RE: [git head] Should X86_PAT really default to yes?


-----Original Message-----
From: Barnes, Jesse
Sent: Monday, May 05, 2008 10:46 AM
To: Frans Pop
Cc: Pallipadi, Venkatesh; linux-kernel@xxxxxxxxxxxxxxx; Ingo
Molnar; Packard, Keith; Yinghai Lu
Subject: Re: [git head] Should X86_PAT really default to yes?

On Monday, May 05, 2008 10:32 am Frans Pop wrote:
Sigh. This is going to get complex...

On Monday 05 May 2008, Jesse Barnes wrote:
If so, it might not be a PAT issue but just a
different memory layout
or something (and therefore it would really just be a
cosmetic bug in
the X driver).

The artifacts may not be a PAT issue directly, but it is a clear
regression for me as I currently have a nice clean
screen when X shuts
down. I'm also 100% sure that it is caused by enabling
PAT. A kernel
with same config and only PAT disabled does not show the
artifacts.

Would you like me to file a bug against X for these artifacts?
If so, against what component? The i810 driver or the server?

I suspect an i810 driver bug is being uncovered here,
since we do have
transient VT switch corruption on some other platforms (we're just
exposing our chip reprogramming on the screen, rather than
keeping it off
the whole time). But there could also be something PAT
specific going
on, I'll have to walk through those code paths...

I suspect it could be vesafb/fbcon related instead. Normally
I boot my
system with 'quiet vga=791', i.e. with vesafb. I then see
the artifacts.

When I boot without 'vga=791', I hit another, unrelated
regression (which
I'll report separately) [1].

When I boot with 'video=vfb:off', I do _not_ get the artifacts when X
exits.

Ahhh, I missed that part of your config. That could
definitely have an effect
on things... You'll probably want something like the attached
(there are
other places in the fb layer that want similar treatment, iirc, maybe
fb_pgprotect?).

Note that the "expected mapping type" errors remain the same
both with and
without framebuffer console.

Oh the messages should be removed or somehow minimized, I
agree. I'm
just not sure if the other bug is serious enough to block
PAT by default
yet, but either way we should fix the bugs!

OK. Thanks. Guess you've also seen "Xorg crash with
xf86MapVidMem error"
that turned out to be due to PAT:
http://www.gossamer-threads.com/lists/linux/kernel/915300 ?

No, I hadn't seen that... Venki/Ingo has that issue been
fixed already?


No. We are following up on that issue to root cause the problem there.
AFAIK, that is the only other open issue related to PAT at this time.

There was another one reported here
http://bugzilla.kernel.org/show_bug.cgi?id=10580
which was root caused as a app bug unmasked by PAT.

Thanks,
Venki
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