Lucas Stach <l.stach at pengutronix.de>
Cc: Marco Felsch <m.felsch at pengutronix.de>
Cc: Martyn Welch <martyn.welch at collabora.com>
Cc: Peng Fan <peng.fan at nxp.com>
Cc: Sam Ravnborg <sam at ravnborg.org>
---
 drivers/gpu/drm/mxsfb/lcdif_kms.c  | 15 +++++++++++++++
 drivers/gpu/drm/mxsfb/lcdif_regs.h |  1 +
 2 files changed, 16 insertions(+)

diff --git a/drivers/gpu/drm/mxsfb/lcdif_kms.c b/drivers/gpu/drm/mxsfb/lcdif_kms.c
index a5302006c02cd..aee7babb5fa5c 100644
--- a/drivers/gpu/drm/mxsfb/lcdif_kms.c
+++ b/drivers/gpu/drm/mxsfb/lcdif_kms.c
@@ -341,6 +341,18 @@ static void lcdif_enable_controller(struct lcdif_drm_private *lcdif)
 	reg = readl(lcdif->base + LCDC_V8_CTRLDESCL0_5);
 	reg |= CTRLDESCL0_5_EN;
 	writel(reg, lcdif->base + LCDC_V8_CTRLDESCL0_5);
+
+	/* Set FIFO Panic watermarks, low 1/3, high 2/3 . */
+	writel(FIELD_PREP(PANIC0_THRES_LOW_MASK, 1 * PANIC0_THRES_RANGE / 3) |
+	       FIELD_PREP(PANIC0_THRES_HIGH_MASK, 2 * PANIC0_THRES_RANGE / 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);
 }
 
 static void lcdif_disable_controller(struct lcdif_drm_private *lcdif)
@@ -348,6 +360,9 @@ static void lcdif_disable_controller(struct lcdif_drm_private *lcdif)
 	u32 reg;
 	int ret;
 
+	/* Disable FIFO Panic NoC priority booster. */
+	writel(0, lcdif->base + LCDC_V8_INT_ENABLE_D1);
+
 	reg = readl(lcdif->base + LCDC_V8_CTRLDESCL0_5);
 	reg &= ~CTRLDESCL0_5_EN;
 	writel(reg, lcdif->base + LCDC_V8_CTRLDESCL0_5);
diff --git a/drivers/gpu/drm/mxsfb/lcdif_regs.h b/drivers/gpu/drm/mxsfb/lcdif_regs.h
index fb74eb5ccbf1d..3d2f81d6f995e 100644
--- a/drivers/gpu/drm/mxsfb/lcdif_regs.h
+++ b/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_RANGE		512
 
 #define LCDIF_MIN_XRES			120
 #define LCDIF_MIN_YRES			120
-- 
2.35.1