Re: [Xen-devel] [PATCH v1 09/47] vidoe: fbdev: atyfb: remove and fix MTRR MMIO "hole" work around

["Luis R. Rodriguez" <mcgrof@xxxxxxxx>]

On Fri, Mar 27, 2015 at 12:43:55PM -0700, Andy Lutomirski wrote:
> On Fri, Mar 27, 2015 at 12:38 PM, Luis R. Rodriguez <mcgrof@xxxxxxxx> wrote:
> > On Sat, Mar 21, 2015 at 11:15:14AM +0200, Ville Syrjälä wrote:
> >> On Fri, Mar 20, 2015 at 04:17:59PM -0700, Luis R. Rodriguez wrote:
> >> > diff --git a/drivers/video/fbdev/aty/atyfb_base.c 
> >> > b/drivers/video/fbdev/aty/atyfb_base.c
> >> > index 8025624..8875e56 100644
> >> > --- a/drivers/video/fbdev/aty/atyfb_base.c
> >> > +++ b/drivers/video/fbdev/aty/atyfb_base.c
> >> > @@ -2630,21 +2630,10 @@ static int aty_init(struct fb_info *info)
> >> >
> >> >  #ifdef CONFIG_MTRR
> >> >     par->mtrr_aper = -1;
> >> > -   par->mtrr_reg = -1;
> >> >     if (!nomtrr) {
> >> > -           /* Cover the whole resource. */
> >> > -           par->mtrr_aper = mtrr_add(par->res_start, par->res_size,
> >> > +           par->mtrr_aper = mtrr_add(info->fix.smem_start,
> >> > +                                     info->fix.smem_len,
> >> >                                       MTRR_TYPE_WRCOMB, 1);
> >>
> >> MTRRs need power of two size, so how is this supposed to work?
> >
> > As per mtrr_add_page() [0] the base and size are just supposed to be in 
> > units
> > of 4 KiB, although the practice is to use powers of 2 in *some* drivers this
> > is not standardized and by no means recorded as a requirement. Obviously
> > powers of 2 will work too and you'd end up neatly aligned as well. 
> > mtrr_add()
> > will use mtrr_check() to verify the the same requirement. Furthermore,
> > as per my commit log message:
> 
> Whatever the code may or may not do, the x86 architecture uses
> power-of-two MTRR sizes.  So I'm confused.

There should be no confusion, I simply did not know that *was* the
requirement for x86, if that is the case we should add a check for that
and perhaps generalize a helper that does the power of two helper changes,
the cleanest I found was the vesafb driver solution.

Thoughts?

 Luis

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