[OLPC-devel] Re: AMD Geode GX framebuffer: support OLPC

[Ronald G Minnich]

Now we need to bother Ollie, he is our real vga expert :-)

Ollie, how should we do this. I have ideas, but I want to hear your 
ideas, they will be better than mine.

ron

Marcelo Tosatti wrote:
> (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. 
> 
> 
> Ronald,
> 
> 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
> arch/i386/kernel/video.S:
> 
> 
> # 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
>         popw    %fs
>         pushw   %cs             # DS is equal to CS
>         popw    %ds
>         pushw   %cs             # ES is equal to CS
>         popw    %es
>         xorw    %ax, %ax
>         movw    %ax, %gs        # GS is zero
>         cld
>         call    basic_detect    # Basic adapter type testing (EGA/VGA/MDA/CGA)
> #ifdef CONFIG_VIDEO_SELECT
>         movw    %fs:(0x01fa), %ax               # User selected video mode
>         cmpw    $ASK_VGA, %ax                   # Bring up the menu
>         jz      vid2
> 
>         call    mode_set                        # Set the mode
>         jc      vid1
> 
>         leaw    badmdt, %si                     # Invalid mode ID
>         call    prtstr
> vid2:   call    mode_menu
> vid1:
> #ifdef CONFIG_VIDEO_RETAIN
>         call    restore_screen                  # Restore screen contents
> #endif /* CONFIG_VIDEO_RETAIN */
>         call    store_edid
> #endif /* CONFIG_VIDEO_SELECT */
>         call    mode_params                     # Store mode parameters
>         popw    %ds                             # Restore original DS
>         ret
> 
> More especifically "basic_detect" and "mode_params".
> 
> Thoughts/comments?