Re: [PATCH 2/3] drm/i915: Do only one posting read on forcewake put sequence

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Ville Syrjälä <ville.syrjala@linux.intel.com> writes:

> On Wed, Jan 28, 2015 at 03:28:56PM +0200, Mika Kuoppala wrote:
>> Chris Wilson <chris@chris-wilson.co.uk> writes:
>> 
>> > On Wed, Jan 28, 2015 at 02:43:25PM +0200, Mika Kuoppala wrote:
>> >> commit 05a2fb157e44a53c79133805d30eaada43911941
>> >> Author: Mika Kuoppala <mika.kuoppala@linux.intel.com>
>> >> Date:   Mon Jan 19 16:20:43 2015 +0200
>> >> 
>> >>     drm/i915: Consolidate forcewake code
>> >> 
>> >> introduced domain handling where each domain has it's own posting
>> >> read registers. This changed the forcewake sequence on 'put' side when
>> >> there is multiple domains as there would be extra read between the domain
>> >> puts. Any posting read should be enough to flush all the changes.
>> >> 
>> >> Do a posting read only once, at the end of the sequence and for
>> >> the first domain. Like it was before.
>> >> 
>> >> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>> >> Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
>> >
>> > fwiw, I would argue that the posting read in _get() is superfluous as we
>> > will serialise the fw with not only the ack, but any subsequent mmio.
>> >
>> > On the _put() side we do want to flush the write so that the hw can
>> > power down as early as possible. So just kill the posting read from _get
>> > and otherwise drop the patch. :)
>> 
>> Yes, both put/get patches should be dropped. I posted a patch removing
>> the posting read on get side and with your explanations in commit message.
>> 
>> This all starts to make so much sense that some gen is bound to break ;)
>
> IIRC the posting read from same cache line actually fixed real bugs. So
> I'm a bit worried about dropping them. But I suppose it's possible only
> the _put side was important for those bugs.

I found these:

commit 6af2d180f82151cf3d58952e35a4f96e45bc453a
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jul 26 16:24:50 2012 +0200

    drm/i915: fix forcewake related hangs on snb

commit 8dee3eea3ccd3b6c00a8d3a08dd715d6adf737dd
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Sat Sep 1 22:59:50 2012 -0700

    drm/i915: Never read FORCEWAKE

https://bugs.freedesktop.org/show_bug.cgi?id=51738
https://bugs.freedesktop.org/show_bug.cgi?id=52424

The snb here seems to survive gem_dummy_reloc_loop and
gem_ring_sync_loop in here with the get side posting removed.

-Mika

>
> -- 
> Ville Syrjälä
> Intel OTC
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