[PATCH] drm/core: Fail atomic IOCTL with no CRTC state but with signaling.

[Andrey Grodzovsky]

On 06/19/2017 03:24 PM, Sean Paul wrote:
> On Mon, Jun 19, 2017 at 11:35:28AM -0400, Harry Wentland wrote:
>> On 2017-06-09 05:30 PM, Andrey Grodzovsky wrote:
>>> Problem:
>>> While running IGT kms_atomic_transition test suite i encountered
>>> a hang in drmHandleEvent immidietly follwoing an atomic_commit.
>> s/immidietly/immediately/g
>> s/follwoing/following/g
>>
>>> 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.
>> s/singnaled/signaled/g
>>
>>> This point to a bug in IGT but also DRM should gracefully
>>> fail  such scenario so no hang on user side will happen.
>>>
>> Can we create an IGT fix for this to make sure this won't happen?
>>
>>> Fix:
>>> Explicitly fail by failing atomic_commit early in
>>> drm_mode_atomic_commit where such problem can be identified.
>>>
>> The change seems reasonable to me but I would like to see some input
>> from someone who's more familiar with the usermode side of things.
> I wonder if there's ever a case where it might be desirable to generate an event
> from a commit without a crtc. I don't know if anyone has explicitly said that an
> event can only be generated from state with crtc.
For a generic event i agree, bit seems to me without active CRTC no way you
can  expect PAGE_FLIP_EVENT to fire.
>
> I usually don't mind letting userspace shoot itself in the foot, so keep that in
> mind :)
>
> Sean

Seems to me you still would try to avoid a bad configuration, returning 
error
will help debugging for user who made a mistake. I also see something 
similar
in drm_mode_atomic_ioctl around line 2122 -

/* can't test and expect an event at the same time. */
if ((arg->flags & DRM_MODE_ATOMIC_TEST_ONLY) &&
(arg->flags & DRM_MODE_PAGE_FLIP_EVENT))
return -EINVAL;

Thanks,
Andrey
>>> Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky at amd.com>
>>> ---
>>>   drivers/gpu/drm/drm_atomic.c | 13 ++++++++++++-
>>>   1 file changed, 12 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
>>> index a567310..32eae1c 100644
>>> --- a/drivers/gpu/drm/drm_atomic.c
>>> +++ b/drivers/gpu/drm/drm_atomic.c
>>> @@ -1933,7 +1933,7 @@ static int prepare_crtc_signaling(struct drm_device *dev,
>>>   {
>>>   	struct drm_crtc *crtc;
>>>   	struct drm_crtc_state *crtc_state;
>>> -	int i, ret;
>>> +	int i, c = 0, ret;
>>>   
>>>   	if (arg->flags & DRM_MODE_ATOMIC_TEST_ONLY)
>>>   		return 0;
>>> @@ -1994,8 +1994,17 @@ static int prepare_crtc_signaling(struct drm_device *dev,
>>>   
>>>   			crtc_state->event->base.fence = fence;
>>>   		}
>>> +
>>> +		c++;
>> Not sure if intentional, but I like it.
>>
>>>   	}
>>>   
>>> +	/*
>>> +	 * Having this flag means user mode pends on event which will never
>>> +	 * reach due to lack of at least one CRTC for signaling
>>> +	 */
>>> +	if (c == 0 && (arg->flags & DRM_MODE_PAGE_FLIP_EVENT))
>>> +		return -EINVAL;
>>> +
>>>   	return 0;
>>>   }
>>>   
>>> @@ -2179,6 +2188,8 @@ int drm_mode_atomic_ioctl(struct drm_device *dev,
>>>   		drm_mode_object_unreference(obj);
>>>   	}
>>>   
>>> +
>>> +
>> Remove these extraneous newlines.
>>
>> Harry
>>
>>>   	ret = prepare_crtc_signaling(dev, state, arg, file_priv, &fence_state,
>>>   				     &num_fences);
>>>   	if (ret)
>>>

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