[PATCH, RFC] displaymodes in devicetree

[Sascha Hauer <s.hauer@pengutronix.de>]

Hi All,

I'd like to have a possibility to describe fixed display modes in the
devicetree. This topic has been discussed before here:

https://lists.ozlabs.org/pipermail/linuxppc-dev/2010-February/080683.html

The result at that time was that EDID data should be considered to use
as this is a format that already exists. I want to come back to this
topic since:

- EDID data is a binary format and as such quite inconvenient to handle.
  There exist several tools to parse EDID data, but I'm not aware of any
  (open source) tool which can generate EDID data.
- EDID blobs are hard to modify and hard to review in patches.
- EDID is designed to describe multiple modes, but fixed displays
  usually only support a single fixed mode.

There are several ways of describing the mode, for this patch I chose to
use the format (and naming) used by the Linux Framebuffer Layer as this
is the only description which does not allow for inconsistent modes. I
added the most common flags like [v|h]sync_active_high. Others can be
added, but these flags are pretty much agreed upon and won't be
superseeded with other flags. The mode from the devicetree can be
converted to the most common modes used in Linux, struct fb_videomode
and struct drm_display_mode.

Comments welcome

Sascha


8<--------------------------------------------

of: Add videomode helper

This patch adds a helper function for parsing videomodes from the devicetree.
The videomode is returned either as a struct drm_display_mode or a
struct fb_videomode.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
---
 .../devicetree/bindings/video/displaymode          |   40 ++++++++
 drivers/of/Kconfig                                 |    5 +
 drivers/of/Makefile                                |    1 +
 drivers/of/of_videomode.c                          |  108 ++++++++++++++++++++
 include/linux/of_videomode.h                       |   19 ++++
 5 files changed, 173 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/video/displaymode
 create mode 100644 drivers/of/of_videomode.c
 create mode 100644 include/linux/of_videomode.h

diff --git a/Documentation/devicetree/bindings/video/displaymode b/Documentation/devicetree/bindings/video/displaymode
new file mode 100644
index 0000000..303cfc8
--- /dev/null
+++ b/Documentation/devicetree/bindings/video/displaymode
@@ -0,0 +1,40 @@
+videomode bindings
+=========
+
+Required properties:
+ - xres, yres: Display resolution
+ - left_margin, right_margin, hsync_len: Horizontal Display timing parameters
+   in pixels
+   upper_margin, lower_margin, vsync_len: Vertical display timing parameters in
+   lines
+ - clock: displayclock in Hz
+
+Optional properties:
+ - width_mm, height_mm: Display dimensions in mm
+ - hsync_active_high (bool): Hsync pulse is active high
+ - vsync_active_high (bool): Vsync pulse is active high
+ - interlaced (bool): This is an interlaced mode
+ - doublescan (bool): This is a doublescan mode
+
+There are different ways of describing a display mode. The devicetree representation
+corresponds to the one used by the Linux Framebuffer framework described here in
+Documentation/fb/framebuffer.txt. This representation has been chosen because it's
+the only format which does not allow for inconsistent parameters.Unlike the Framebuffer
+framework the devicetree has the clock in Hz instead of ps.
+
+Example:
+
+	display@0 {
+		/* 1920x1080p24 */
+		clock = <52000000>;
+		xres = <1920>;
+		yres = <1080>;
+		left_margin = <25>;
+		right_margin = <25>;
+		hsync_len = <25>;
+		lower_margin = <2>;
+		upper_margin = <2>;
+		vsync_len = <2>;
+		hsync_active_high;
+	};
+
diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
index dfba3e6..a3acaa3 100644
--- a/drivers/of/Kconfig
+++ b/drivers/of/Kconfig
@@ -83,4 +83,9 @@ config OF_MTD
 	depends on MTD
 	def_bool y
 
