Re: [PATCH 11/21] drm/i915: disable GTT cache for 2M pages

From: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
To: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
	Matthew Auld <matthew.auld@intel.com>
Subject: Re: [PATCH 11/21] drm/i915: disable GTT cache for 2M pages
Date: Tue, 03 Oct 2017 15:32:31 +0300	[thread overview]
Message-ID: <1507033951.4728.52.camel@linux.intel.com> (raw)
In-Reply-To: <CAM0jSHMiZ1RBN_=6PhsLOnwer7zn7EkZWsj=UkRaizHsvPeBAg@mail.gmail.com>

+ Mika for more comments as the original reviewer

On Mon, 2017-10-02 at 14:52 +0100, Matthew Auld wrote:
> On 2 October 2017 at 13:31, Joonas Lahtinen
> <joonas.lahtinen@linux.intel.com> wrote:
> > On Fri, 2017-09-29 at 17:10 +0100, Matthew Auld wrote:
> > > When SW enables the use of 2M/1G pages, it must disable the GTT cache.
> > > 
> > > v2: don't disable for Cherryview which doesn't even support 48b PPGTT!
> > > 
> > > v3: explicitly check that the system does support 2M/1G pages
> > > 
> > > Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> > > Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > > Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > > Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> > > Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> > 
> > <SNIP>
> > 
> > > @@ -8483,10 +8483,11 @@ static void bdw_init_clock_gating(struct drm_i915_private *dev_priv)
> > > 
> > >       /*
> > >        * WaGttCachingOffByDefault:bdw
> > > -      * GTT cache may not work with big pages, so if those
> > > -      * are ever enabled GTT cache may need to be disabled.
> > > +      * The GTT cache must be disabled if the system is using 2M/1G pages.
> > >        */
> > > -     I915_WRITE(HSW_GTT_CACHE_EN, GTT_CACHE_EN_ALL);
> > > +     I915_WRITE(HSW_GTT_CACHE_EN,
> > > +                HAS_PAGE_SIZES(dev_priv, I915_GTT_PAGE_SIZE_2M) ? 0 :
> > > +                GTT_CACHE_EN_ALL);
> > 
> > Umm, this is mixing a known W/A with decision logic.
> > 
> >         bool can_use_gtt_cache = !HAS_PAGE_SIZES(dev_priv, I915_GTT_PAGE_SIZE_2M);
> > 
> >         /* WaGttCachingOffByDefault:bdw */
> >         I915_WRITE(HSW_GTT_CACHE_EN, can_use_gtt_cache ? GTT_CACHE_EN_ALL : 0);
> > 
> > The big question is that if everyone else has GTT caching enabled by
> > default, should not we actively be disabling it on other code paths?
> 
> AFAIK it's *disabled* by default, bdw and chv are the only platforms
> which seek to enable it. I don't know why other platforms don't also
> follow suit...
> 
> That WA is literally just: "WA to enable the caching if off by
> defaultboth at driver init and Resume".
> 
> > I'm also feeling 'init_clock_gating' is not the best home for the code.
> > 
> > We can find a new home later, but do separate the decision logic from
> > the W/A.
> > 
> > Also, do 1G pages imply 2M page support? It's bit on the theoritical
> > side of science of course.
> 
> I just assume not, since it doesn't explicitly state that anywhere.
> 
> > 
> > Regards, Joonas
> > --
> > Joonas Lahtinen
> > Open Source Technology Center
> > Intel Corporation
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
-- 
Joonas Lahtinen
Open Source Technology Center
Intel Corporation
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