linux-kernel - Re: [PATCH] fbdev: fix color component field length documentation

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Message-ID: <20090331222835.GA8482@tria>
Date:	Wed, 1 Apr 2009 00:28:35 +0200
From:	Michal Januszewski <spock@...too.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	linux-kernel@...r.kernel.org,
	Krzysztof Helt <krzysztof.h1@...zta.fm>,
	Ville Syrjälä <syrjala@....fi>,
	linux-fbdev-devel@...ts.sourceforge.net
Subject: Re: [PATCH] fbdev: fix color component field length documentation

The documentation about the meaning of the color component bitfield lengths
in pseudocolor modes is inconsistent.  Fix it, so that it indicates the
correct interpretation everywhere, i.e. that 1 << length is the number
of palette entries.

Signed-off-by: Michal Januszewski <spock@...too.org>
Cc: Krzysztof Helt <krzysztof.h1@...zta.fm>
Cc: Ville Syrjälä <syrjala@....fi>
Cc: Geert Uytterhoeven <geert.uytterhoeven@...il.com>
---
This version of the patch incorporates Geert's comments about the
meaning of the offset field for pseudocolor modes.

diff --git a/drivers/video/skeletonfb.c b/drivers/video/skeletonfb.c
index df53365..63cf3bf 100644
--- a/drivers/video/skeletonfb.c
+++ b/drivers/video/skeletonfb.c
@@ -308,9 +308,11 @@ static int xxxfb_setcolreg(unsigned regno, unsigned red, unsigned green,
      *   color depth = SUM(var->{color}.length)
      *
      * Pseudocolor:
-     *    var->{color}.offset is 0
-     *    var->{color}.length contains width of DAC or the number of unique
-     *                        colors available (color depth)
+     *    var->{color}.offset is 0 unless the palette index takes less than
+     *                        bits_per_pixel bits and is stored in the upper
+     *                        bits of the pixel value
+     *    var->{color}.length is set so that 1 << length is the number of
+     *                        available palette entries
      *    pseudo_palette is not used
      *    RAMDAC[X] is programmed to (red, green, blue)
      *    color depth = var->{color}.length
diff --git a/drivers/video/vfb.c b/drivers/video/vfb.c
index 93fe08d..7d58c8a 100644
--- a/drivers/video/vfb.c
+++ b/drivers/video/vfb.c
@@ -318,13 +318,16 @@ static int vfb_setcolreg(u_int regno, u_int red, u_int green, u_int blue,
 	 *   {hardwarespecific} contains width of RAMDAC
 	 *   cmap[X] is programmed to (X << red.offset) | (X << green.offset) | (X << blue.offset)
 	 *   RAMDAC[X] is programmed to (red, green, blue)
-	 * 
+	 *
 	 * Pseudocolor:
-	 *    uses offset = 0 && length = RAMDAC register width.
-	 *    var->{color}.offset is 0
-	 *    var->{color}.length contains widht of DAC
+	 *    var->{color}.offset is 0 unless the palette index takes less than
+	 *                        bits_per_pixel bits and is stored in the upper
+	 *                        bits of the pixel value
+	 *    var->{color}.length is set so that 1 << length is the number of available
+	 *                        palette entries
 	 *    cmap is not used
 	 *    RAMDAC[X] is programmed to (red, green, blue)
+	 *
 	 * Truecolor:
 	 *    does not use DAC. Usually 3 are present.
 	 *    var->{color}.offset contains start of bitfield
diff --git a/include/linux/fb.h b/include/linux/fb.h
index 31527e1..0d52630 100644
--- a/include/linux/fb.h
+++ b/include/linux/fb.h
@@ -172,8 +172,12 @@ struct fb_fix_screeninfo {
 /* Interpretation of offset for color fields: All offsets are from the right,
  * inside a "pixel" value, which is exactly 'bits_per_pixel' wide (means: you
  * can use the offset as right argument to <<). A pixel afterwards is a bit
- * stream and is written to video memory as that unmodified. This implies
- * big-endian byte order if bits_per_pixel is greater than 8.
+ * stream and is written to video memory as that unmodified.
+ *
+ * For pseudocolor: offset and length should be the same for all color
+ * components. Offset specifies the position of the least significant bit
+ * of the pallette index in a pixel value. Length indicates the number
+ * of available palette entries (i.e. # of entries = 1 << length).
  */
 struct fb_bitfield {
 	__u32 offset;			/* beginning of bitfield	*/
--
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