+config OF_VIDEOMODE
+	def_bool y
+	help
+	  helper to parse videomodes from the devicetree
+
 endmenu # OF
diff --git a/drivers/of/Makefile b/drivers/of/Makefile
index e027f44..80e6db3 100644
--- a/drivers/of/Makefile
+++ b/drivers/of/Makefile
@@ -11,3 +11,4 @@ obj-$(CONFIG_OF_MDIO)	+= of_mdio.o
 obj-$(CONFIG_OF_PCI)	+= of_pci.o
 obj-$(CONFIG_OF_PCI_IRQ)  += of_pci_irq.o
 obj-$(CONFIG_OF_MTD)	+= of_mtd.o
+obj-$(CONFIG_OF_VIDEOMODE)	+= of_videomode.o
diff --git a/drivers/of/of_videomode.c b/drivers/of/of_videomode.c
new file mode 100644
index 0000000..859aefb
--- /dev/null
+++ b/drivers/of/of_videomode.c
@@ -0,0 +1,108 @@
+/*
+ * OF helpers for parsing display modes
+ *
+ * Copyright (c) 2012 Sascha Hauer <s.hauer@pengutronix.de>, Pengutronix
+ *
+ * This file is released under the GPLv2
+ */
+#include <linux/of.h>
+#include <linux/fb.h>
+#include <linux/export.h>
+#include <drm/drmP.h>
+#include <drm/drm_crtc.h>
+
+int of_get_video_mode(struct device_node *np, struct drm_display_mode *dmode,
+		struct fb_videomode *fbmode)
+{
+	int ret = 0;
+	u32 left_margin, xres, right_margin, hsync_len;
+	u32 upper_margin, yres, lower_margin, vsync_len;
+	u32 width_mm = 0, height_mm = 0;
+	u32 clock;
+	bool hah = false, vah = false, interlaced = false, doublescan = false;
+
+	if (!np)
+		return -EINVAL;
+
+	ret |= of_property_read_u32(np, "left_margin", &left_margin);
+	ret |= of_property_read_u32(np, "xres", &xres);
+	ret |= of_property_read_u32(np, "right_margin", &right_margin);
+	ret |= of_property_read_u32(np, "hsync_len", &hsync_len);
+	ret |= of_property_read_u32(np, "upper_margin", &upper_margin);
+	ret |= of_property_read_u32(np, "yres", &yres);
+	ret |= of_property_read_u32(np, "lower_margin", &lower_margin);
+	ret |= of_property_read_u32(np, "vsync_len", &vsync_len);
+	ret |= of_property_read_u32(np, "clock", &clock);
+	if (ret)
+		return -EINVAL;
+
+	of_property_read_u32(np, "width_mm", &width_mm);
+	of_property_read_u32(np, "height_mm", &height_mm);
+
+	hah = of_property_read_bool(np, "hsync_active_high");
+	vah = of_property_read_bool(np, "vsync_active_high");
+	interlaced = of_property_read_bool(np, "interlaced");
+	doublescan = of_property_read_bool(np, "doublescan");
+
+	if (dmode) {
+		memset(dmode, 0, sizeof(*dmode));
+
+		dmode->hdisplay = xres;
+		dmode->hsync_start = xres + right_margin;
+		dmode->hsync_end = xres + right_margin + hsync_len;
+		dmode->htotal = xres + right_margin + hsync_len + left_margin;
+
+		dmode->vdisplay = yres;
+		dmode->vsync_start = yres + lower_margin;
+		dmode->vsync_end = yres + lower_margin + vsync_len;
+		dmode->vtotal = yres + lower_margin + vsync_len + upper_margin;
+
+		dmode->width_mm = width_mm;
+		dmode->height_mm = height_mm;
+
+		dmode->clock = clock / 1000;
+
+		if (hah)
+			dmode->flags |= DRM_MODE_FLAG_PHSYNC;
+		else
+			dmode->flags |= DRM_MODE_FLAG_NHSYNC;
+		if (vah)
+			dmode->flags |= DRM_MODE_FLAG_PVSYNC;
+		else
+			dmode->flags |= DRM_MODE_FLAG_NVSYNC;
+		if (interlaced)
+			dmode->flags |= DRM_MODE_FLAG_INTERLACE;
+		if (doublescan)
+			dmode->flags |= DRM_MODE_FLAG_DBLSCAN;
+
+		drm_mode_set_name(dmode);
+	}
+
+	if (fbmode) {
+		memset(fbmode, 0, sizeof(*fbmode));
+
+		fbmode->xres = xres;
+		fbmode->left_margin = left_margin;
+		fbmode->right_margin = right_margin;
+		fbmode->hsync_len = hsync_len;
+
+		fbmode->yres = yres;
+		fbmode->upper_margin = upper_margin;
+		fbmode->lower_margin = lower_margin;
+		fbmode->vsync_len = vsync_len;
+
+		fbmode->pixclock = KHZ2PICOS(clock / 1000);
+
+		if (hah)
+			fbmode->sync |= FB_SYNC_HOR_HIGH_ACT;
+		if (vah)
+			fbmode->sync |= FB_SYNC_VERT_HIGH_ACT;
+		if (interlaced)
+			fbmode->vmode |= FB_VMODE_INTERLACED;
+		if (doublescan)
+			fbmode->vmode |= FB_VMODE_DOUBLE;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(of_get_video_mode);
diff --git a/include/linux/of_videomode.h b/include/linux/of_videomode.h
new file mode 100644
index 0000000..a988429
--- /dev/null
+++ b/include/linux/of_videomode.h
@@ -0,0 +1,19 @@
+/*
+ * Copyright 2012 Sascha Hauer <s.hauer@pengutronix.de>
+ *
+ * OF helpers for videomodes.
+ *
+ * This file is released under the GPLv2
+ */
+
+#ifndef __LINUX_OF_VIDEOMODE_H
+#define __LINUX_OF_VIDEOMODE_H
+
+struct device_node;
+struct fb_videomode;
+struct drm_display_mode;
+
+int of_get_video_mode(struct device_node *np, struct drm_display_mode *dmode,
+		struct fb_videomode *fbmode);
+
+#endif /* __LINUX_OF_VIDEOMODE_H */
-- 
1.7.10

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