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 11:56:55PM +0200, Ville Syrjälä wrote:
> On Fri, Mar 27, 2015 at 08:57:59PM +0100, Luis R. Rodriguez wrote:
> > 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?
> 
> The vesafb solution is bad since you'll only end up covering only
> the first 4MB of the framebuffer instead of the almost 8MB you want.

OK so the power of 2 requirement implicates us *having* to use a large
MTRR that includes the MMIo region in the shared PCI case?

Andy, Ville, are we 100% certain about this power of two requirement?
Is that for the base and size or just the size?

  Luis

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