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

[Andy Lutomirski]

On Fri, Mar 27, 2015 at 2:56 PM, Ville SyrjÃlà <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 SyrjÃlà 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.

ioremap_nocache is UC- (even on non-PAT unless I misunderstood how
this stuff works), so ioremap_nocache by itself isn't good enough.

--Andy
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