Re: [PATCH v4 3/4] drm: Document variable refresh properties

["Kazlauskas, Nicholas" <Nicholas.Kazlauskas@amd.com>]

On 10/26/18 3:13 PM, Manasi Navare wrote:
> On Fri, Oct 26, 2018 at 05:34:15PM +0000, Kazlauskas, Nicholas wrote:
>> On 10/26/18 10:53 AM, Ville Syrjälä wrote:
>>> On Fri, Oct 26, 2018 at 02:49:31PM +0000, Kazlauskas, Nicholas wrote:
>>>> On 10/26/18 7:37 AM, Pekka Paalanen wrote:
>>>>> On Thu, 11 Oct 2018 12:39:41 -0400
>>>>> Nicholas Kazlauskas <nicholas.kazlauskas@amd.com> wrote:
>>>>>
>>>>>> These include the drm_connector 'vrr_capable' and the drm_crtc
>>>>>> 'vrr_enabled' properties.
>>>>>>
>>>>>> Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
>>>>>> ---
>>>>>>     Documentation/gpu/drm-kms.rst   |  7 +++++++
>>>>>>     drivers/gpu/drm/drm_connector.c | 22 ++++++++++++++++++++++
>>>>>>     2 files changed, 29 insertions(+)
>>>>>>
>>>>>> diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
>>>>>> index 4b1501b4835b..8da2a178cf85 100644
>>>>>> --- a/Documentation/gpu/drm-kms.rst
>>>>>> +++ b/Documentation/gpu/drm-kms.rst
>>>>>> @@ -575,6 +575,13 @@ Explicit Fencing Properties
>>>>>>     .. kernel-doc:: drivers/gpu/drm/drm_atomic_uapi.c
>>>>>>        :doc: explicit fencing properties
>>>>>>     
>>>>>> +
>>>>>> +Variable Refresh Properties
>>>>>> +---------------------------
>>>>>> +
>>>>>> +.. kernel-doc:: drivers/gpu/drm/drm_connector.c
>>>>>> +   :doc: Variable refresh properties
>>>>>> +
>>>>>>     Existing KMS Properties
>>>>>>     -----------------------
>>>>>>     
>>>>>> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
>>>>>> index f0deeb7298d0..2a12853ca917 100644
>>>>>> --- a/drivers/gpu/drm/drm_connector.c
>>>>>> +++ b/drivers/gpu/drm/drm_connector.c
>>>>>> @@ -1254,6 +1254,28 @@ int drm_mode_create_scaling_mode_property(struct drm_device *dev)
>>>>>>     }
>>>>>>     EXPORT_SYMBOL(drm_mode_create_scaling_mode_property);
>>>>>>     
>>>>>> +/**
>>>>>> + * DOC: Variable refresh properties
>>>>>> + *
>>>>>> + * Variable refresh rate control is supported via properties on the
>>>>>> + * &drm_connector and &drm_crtc objects.
>>>>>> + *
>>>>>> + * "vrr_capable":
>>>>>> + *	Optional &drm_connector boolean property that drivers should attach
>>>>>> + *	with drm_connector_attach_vrr_capable_property() on connectors that
>>>>>> + *	could support variable refresh rates. Drivers should update the
>>>>>> + *	property value by calling drm_connector_set_vrr_capable_property().
>>>>>> + *
>>>>>> + *	Absence of the property should indicate absence of support.
>>>>>> + *
>>>>>> + * "vrr_enabled":
>>>>>> + *	Default &drm_crtc boolean property that notifies the driver that the
>>>>>> + *	variable refresh rate adjustment should be enabled for the CRTC.
>>>>>
>>>>> Hi,
>>>>>
>>>>> where is the documentation that explains how drivers must implement
>>>>> "variable refresh rate adjustment"?
>>>>>
>>>>> What should and could userspace expect to get if it flicks this switch?
>>>>>
>>>>> I also think the kernel documentation should include a description of
>>>>> what VRR actually is and how it conceptually works as far as userspace
>>>>> is concerned.
>>>>>
>>>>> That is, the kernel documentation should describe what this thing does,
>>>>> so that we avoid every driver implementing a different thing. For
>>>>> example, one driver could prevent the luminance flickering itself by
>>>>> tuning the timings while another driver might not do that, which means
>>>>> that an application tested on the former driver will look just fine
>>>>> while it is unbearable to watch on the latter.
>>>>>
>>>>>
>>>>> Thanks,
>>>>> pq
>>>>
>>>> I felt it was best to leave this more on the vague side to not impose
>>>> restrictions yet on what a driver must do.
>>>>
>>>> If you think it's worth defining what the "baseline" expectation is for
>>>> userspace, I would probably describe it as "utilizing the monitor's
>>>> variable refresh rate to reduce stuttering or tearing that can occur
>>>> during flipping". This is currently what the amdgpu driver has enabled
> 
> I would also mention that without VRR, the display engine processes the flips
> independently of the rendering speed which might result into stuttering or tearing.
> 
> Might also be worth giving a quick example with numbers in the documentation.
> Something like: For Eg if the rendering speed is 40Hz, without VRR, display engine
> will flip at constant 60Hz, which means that same frame will be displayed twice which
> will be observed as stutter/repetition.
> With VRR enabled, the vertical front porch will be extended and the flip will
> be processed when its ready or at max blanking time resulting a smooth transition without repetition.

Good points about mentioning the problems it solves in the documentation.

> 
>>>> for support. The implementation also isn't much more complex than just
>>>> passing the variable refresh range to the hardware.
>>>>
>>>> I wouldn't want every driver to be forced to implement some sort of
>>>> luminance flickering by default. It's not noticeable on many panels and
>>>> any tuning would inherently add latency to the output. It would probably
>>>> be better left as a new property or a driver specific feature.
>>>>
>>>> In general I would imagine that most future VRR features would end up as
>>>> new properties. Anything that's purely software could be implemented as
>>>> a drm helper that every driver can use. I think the target presentation
>>>> timestamp feature is a good example for that.
>>>
>>> Speaking of timestamps. What is the expected behaviour of vblank
>>> timestamps when vrr is enabled?
>>>
>>
>> When vrr is enabled the duration of the vertical front porch will be
>> extended until flip or timeout occurs. The vblank timestamp will vary
>> based on duration of the vertical front porch. The min/max duration for
>> the front porch can be specified by the driver via the min/max range.
> 
> This min max range that is read from monitor descriptor is only used to program
> the HW registers right? Its not exposed to the userspace, correct?
> 
> Manasi

Currently the range isn't exposed to userspace.

I think I wouldn't mind having them for testing purposes but that can 
just be done via debugfs. They might make more sense as properties but I 
don't have any integration patches to utilize them in userspace.

> 
>>
>> No changes to vblank timestamping handling should be necessary to
>> accommodate variable refresh rate.
>>
>> I think it's probably best to update the documentation for vrr_enable
>> with some of the specifics I described above. That should help clarify
>> userspace expectations as well.
>>
>> Nicholas Kazlauskas

Nicholas Kazlauskas
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