[OLPC-devel] Re: AMD Geode GX framebuffer: support OLPC
mtosatti at redhat.com
Mon Aug 14 17:40:57 EDT 2006
(switching to olpc-devel)
On Mon, Aug 14, 2006 at 01:14:04PM -0600, Jordan Crouse wrote:
> On 14/08/06 14:49 -0400, David Zeuthen wrote:
> > > since VGA probing must be disabled in OLPC's kernel.
> > > Unfortunately, as far as I could gather, there exists no native
> > > framebuffer driver for the emulated Cirrus CLGD 5446 PCI VGA device.
> > How about disabling vga probing only if we know we are on the OLPC
> > hardware? Surely this must be fixable...
> VGA probing happens pretty darn early in the boot process, so we would
> probably have to communicate the existence of the OLPC hardware from the
> BIOS through one of the usual methods.
> The reasoning behind the patch was that we were purposely ignoring VGA
> all together and I wanted to eliminate the possibility that random bits would
> be returned to the probing function which may or may not cause the code to
> do stupid things. Far better, I reasoned, to just ignore the VGA stuff
> all together (plus it saved us a bit of space).
> We could probably avoid this patch all together if the kernel gurus agree
> that nothing bad would happen if we left it in, or if we did the extra work
> to return sane values in LinuxBIOS.
The aim is to kill the CONFIG_VGA_NOPROBE option which was added by
Jordan for safety reasons (see comments above).
For that to happen, it must be certain that LinuxBIOS returns
sane values for the following Linux VGA detection routines, at
# This is the main entry point called by setup.S
# %ds *must* be pointing to the bootsector
video: pushw %ds # We use different segments
pushw %ds # FS contains original DS
pushw %cs # DS is equal to CS
pushw %cs # ES is equal to CS
xorw %ax, %ax
movw %ax, %gs # GS is zero
call basic_detect # Basic adapter type testing (EGA/VGA/MDA/CGA)
movw %fs:(0x01fa), %ax # User selected video mode
cmpw $ASK_VGA, %ax # Bring up the menu
call mode_set # Set the mode
leaw badmdt, %si # Invalid mode ID
vid2: call mode_menu
call restore_screen # Restore screen contents
#endif /* CONFIG_VIDEO_RETAIN */
#endif /* CONFIG_VIDEO_SELECT */
call mode_params # Store mode parameters
popw %ds # Restore original DS
More especifically "basic_detect" and "mode_params".
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