Re: [PATCH] VESA framebuffer w/MTRR locks 2.4.0 on init

[David Wragg <dpw@doc.ic.ac.uk>]

Alan Cox <alan@lxorguk.ukuu.org.uk> writes:
> > loop with no exit, as each size mtrr fails.
> >                  while (mtrr_add(video_base, temp_size, MTRR_TYPE_WRCOMB,
> > 1)==-EINVAL) {
> >                          temp_size >>= 1;
> >                  }
> 
> Ok that one is the bug.

Even with the obvious bug fixed, that code is strange.  "temp_size >>=
1" does little to improve the chances of the mtrr_add succeeding.
Something like this would be better:

if (mtrr_add(video_base, temp_size, MTRR_TYPE_WRCOMB, 1) == -EINVAL) {
        /* Find the largest power-of-two */
        while (temp_size & (temp_size - 1))
                temp_sze &= (temp_size - 1);

        mtrr_add(video_base, temp_size, MTRR_TYPE_WRCOMB, 1);
}


(But this is just a very crude way to work around the inflexibility of
the MTRRs.  Rather than cluttering up calls to mtrr_add, it would be
better to fix this properly, either by implementing PAT support
(Zoltán Böszörményi said he was working on that), or by having a
user-space helper program to make more intelligent MTRR allocations,
or both.)


David Wragg
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