Re: Problem with current fb_get_color_depth function

["Antonino A. Daplas" <adaplas@hotpop.com>]

On Monday 11 October 2004 06:59, Joshua Kwan wrote:
> because on advice from Andrew Suffield and Matthew Garrett the first
> conditional was a roundabout way to check for grayscale displays, where
> you can't actually have 24 bits (8 + 8 + 8) of gray. So it is suitable
> to just return one of the values arbitrarily. But I noticed there was
> also a grayscale variable, so I substituted that for the conditional
> and my logo reappeared again :)
>
> So is radeonfb or fb_get_color_depth at fault here? Or was the logo
> never appropriate for my display in the first place? (Unlikely, the
> CLUT224 linux logo always displayed perfectly for me until now.)
>

This is because radeonfb chooses directcolor for its visual.  With RGB565,
the depth == 16, true.  However, unlike truecolor where color values are
hardwired to linear values, directcolor looks at the hardware CLUT.

So, with directcolor RGB656, these are the CLUT lengths:

Red and Blue = 32 (2 ^ 5)
Green        = 64 (2 ^ 6)

(And you can see this in radeonfb_base:radeonfb_setcolreg())

		if (rinfo->bpp == 16) {
			pindex = regno * 8;

			if (rinfo->depth == 16 && regno > 63)
				return 1;
			if (rinfo->depth == 15 && regno > 31)
				return 1;

However, linux_logo_224 requires a CLUT with these lengths:

Red = Green = Blue = 224;

So, linux_logo_224 cannot be drawn when visual is directcolor at RGB555 or
RGB565 because the logo clut requirements exceeds the hardware clut
capability. You need to use a logo image with a lower depth such as the
16-color logo, linux_logo_16.

In short, fb_get_color_depth() and radeonfb both do the correct thing, but
you need a logo with a lower color depth. Or choose RGB888 or higher, or
set the visual to truecolor.

Tony

PS: Note that this behavior is the same as 2.4 behavior (224-color logo is
only chosen if directcolor and bpp >= 24).  It might have worked before, and
this is probably due to the directcolor clut being ramped to truecolor
values. However, the correct solution is to use a 16-color logo.