[Intel-gfx] [PATCH] drm/i915: Revert async unpin and nonblocking atomic commit

[Maarten Lankhorst]

Op 25-05-16 om 09:35 schreef Daniel Vetter:
> On Wed, May 25, 2016 at 08:57:49AM +0200, Maarten Lankhorst wrote:
>> Op 24-05-16 om 17:49 schreef Daniel Vetter:
>>> This reverts the following patches:
>>>
>>> d55dbd06bb5e1399aba9ab5227465339d1bbefff drm/i915: Allow nonblocking update of pageflips.
>>> 15c86bdb760185e871c7a0f559978328aa500971 drm/i915: Check for unpin correctness.
>>> 95c2ccdc82d520f59ae3b6fdc097b63c9b7082bb Reapply "drm/i915: Avoid stalling on pending flips for legacy cursor updates"
>>> a6747b7304a9d66758a196d885dab8bbfa5e7d1f drm/i915: Make unpin async.
>>> 03f476e1fcb42fca88fc50b94b0d3adbdbe887f0 drm/i915: Prepare connectors for nonblocking checks.
>>> 2099deffef4404f949ba1b68d2b17e0608190bc2 drm/i915: Pass atomic states to fbc update functions.
>>> ee7171af72c39c18b7d7571419a4ac6ca30aea66 drm/i915: Remove reset_counter from intel_crtc.
>>> 2ee004f7c59b2e642f0bb2834f847d756f2dd7b7 drm/i915: Remove queue_flip pointer.
>>> b8d2afae557dbb9b9c7bc6f6ec4f5278f3c4c34e drm/i915: Remove use_mmio_flip kernel parameter.
>>> 8dd634d922615ec3a9af7976029110ec037f8b50 drm/i915: Remove cs based page flip support.
>>> 143f73b3bf48c089b40f58462dd7f7c199fd4f0f drm/i915: Rework intel_crtc_page_flip to be almost atomic, v3.
>>> 84fc494b64e8c591be446a966b7447a9db519c88 drm/i915: Add the exclusive fence to plane_state.
>>> 6885843ae164e11f6c802209d06921e678a3f3f3 drm/i915: Convert flip_work to a list.
>>> aa420ddd8eeaa5df579894a412289e4d07c2fee9 drm/i915: Allow mmio updates on all platforms, v2.
>>> afee4d8707ab1f21b7668de995be3a5961e83582 Revert "drm/i915: Avoid stalling on pending flips for legacy cursor updates"
>>>
>>> "drm/i915: Allow nonblocking update of pageflips" should have been
>>> split up, misses a proper commit message and seems to cause issues in
>>> the legacy page_flip path as demonstrated by kms_flip.
>>>
>>> "drm/i915: Make unpin async" doesn't handle the unthrottled cursor
>>> updates correctly, leading to an apparent pin count leak. This is
>>> caught by the WARN_ON in i915_gem_object_do_pin which screams if we
>>> have more than DRM_I915_GEM_OBJECT_MAX_PIN_COUNT pins.
>>>
>>> Unfortuantely we can't just revert these two because this patch series
>>> came with a built-in bisect breakage in the form of temporarily
>>> removing the unthrottled cursor update hack for legacy cursor ioctl.
>>> Therefore there's no other option than to revert the entire pile :(
>>>
>>> There's one tiny conflict in intel_drv.h due to other patches, nothing
>>> serious.
>>>
>>> Normally I'd wait a bit longer with doing a maintainer revert, but
>>> since the minimal set of patches we need to revert (due to the bisect
>>> breakage) is so big, time is running out fast. And very soon
>>> (especially after a few attempts at fixing issues) it'll be really
>>> hard to revert things cleanly.
>>>
>>> Lessons learned:
>>> - Not a good idea to rush the review (done by someone fairly new to
>>>   the area) and not make sure domain experts had a chance to read it.
>>>
>>> - Patches should be properly split up. I only looked at the two
>>>   patches that should be reverted in detail, but both look like the
>>>   mix up different things in one patch.
>>>
>>> - Patches really should have proper commit messages. Especially when
>>>   doing more than one thing, and especially when touching critical and
>>>   tricky core code.
>>>
>>> - Building a patch series and r-b stamping it when it has a built-in
>>>   bisect breakage is not a good idea.
>>>
>>> - I also think we need to stop building up technical debt by
>>>   postponing atomic igt testcases even longer. I think it's clear that
>>>   there's enough corner cases in this beast that we really need to
>>>   have the testcases _before_ the next step lands.
>> Acked-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Ok applied&pushed with irc acks from Dave, Jani&Ville added.
>
>> It should be possible to make avoid stalling work without much issue after reworking intel_crtc_page_flip to be almost atomic,
>> but it would be better to edit the mmio updates on all platforms patch for lesson #4..
> Yeah, this entire story would have been much less tricky without that.
> -Daniel

Yeah, I've changed page flip to prevent breakage now, and testing mmio flips on trybot first before submitting again.
Should prevent breakage along the way. The patch that did the revert was originally added because the legacy cursor
update was part of my tree, but Chris Wilson hit and submitted a patch for upstream too. It was easier to just revert
since the fix was in my tree anyway, and I submitted it around the same time. Of course with the long review times it
the patches should just have the fix integrated in the first place.

~Maarten