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

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

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:

---
    The last thing we do must do to remain sane is ensure we
    use the info->fix.smem_start and info->fix.smem_len for
    the framebuffer MTRR as we know that is always well adjusted.
    The *one* concern here would be if the MTRR is not in units
    of 4K __but__ we already know that in the PCI case this cannot
    happen, in the shared space setting the MTRR would be up to
    0x7ff000 and assuming a 4K page:
    
    ; 0x7ff000 / 0x1000
        2047
    
    Also, internally when MTRR is used mtrr_add() will use mtrr_check()
    and that should splat a warning when the MTRR base and size are
    not compatible with what is expected for MTRR usage.
---

If any of this is too risky we can use the __arch_phys_wc_add() (or as
Andy suggested perhaps use set_page_* stuff, although I am still evaluating
this) but I did this change to show the effort required for a change when
the registers / framebuffer is on the same PCI BAR but at different offsets.

[0] scripts/kernel-doc -man -function mtrr_add_page 
arch/x86/kernel/cpu/mtrr/main.c | nroff -man | less

  Luis

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