Re: [PATCH] Use MTRRs by default for vesafb on x86-64

[ebiederm@xmission.com (Eric W. Biederman)]

Andi Kleen <ak@muc.de> writes:

> x86-64 cannot call the 32bit VESA BIOS. This means when vesafb is active
> it does software copying in the vesa frame buffer. This is insanely slow
> when the frame buffer is not marked for write combining. 
> 
> Some discussion showed that the use_mtrr flag was only off for some 
> old broken ET4000 ISA card. x86-64 has no ISA, so this is no concern.
> Make the default depend on CONFIG_ISA. 
> 
> Patch for 2.5.69.  Originally suggested by Gerd Knorr.

I don't know if this affects the frame buffers per se.

But often BIOS's on systems with large amounts of memory configure
overlapping mtrrs (where an uncacheable mtrr would override a larger
cacheable range).  To date this has confused the linux mtrr code when
it tries to modify things, and you cannot properly setup mtrrs.    I
believe this applies to both the fb case as well as X.

Eric