[PATCH 1/4 v2] drm/pl111: Support grayscale

[Linus Walleij]

Migrating the TI nspire calculators to use the PL111 driver for
framebuffer requires grayscale support for the elder panel
which uses 8bit grayscale only.

DRM does not support 8bit grayscale framebuffers in memory,
but by defining the bus format to be MEDIA_BUS_FMT_Y8_1X8 we
can get the hardware to turn on a grayscaling feature and
convert the RGB framebuffer to grayscale for us.

Cc: Daniel Tang <dt.tangr at gmail.com>
Cc: Fabian Vogt <fabian at ritter-vogt.de>
Tested-by: Fabian Vogt <fabian at ritter-vogt.de>
Acked-by: Sam Ravnborg <sam at ravnborg.org>
Signed-off-by: Linus Walleij <linus.walleij at linaro.org>
---
ChangeLog v1->v2:
- Now tested
---
 drivers/gpu/drm/pl111/pl111_display.c | 29 +++++++++++++++++++++++++--
 include/linux/amba/clcd-regs.h        |  1 +
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/pl111/pl111_display.c b/drivers/gpu/drm/pl111/pl111_display.c
index 450166d167f6..1751fca6f6f5 100644
--- a/drivers/gpu/drm/pl111/pl111_display.c
+++ b/drivers/gpu/drm/pl111/pl111_display.c
@@ -126,6 +126,7 @@ static void pl111_display_enable(struct drm_simple_display_pipe *pipe,
 	struct drm_framebuffer *fb = plane->state->fb;
 	struct drm_connector *connector = priv->connector;
 	struct drm_bridge *bridge = priv->bridge;
+	bool grayscale = false;
 	u32 cntl;
 	u32 ppl, hsw, hfp, hbp;
 	u32 lpp, vsw, vfp, vbp;
@@ -185,6 +186,20 @@ static void pl111_display_enable(struct drm_simple_display_pipe *pipe,
 		if (connector->display_info.bus_flags &
 		    DRM_BUS_FLAG_PIXDATA_DRIVE_NEGEDGE)
 			tim2 |= TIM2_IPC;
+
+		if (connector->display_info.num_bus_formats == 1 &&
+		    connector->display_info.bus_formats[0] ==
+		    MEDIA_BUS_FMT_Y8_1X8)
+			grayscale = true;
+
+		/*
+		 * The AC pin bias frequency is set to max count when using
+		 * grayscale so at least once in a while we will reverse
+		 * polarity and get rid of any DC built up that could
+		 * damage the display.
+		 */
+		if (grayscale)
+			tim2 |= TIM2_ACB_MASK;
 	}
 
 	if (bridge) {
@@ -216,8 +231,18 @@ static void pl111_display_enable(struct drm_simple_display_pipe *pipe,
 
 	writel(0, priv->regs + CLCD_TIM3);
 
-	/* Hard-code TFT panel */
-	cntl = CNTL_LCDEN | CNTL_LCDTFT | CNTL_LCDVCOMP(1);
+	/*
+	 * Detect grayscale bus format. We do not support a grayscale mode
+	 * toward userspace, instead we expose an RGB24 buffer and then the
+	 * hardware will activate its grayscaler to convert to the grayscale
+	 * format.
+	 */
+	if (grayscale)
+		cntl = CNTL_LCDEN | CNTL_LCDMONO8;
+	else
+		/* Else we assume TFT display */
+		cntl = CNTL_LCDEN | CNTL_LCDTFT | CNTL_LCDVCOMP(1);
+
 	/* On the ST Micro variant, assume all 24 bits are connected */
 	if (priv->variant->st_bitmux_control)
 		cntl |= CNTL_ST_CDWID_24;
diff --git a/include/linux/amba/clcd-regs.h b/include/linux/amba/clcd-regs.h
index 516a6fda83c5..421b0fa90d6a 100644
--- a/include/linux/amba/clcd-regs.h
+++ b/include/linux/amba/clcd-regs.h
@@ -42,6 +42,7 @@
 #define TIM2_PCD_LO_MASK	GENMASK(4, 0)
 #define TIM2_PCD_LO_BITS	5
 #define TIM2_CLKSEL		(1 << 5)
+#define TIM2_ACB_MASK		GENMASK(10, 6)
 #define TIM2_IVS		(1 << 11)
 #define TIM2_IHS		(1 << 12)
 #define TIM2_IPC		(1 << 13)
-- 
2.21.0