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

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Date:	Fri, 27 Mar 2015 14:21:34 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>
Cc:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Juergen Gross <jgross@...e.com>,
	Jan Beulich <JBeulich@...e.com>, Borislav Petkov <bp@...e.de>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	venkatesh.pallipadi@...el.com, Dave Airlie <airlied@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
	X86 ML <x86@...nel.org>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Antonino Daplas <adaplas@...il.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>
Subject: Re: [PATCH v1 09/47] vidoe: fbdev: atyfb: remove and fix MTRR MMIO
 "hole" work around

On Fri, Mar 27, 2015 at 1:12 PM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
> On Fri, Mar 20, 2015 at 04:52:18PM -0700, Andy Lutomirski wrote:
>> On Fri, Mar 20, 2015 at 4:17 PM, Luis R. Rodriguez
>> <mcgrof@...not-panic.com> 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);
>> > -               if (par->mtrr_aper >= 0 && !par->aux_start) {
>> > -                       /* Make a hole for mmio. */
>> > -                       par->mtrr_reg = mtrr_add(par->res_start + 0x800000 -
>> > -                                                GUI_RESERVE, GUI_RESERVE,
>> > -                                                MTRR_TYPE_UNCACHABLE, 1);
>> > -                       if (par->mtrr_reg < 0) {
>> > -                               mtrr_del(par->mtrr_aper, 0, 0);
>> > -                               par->mtrr_aper = -1;
>> > -                       }
>> > -               }
>> >         }
>> >  #endif
>> >
>> > @@ -2776,10 +2765,6 @@ aty_init_exit:
>> >         par->pll_ops->set_pll(info, &par->saved_pll);
>> >
>> >  #ifdef CONFIG_MTRR
>> > -       if (par->mtrr_reg >= 0) {
>> > -               mtrr_del(par->mtrr_reg, 0, 0);
>> > -               par->mtrr_reg = -1;
>> > -       }
>> >         if (par->mtrr_aper >= 0) {
>> >                 mtrr_del(par->mtrr_aper, 0, 0);
>> >                 par->mtrr_aper = -1;
>> > @@ -3466,7 +3451,7 @@ static int atyfb_setup_generic(struct pci_dev *pdev, struct fb_info *info,
>> >         }
>> >
>> >         info->fix.mmio_start = raddr;
>> > -       par->ati_regbase = ioremap(info->fix.mmio_start, 0x1000);
>> > +       par->ati_regbase = ioremap_nocache(info->fix.mmio_start, 0x1000);
>>
>> Double-check me, but I think that ioremap_nocache + WC MTRR = WC.
>
> Precicely, in this case the WC hole was obtained by using MTRR WC. This
> patch removes that WC hole trick and now we can be explciit about
> only wanting ioremap_nocache() on the registers, that is WC is not
> desired here and is not used. The patch does not highlight the fact
> that there was left in place another ioremap() call for the framebuffer:
>
> info->screen_base = ioremap(info->fix.smem_start, info->fix.smem_len);
>
> That is the one that later after this patch we use ioremap_wc() for.
> This patch just removes the hole solution. That's all.
>

I don't understand.

If I read it right, there's a 2^n byte BAR.  You're requesting WC for
the whole think using arch_phys_wc_add.  On a PAT system that has no
effect and all is well.  On a non-PAT system, it adds an MTRR.  That
means that you need to override the MTRR somehow for the mmio regs,
and UC- won't do the trick.

Or am I missing something here?

--Andy

>   Luis



-- 
Andy Lutomirski
AMA Capital Management, LLC
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