[PATCH v2] video: simplefb: add mode parsing function

[Alexandre Courbot <acourbot-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>]

The naming scheme of simplefb's mode is precise enough to allow building
the mode structure from it instead of using a static list of modes. This
patch introduces a function that does this. In case exotic modes that
cannot be represented from their name alone are needed, the static list
of modes is still available as a backup.

It also changes the order in which colors are declared from MSB first to
the more standard LSB first.

Signed-off-by: Alexandre Courbot <acourbot-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
---
Changes from v1:
- amended documentation following Stephen's suggestion
- allow parsing of bitfields larger than 9 bits
- made it clear that the parsing order of bits is changed with respect
  to the original patch

Andrew, since this patch introduces a (small) change in the DT bindings,
could we try to merge it during the -rc cycle so we don't have to come
with a more complex solution in the future?

 .../bindings/video/simple-framebuffer.txt          | 12 +++-
 drivers/video/simplefb.c                           | 72 +++++++++++++++++++++-
 2 files changed, 80 insertions(+), 4 deletions(-)

diff --git a/Documentation/devicetree/bindings/video/simple-framebuffer.txt b/Documentation/devicetree/bindings/video/simple-framebuffer.txt
index 3ea4605..18d03e2 100644
--- a/Documentation/devicetree/bindings/video/simple-framebuffer.txt
+++ b/Documentation/devicetree/bindings/video/simple-framebuffer.txt
@@ -10,8 +10,16 @@ Required properties:
 - width: The width of the framebuffer in pixels.
 - height: The height of the framebuffer in pixels.
 - stride: The number of bytes in each line of the framebuffer.
-- format: The format of the framebuffer surface. Valid values are:
-  - r5g6b5 (16-bit pixels, d[15:11]=r, d[10:5]=g, d[4:0]=b).
+- format: The format of the framebuffer surface. Described as a sequence of
+	channel/num-bits pairs, where each pair describes how many bits are used
+	by a given color channel. Value channels are "r" (red), "g" (green),
+	"b" (blue), "a" (alpha) and "x" (unused). Channels are listed in bit
+	order, starting from the LSB. For instance, a format named "r5g6b5"
+	describes a 16-bit format where red is encoded in the 5 less significant
+	bits, green in the 6 following ones, and blue in the 5 last:
+				BBBBBGGG GGGRRRRR
+				^               ^
+			       MSB             LSB
 
 Example:
 
diff --git a/drivers/video/simplefb.c b/drivers/video/simplefb.c
index e2e9e3e..1430752 100644
--- a/drivers/video/simplefb.c
+++ b/drivers/video/simplefb.c
@@ -21,6 +21,7 @@
  */
 
 #include <linux/errno.h>
+#include <linux/ctype.h>
 #include <linux/fb.h>
 #include <linux/io.h>
 #include <linux/module.h>
@@ -82,8 +83,72 @@ struct simplefb_format {
 	struct fb_bitfield transp;
 };
 
+static struct simplefb_format *simplefb_parse_format(struct device *dev,
+						     const char *str)
+{
+	struct simplefb_format *format;
+	unsigned int offset = 0;
+	unsigned int i = 0;
+
+	format = devm_kzalloc(dev, sizeof(*format), GFP_KERNEL);
+	if (!format)
+		return ERR_PTR(-ENOMEM);
+
+	while (str[i] != 0) {
+		struct fb_bitfield *field = NULL;
+		int length = 0;
+
+		switch (str[i++]) {
+		case 'r':
+		case 'R':
+			field = &format->red;
+			break;
+		case 'g':
+		case 'G':
+			field = &format->green;
+			break;
+		case 'b':
+		case 'B':
+			field = &format->blue;
+			break;
+		case 'a':
+		case 'A':
+			field = &format->transp;
+			break;
+		case 'x':
+		case 'X':
+			break;
+		default:
+			goto error;
+		}
+
+		if (!isdigit(str[i]))
+			goto error;
+
+		while (isdigit(str[i])) {
+			length = length * 10 + (str[i++] - '0');
+		}
+
+		if (field) {
+			field->offset = offset;
+			field->length = length;
+		}
+
+		offset += length;
+	}
+
+	format->bits_per_pixel = (offset + 7) & ~0x7;
+	format->name = str;
+	return format;
+
+error:
+	dev_err(dev, "Invalid format string\n");
+	return ERR_PTR(-EINVAL);
+}
+
+/* if you use exotic modes that simplefb_parse_format cannot decode, you can
+   specify them here. */
 static struct simplefb_format simplefb_formats[] = {
-	{ "r5g6b5", 16, {11, 5}, {5, 6}, {0, 5}, {0, 0} },
 };
 
 struct simplefb_params {
@@ -131,7 +196,10 @@ static int simplefb_parse_dt(struct platform_device *pdev,
 		params->format = &simplefb_formats[i];
 		break;
 	}
-	if (!params->format) {
+	if (!params->format)
+		params->format = simplefb_parse_format(&pdev->dev, format);
+
+	if (IS_ERR(params->format)) {
 		dev_err(&pdev->dev, "Invalid format value\n");
 		return -EINVAL;
 	}
-- 
1.8.3