Re: Bug in vga16fb?

[Ondrej Zajicek <santiago@mail.cz>]

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On Thu, Apr 06, 2006 at 12:48:48AM +0200, Petr Vandrovec wrote:
> Ondrej Zajicek wrote:
> >Hello
> >
> >I tried vga16fb driver in 320x200x4 nonstd mode (on i386 - little endian). 
> >In
> >this mode framebuffer reports PACKED PIXELS type. Drawing through 
> >framebuffer
> >is OK, but console is broken - it looks like that even and odd columns are
> >exchanged.
> >
> >Framebuffer structure of this mode is that even pixel is in high nibble and
> >odd pixel is in low nibble.
> >
> >It looks like vga16fb use cfbimgblt.c/slow_imageblit in this mode for
> >image-blit and this function in CFB4 writes even pixel in low nibble and
> >odd pixel in high nibble.
> >
> >What is correct framebuffer structure for CFB4 mode?
> 
> Unfortunately there is no standard, as CFB4 itself is quite non-standard 
> VGA mode.  Some videocards first output low nibble, followed by high, and 
> some do otherwise.  I've experimented with this back in 2000, and I've 
> found that Matrox Millennium does exactly opposite than some ATI Mach64 
> card I had in another box at that time.
> 
> So your only option is providing two CFB4 modes.  Unfortunately I have no 
> idea how to autodetect what your hardware actually does without adding 
> camera to the system...

Even if we find some possibility to autodetect what hardware does (or in
case of another, non-VGA hardware using CFB4 mode) there would be
problem that driver cannot say it to userspace (or generic kernel
functions like cfb_imageblit). Maybe there should be some flag in
type_aux specifying whether given packed pixel mode is LSB-first or
MSB-first (relevant only for bpp % 8 != 0) ?

-- 
Elen sila lumenn' omentielvo

Ondrej 'SanTiago' Zajicek (email: santiago@mail.cz, jabber: santiago@njs.netlab.cz)
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"To err is human -- to blame it on a computer is even more so."

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