Re: DRM & MTRR's

["Antonino A. Daplas" <adaplas@gmail.com>]

Alan Hourihane wrote:
> Has anyone any objections to me removing the MTRR code from the DRM.
> 
> It doesn't have the intimate knowledge needed to properly configure
> MTRR's and fails in many cases.
> 
> There are two cases currently, one for the framebuffer and a second for
> the entire AGP space.
> 
> Certainly in Intel hardware this is the same memory space and you always
> get bogus error messages in your kernel logs as things fail due to lack
> of boundary checks.
> 
> I'm more of the opinion that it should be left up to userspace to
> configure MTRR's if it indeed wants them and we shouldn't force them
> within the DRM.
> 
> Additionally, the Xserver (for user-space) already sets up the MTRR's,
> as should Xgl (Xegl) or other user space apps.
> 
> What makes the situation a little worse is that vesafb (and other *fb
> drivers) also setup an mtrr which frequently stops subsequent processes
> from adding a new one that overlaps an existing write-combining range.
> But the *fb drivers do provide a nomtrr option in many cases. (But I'd
> like to remove it from them too :-) or at least default those to off)

I don't know the repercussions of defaulting to nomtrr. But I would
surmise that a large percentage of users gives more importance to X/DRI
performance than console performance (majority of vesafb users still
use redraw for scrolling where ypan is magnitudes faster, so I'm pretty
sure defaulting vesafb to nomtrr will not be noticeable to most users,
they probably go straight to X anyway :-)

I cannot decide on this alone, so I'm going to CC the fbdev-devel list.
Perhaps I can submit a patch that defaults all drivers to nomtrr and a
big warning about mtrr not being set in dmesg. If I don't get any
backlash for a few patch cycles, I can make this permanent.

Tony


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