Re: Large framebuffers and HIGHMEM systems

[Sven Luther <sven.luther@wanadoo.fr>]

On Wed, Sep 10, 2003 at 08:01:00AM -0700, Jon Smirl wrote:
> --- Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > Or modify ioremap() to print the message? This problem is not only limited to
> > frame buffer devices, but to every driver that needs to ioremap() large
> > regions, right?
> > 
> ioremap() is supposed to map the region into the bottom 1GB or fail. After
> ioremap() fails you need to use a different API to map to the highmem regions.
> The way Linux is set up there is always about 20-30MB of free address space
> below 1GB. All of the hardware I've seen can map into that space. The only
> exception is the huge amount of address space needed for kernel mapped video
> frame buffers.
> 
> Memory below 1GB is important to the kernel for keeping page tables. Page table
> can be pushed above 1GB but you incur performance penalties. There is no
> problem creating a 256MB hole with reserve=XXXX in the 1GB address space to
> allow the latest ATI/NV cards to run with framebuffers; but doing this will
> slow your system down. It is also a very non-obvious solution to why a
> framebuffer fails to load.
> 
> I don't think we should be forcing non-performance sensitve framebuffer memory
> into the bottom 1GB and pushing performance sensitive page tables into highmem.
> Besides, we are going that have to do something, we are likely to have 1GB
> video cards two years from now.

We already have 512MB ones, i guess you will not have to wait for as
long as two years for 1GB video cards.

Friendly,

Sven Luther


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