Sounds like it might be possible to use a VGA PCI card with a LFB as a fast
swap device.
On 06-Apr-2000 Helge Hafting wrote:
>> The video RAM reported on video boards as "2Mb, 4Mb,... NMb.", is
>> not RAM that can be directly accessed via ISA or PCI et all, buses.
>> It's SRAM mappable to the Video device chip, and assessible only
>> through a port from the "outside". It isn't even the "Screen regen buffer"
>> you see at 0xB8000 which is typically only 16 kb. To access that RAM, you
>> have to enable "graphics mode" and then it's available as 64k pages
>> from 0xA0000 by writing to page registers through a port. This is only
>> 64k per page and the kernel has no way of "knowing" when to change the
>
> Most new cards have all the video memory directly acessible via PCI (or
> AGP)
> busses. They have the ugly 0xA0000 64k-block stuff too, but that's only
> for
> backward compatibility with VGA. Most cards support a
> directly accessible linear framebuffer.
>
> And there is a hack for those old cards only acessible in 64k chunks:
> Set up a linear mapping in the page tables, but mark all pages except
> the
> ones in the current 64k buffer as "not present" Programs can now acess
> this
> "linear framebuffer", and the kernel will simply get a page fault
> whenever
> a program steps outside the 64k block. The swapper will then change
> block on the video card, and mark the new pages present and the old ones
> gone. Then the process is restarted. An old trick that simplifies
> programs
> a lot, don't know if it is done under linux though. It is usually not
> needed any more, thanks to pci/agp buses with enough adress space for
> any video card.
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