drm: mxsfb: Increase number of outstanding requests on V4 and newer HW

In case the DRAM is under high load, the MXSFB FIFO might underflow
and that causes visible artifacts. This could be triggered on i.MX8MM
using e.g. "$ memtester 128M" on a device with 1920x1080 panel. The
first "Stuck Address" test of the memtester will completely corrupt
the image on the panel and leave the MXSFB FIFO in odd state.

To avoid this underflow, increase number of outstanding requests to
DRAM from 2 to 16, which is the maximum. This mitigates the issue
and it can no longer be triggered.

Fixes: 45d59d7 ("drm: Add new driver for MXSFB controller")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Abrecht <public@danielabrecht.ch>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Stefan Agner <stefan@agner.ch>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210620224759.189351-1-marex@denx.de

drivers/gpu/drm/mxsfb/mxsfb_drv.c

@@ -51,6 +51,7 @@ static const struct mxsfb_devdata mxsfb_devdata[] = {
 		.hs_wdth_mask	= 0xff,
 		.hs_wdth_shift	= 24,
 		.has_overlay	= false,
+		.has_ctrl2	= false,
 	},
 	[MXSFB_V4] = {
 		.transfer_count	= LCDC_V4_TRANSFER_COUNT,
@@ -59,6 +60,7 @@ static const struct mxsfb_devdata mxsfb_devdata[] = {
 		.hs_wdth_mask	= 0x3fff,
 		.hs_wdth_shift	= 18,
 		.has_overlay	= false,
+		.has_ctrl2	= true,
 	},
 	[MXSFB_V6] = {
 		.transfer_count	= LCDC_V4_TRANSFER_COUNT,
@@ -67,6 +69,7 @@ static const struct mxsfb_devdata mxsfb_devdata[] = {
 		.hs_wdth_mask	= 0x3fff,
 		.hs_wdth_shift	= 18,
 		.has_overlay	= true,
+		.has_ctrl2	= true,
 	},
 };
 

drivers/gpu/drm/mxsfb/mxsfb_drv.h

@@ -22,6 +22,7 @@ struct mxsfb_devdata {
 	unsigned int	hs_wdth_mask;
 	unsigned int	hs_wdth_shift;
 	bool		has_overlay;
+	bool		has_ctrl2;
 };
 
 struct mxsfb_drm_private {

drivers/gpu/drm/mxsfb/mxsfb_kms.c

@@ -107,6 +107,14 @@ static void mxsfb_enable_controller(struct mxsfb_drm_private *mxsfb)
 		clk_prepare_enable(mxsfb->clk_disp_axi);
 	clk_prepare_enable(mxsfb->clk);
 
+	/* Increase number of outstanding requests on all supported IPs */
+	if (mxsfb->devdata->has_ctrl2) {
+		reg = readl(mxsfb->base + LCDC_V4_CTRL2);
+		reg &= ~CTRL2_SET_OUTSTANDING_REQS_MASK;
+		reg |= CTRL2_SET_OUTSTANDING_REQS_16;
+		writel(reg, mxsfb->base + LCDC_V4_CTRL2);
+	}
+
 	/* If it was disabled, re-enable the mode again */
 	writel(CTRL_DOTCLK_MODE, mxsfb->base + LCDC_CTRL + REG_SET);
 

drivers/gpu/drm/mxsfb/mxsfb_regs.h

@@ -15,6 +15,7 @@
 #define LCDC_CTRL			0x00
 #define LCDC_CTRL1			0x10
 #define LCDC_V3_TRANSFER_COUNT		0x20
+#define LCDC_V4_CTRL2			0x20
 #define LCDC_V4_TRANSFER_COUNT		0x30
 #define LCDC_V4_CUR_BUF			0x40
 #define LCDC_V4_NEXT_BUF		0x50
@@ -61,6 +62,13 @@
 #define CTRL1_CUR_FRAME_DONE_IRQ_EN	BIT(13)
 #define CTRL1_CUR_FRAME_DONE_IRQ	BIT(9)
 
+#define CTRL2_SET_OUTSTANDING_REQS_1	0
+#define CTRL2_SET_OUTSTANDING_REQS_2	(0x1 << 21)
+#define CTRL2_SET_OUTSTANDING_REQS_4	(0x2 << 21)
+#define CTRL2_SET_OUTSTANDING_REQS_8	(0x3 << 21)
+#define CTRL2_SET_OUTSTANDING_REQS_16	(0x4 << 21)
+#define CTRL2_SET_OUTSTANDING_REQS_MASK	(0x7 << 21)
+
 #define TRANSFER_COUNT_SET_VCOUNT(x)	(((x) & 0xffff) << 16)
 #define TRANSFER_COUNT_GET_VCOUNT(x)	(((x) >> 16) & 0xffff)
 #define TRANSFER_COUNT_SET_HCOUNT(x)	((x) & 0xffff)