Re: [Linux-fbdev-devel] Supporting various depth

[Petr V. <VAN...@vc...>]

On 14 Jun 01 at 18:12, Romain Dolbeau wrote:

> Unfortunately, fbset -depth X should really be fbset -bpp X as it
> means 'set the framebuffer to that number of bits per pixels in
> memory, or more'. For TrueColor mode, there's an option -rgba R,G,B,A
> to give the number of significant bits for each color component. So to
> get Depth 15 in 16 BitsPerPixels, i.e 5 bits for each of red, green
> and blue plus a spare bit, one should use: fbset -depth 16 -rgba
> 5,5,5,1.

For Palletized mode option -rgba reports/selects DAC width - on VGA
either 6 or 8. Not all drivers supports selecting DAC width, but all
must report real width (on normal hardware width of R,G,B should be same,
and width of A should be zero - unless your DAC produces alpha channel).

And I think that we should add into spec:

If values required in rgba are not supported by driver, driver should
do its best to find another possible rgba values.

Reason: In past rgba values were ignored by fb_set_var, and so some
tools (fbset comes to my mind) tries to use '-depth 32 -rgba 5,6,5,0'
when doing 16->32bpp, and other invalid combinations. Requested value
for A channel width must be ignored by driver if requested -depth
cannot have alpha channel on this hardware (and must report A width=0).
                                        
                                        Petr Vandrovec
                                        van...@vc...