[Linux-fbdev-devel] color map vs. pseudo-palette vs. RAMDAC registers

[Rolf O. <rol...@gm...>]

Hi All,
I tried to understand the skeletonfb driver code, and I am confused by some=
=20
comments in it:

I do not understand what the color map and pseudo palette stuff is used for.

=46rom the c file:
=2D-------------------------------------------------------------------
Pseudocolor:
* cmap is not used
* DAC[X] is programmed to (red, green, blue)
=2D-------------------------------------------------------------------
Does that mean, I do not need to allocate a color map, if my driver would o=
nly=20
support pseudocolor visual? Is pseudocolor always used for 8bpp or are ther=
e=20
situations where 16bpp pseudocolor makes sense?

=2D------------------------------------------------------------------------=
=2D---
Truecolor:
     *    does not use RAMDAC (usually has 3 of them).
     *    var->{color}.offset contains start of bitfield
     *    var->{color}.length contains length of bitfield
     *    cmap is programmed to (red << red.offset) |=20
				(green << green.offset) |
    				(blue << blue.offset) |=20
				(transp << transp.offset)
     *    RAMDAC does not exist
=2D------------------------------------------------------------------------=
=2D---
Now I am lost:
=2D> does not use RAMDAC.=20
I understand this. It states the same in my chip reference. But what means=
=20
"usually has 3 of them" in this context?

=2D> cmap is programmed to...
In the code that follows this comment, not the cmap is programmed, but the=
=20
pseudo palette.
1. Why do I need a pseudo palette and where does its size 16 comes from?
2. What do I need a color map for?

=2D> RAMDAC does not exist <-> "usually has 3 of them"
???

Please shed some light on me.

Thanks,
Rolf