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To:     Jean Delvare <jdelvare@suse.de>, David Airlie <airlied@redhat.com>
Cc:     christian.koenig@amd.com,
        dri-devel <dri-devel@lists.freedesktop.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Takashi Iwai <tiwai@suse.de>
References: <20181101162707.598c1718@endymion>
 <20181108130509.3b2d42ff@endymion>
 <CAMwc25oWstf8bFfWfb=FFR8GfFhx+jM77TzqE-qfi+ev2pMvFw@mail.gmail.com>
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From:   Thomas Zimmermann <tzimmermann@suse.de>
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Subject: Re: Performance regression in ast drm driver
Message-ID: <c25b99fc-0762-ab88-7c08-f71510383e9e@suse.de>
Date:   Mon, 12 Nov 2018 20:36:00 +0100
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From: Thomas Zimmermann <tzimmermann@suse.de>
To: Jean Delvare <jdelvare@suse.de>, David Airlie <airlied@redhat.com>
Cc: christian.koenig@amd.com, dri-devel <dri-devel@lists.freedesktop.org>,
 linux-kernel <linux-kernel@vger.kernel.org>, Takashi Iwai <tiwai@suse.de>
Message-ID: <c25b99fc-0762-ab88-7c08-f71510383e9e@suse.de>
Subject: Re: Performance regression in ast drm driver
References: <20181101162707.598c1718@endymion>
 <20181108130509.3b2d42ff@endymion>
 <CAMwc25oWstf8bFfWfb=FFR8GfFhx+jM77TzqE-qfi+ev2pMvFw@mail.gmail.com>
 <1542033367.2661.33.camel@suse.de>
In-Reply-To: <1542033367.2661.33.camel@suse.de>

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Hi Jean

Am 12.11.18 um 15:36 schrieb Jean Delvare:
> Hi David,
>=20
> On Fri, 2018-11-09 at 10:04 +1000, David Airlie wrote:
>> On Thu, Nov 8, 2018 at 10:05 PM Jean Delvare <jdelvare@suse.de> wrote:=

>>>
>>> On Thu, 1 Nov 2018 16:27:07 +0100, Jean Delvare wrote:
>>>> Hi David,
>>>>
>>>> The following commit:
>>>>
>>>> commit 7cf321d118a825c1541b43ca45294126fd474efa
>>>> Author: Dave Airlie
>>>> Date:   Mon Oct 24 15:37:48 2016 +1000
>>>>
>>>>     drm/drivers: add support for using the arch wc mapping API.
>>>>
>>>> is causing a huge performance regression for the ast drm driver. In =
a
>>>> text console, if I call "cat" on a large text file, it takes almost
>>>> twice as much time to be displayed and scrolled completely.
>>>>
>>>> Can you please check that the ast driver portion of that commit is b=
oth
>>>> correct and complete?
>>>
>>> And in the meantime, what bad will happen if we just revert the ast
>>> portion of that commit?
>>
>> This seems likely to be a hw problem with PCI writes to the AST "GPU",=

>> since it's just some sort of RAM + ARM on the end of a PCIE bus, we've=

>> definitely seen possible issues in the past with write combining
>> around some of the mga GPUs with some CPUs.
>=20
> Takashi asked me to compare the contents of
> /sys/kernel/debug/x86/pat_memtype_list before and after the commit
> above. Before the commit, we have:
>=20
> uncached-minus @ 0xfafe0000-0xfb000000
> uncached-minus @ 0xfb000000-0xfb005000
> write-combining @ 0xfb005000-0xfb584000
>=20
> After the commit, we have:
>=20
> uncached-minus @ 0xfafe0000-0xfb000000
> uncached-minus @ 0xfb000000-0xfb005000
> uncached-minus @ 0xfb000000-0xfb800000
> uncached-minus @ 0xfb005000-0xfb584000
>=20
> The corresponding lines in /proc/iomem are:
>=20
> f0000000-fed8ffff : PCI Bus 0000:00
> =C2=A0 faf00000-fb7fffff : PCI Bus 0000:01
> =C2=A0=C2=A0=C2=A0=C2=A0fafe0000-faffffff : 0000:01:01.0
> =C2=A0=C2=A0=C2=A0=C2=A0fb000000-fb7fffff : 0000:01:01.0
>=20
> Does it help? Is the change of type expected? Is it not a problem that
> one of the ranges is overlapping with 2 other ranges?
>=20

I debugged this problem with an AST 2000. What happens is that the
vesafb driver attaches first to the graphics device. It disables
write-combining and falls back to uncached access; unless the kernel has
been booted with video=3Dvesafb:mtrr:3. Then it will set-up WC correctly.=

The respective code is at [1].

Best regards
Thomas

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/d=
rivers/video/fbdev/vesafb.c?h=3Dv4.20-rc2#n415

--=20
Thomas Zimmermann
Graphics Driver Developer
SUSE Linux GmbH, Maxfeldstr. 5, D-90409 N=C3=BCrnberg
Tel: +49-911-74053-0; Fax: +49-911-7417755;  https://www.suse.com/
SUSE Linux GmbH, GF: Felix Imend=C3=B6rffer, Jane Smithard,
Graham Norton, HRB 21284 (AG N=C3=BCrnberg)


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