linux-kernel - Re: [git pull] drm pull for v4.12

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Date:   Sun, 7 May 2017 14:33:15 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dave Airlie <airlied@...il.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [git pull] drm pull for v4.12

On Sat, May 6, 2017 at 10:02 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, May 2, 2017 at 8:44 PM, Dave Airlie <airlied@...il.com> wrote:
>> i915:
>> vblank evasion improvements
>
> These may be "improvements", but they end up being very noisy.
>
> I geta fair amount of messages like
>
>   [drm] Atomic update on pipe (A) took 161 us, max time under evasion is 100 us
>
> on my desktop (i7-6700K) and I've seen it once on my laptop (i7-6560U) too.
>
> The commit message says "This will make it easier to find cases where
> we potentially miss vblanks." but I'm not sure how that is the case.
> There's nothing else helpful in the log, and it doesn't seem to be
> associated with anything in particular (I'm just at the desktop,
> running a few xterms and google-chrome).

We did improve evasion a lot to the point that it didn't show up in
our CI machines anymore (and we have a pile of those crunching tests
24/7), so we felt we could risk enabling this everywhere. But of
course it pops up all over the place as soon as drm-next hits
mainline, so we'll shut it down again except when debugging is enabled
(to make sure we catch regressions in our CI).

The evasion stuff is duct-tape around hw design issues and mostly
harmless (i.e. very unlikely it'll result in a real issue for you,
especially on normal Linux desktops that don't use overlay planes) if
you hit it, worst case should be flickering or some other temporary
visual corruptions. And the message happens when there's simply a
potential for a miss, not when we observed the race happening (i.e.
like lockdep splats about theoretical stuff).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch