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

[Ville Syrjälä]

On Sat, Mar 28, 2015 at 01:28:18AM +0100, Luis R. Rodriguez wrote:
> On Fri, Mar 27, 2015 at 03:02:10PM -0700, Andy Lutomirski wrote:
> > On Fri, Mar 27, 2015 at 2:56 PM, Ville Syrjälä <syrjala@xxxxxx> 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.
> > > Which in practice will mean throwing away half the VRAM since you really
> > > don't want the massive performance hit from accessing it as UC. And that
> > > would mean giving up decent display resolutions as well :(
> > >
> > > And the other option of trying to cover the remainder with multiple ever
> > > smaller MTRRs doesn't work either since you'll run out of MTRRs very
> > > quickly.
> > >
> > > This is precisely why I used the hole method in atyfb in the first
> > > place.
> > >
> > > I don't really like the idea of any new mtrr code not supporting that
> > > use case, especially as these things tend to be present in older machines
> > > where PAT isn't an option.
> > 
> > According to the Intel SDM, volume 3, section 11.5.2.1, table 11-6,
> > non-PAT CPUs that have a WC MTRR, PCD = 1, and PWT = 1 (aka UC) have
> > an effective memory type of UC.  Hence my suggestion to add
> > ioremap_x86_uc and/or set_memory_x86_uc to punch a UC hole in an
> > otherwise WC MTRR-covered region.
> 
> OK I think I get it now.
> 
> And I take it this would hopefully only be used for non-PAT systems?
> Would there be a use case for PAT systems? I wonder if we can wrap
> this under some APIs to make it clean and hide this dirty thing
> behind the scenes, it seems a fragile and error prone and my hope
> would be that we won't need more specialization in this area for
> PAT systems.

One potential complication is kernel vs. userspace mmap. MTRR applies to
the physical address, but PAT applies to the virtual address, so with
the WC MTRR you get WC for userspace "for free" as well. Also the
userspace mmaps request will have the length of smem_len (at most), so
it won't be the nice power of two in that case.

Also on PAT systems w/o a BIOS provided WC MTRR, the fbdev mmap seems
to be total crap at the moment. IIRC I have a patch to fix things a bit...