Subject: Re: [PATCH] drm/core: Fail atomic IOCTL with no CRTC state but with
 signaling.
To: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        intel-gfx@lists.freedesktop.org
References: <1497043819-28591-1-git-send-email-Andrey.Grodzovsky@amd.com>
 <7c001485-0485-5ba6-0be1-8738523d3fe5@linux.intel.com>
Cc: amd-gfx@lists.freedesktop.org, harry.wentland@amd.com
From: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Message-ID: <92a4ac19-ce78-47bd-f678-8ea84cc29d85@amd.com>
Date: Mon, 12 Jun 2017 10:12:23 -0400
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On 06/12/2017 07:08 AM, Maarten Lankhorst wrote:
> Op 09-06-17 om 23:30 schreef Andrey Grodzovsky:
>> Problem:
>> While running IGT kms_atomic_transition test suite i encountered
>> a hang in drmHandleEvent immidietly follwoing an atomic_commit.
>> After dumping the atomic state I relized that in this case there was
>> not even one CRTC attached to the state and only disabled
>> planes. This probably due to a commit which hadn't changed any property
>> which would require attaching crtc state. This means drmHandleEvent
>> will never wake up from read since without CRTC in atomic state
>> the event fd will not be singnaled.
>> This point to a bug in IGT but also DRM should gracefully
>> fail  such scenario so no hang on user side will happen.
>>
>> Fix:
>> Explicitly fail by failing atomic_commit early in
>> drm_mode_atomic_commit where such problem can be identified.
>>
>> Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
>> ---
>>   drivers/gpu/drm/drm_atomic.c | 13 ++++++++++++-
>>   1 file changed, 12 insertions(+), 1 deletion(-)
> Patch itself looks sane, but I'm worried about failing with -EINVAL when the same configuration with TEST_ONLY would otherwise succeed on it.
DRM_MODE_ATOMIC_TEST_ONLY flag is checked later, after this, so if this 
fails , test only will return EINVAL also.
> Not sure whether we should fail or not, since sending 0 events could still be considered success.
>
> I don't mind either way, but definitely something that should be discussed before applying.
Sending 0 events is no problem at all on it's own but if user provides 
DRM_MODE_PAGE_FLIP_EVENT in args
then when it becomes a problem , doesn't it mean he expects to receive 
at least one event ?

Thanks.