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

[Geert U. <ge...@li...>]

On Wed, 13 Jun 2001, Sven LUTHER wrote:
> On Wed, Jun 13, 2001 at 08:47:21AM -0700, James Simmons wrote:
> > > So i suppose saying fbset -depth 15 could be done to switch the fb to
> > > RGBA5551 mode ? 
> > > 
> > > BTW, what is fbset -depth 24 supposed to do, especially if the chip only
> > > support 8, 16, and 32 bpp ?
> 
> > > should only fbset -depth 32 be supported ?
> 
> mmm, well, the problem is that most current 'millions of color' modes are in
> truth depth 24 ones, which are internally either 32bpp (as in the permedia3
> case) or 24bpp (saves bandwith, but no alpha channel and you may be forced to
> do some shifting).
> 
> Now, if a user wants to set a depth of 24, what does he mean ? mostly it is
> RGB 888, but for the user the presence of an alpha channel (32bpp) or not
> (24bpp) may not be that important. Especially Since X is using per default
> depth 24 with bpp 32.
> 
> So what should happen if you try to set depth 24 with fbset ? Silently set 32
> bpp ? reject it ?

Round it up to 32.

> Same goes for depth 15 ? should it silently be set to bpp 16, RGBA 5551, or
> rejected ?

You cannot specify `depth 15', only `bits per pixel 15', which will be rounded
up to the driver to 16 (in most cases).

Set {red,green,blue,transp}.length to indicate which 16 bpp mode you really
want.

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@li...

In personal conversations with technical people, I call myself a hacker. But
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							    -- Linus Torvalds