Re: [PATCH v2 4/4] drm/simple-kms: Let DRM core send VBLANK events by default

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 22.01.20 um 09:11 schrieb Daniel Vetter:
> On Fri, Jan 17, 2020 at 08:17:10AM +0100, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 17.01.20 um 00:59 schrieb Daniel Vetter:
>>> On Thu, Jan 16, 2020 at 05:22:34PM +0000, Emil Velikov wrote:
>>>> Hi all,
>>>>
>>>> On Thu, 16 Jan 2020 at 07:37, Thomas Zimmermann <tzimmermann@suse.de> wrote:
>>>>
>>>>>> diff --git a/drivers/gpu/drm/drm_atomic_state_helper.c b/drivers/gpu/drm/drm_atomic_state_helper.c
>>>>>> index 7cf3cf936547..23d2f51fc1d4 100644
>>>>>> --- a/drivers/gpu/drm/drm_atomic_state_helper.c
>>>>>> +++ b/drivers/gpu/drm/drm_atomic_state_helper.c
>>>>>> @@ -149,6 +149,11 @@ void __drm_atomic_helper_crtc_duplicate_state(struct drm_crtc *crtc,
>>>>>>       /* Self refresh should be canceled when a new update is available */
>>>>>>       state->active = drm_atomic_crtc_effectively_active(state);
>>>>>>       state->self_refresh_active = false;
>>>>>> +
>>>>>> +     if (drm_dev_has_vblank(crtc->dev))
>>>>>> +             state->no_vblank = true;
>>>>>> +     else
>>>>>> +             state->no_vblank = false;
>>>>>>  }
>>>>>>  EXPORT_SYMBOL(__drm_atomic_helper_crtc_duplicate_state);
>>>>>
>>>>> I think the if/else branches are in the wrong order.
>>>
>>> Yeah fumbled that.
>>>
>>>>> But generally speaking, is it really that easy? The xen driver already
>>>>> has to work around simple-kms's auto-enabling of no_vblank (see patch
>>>>> 4). Maybe this settings interferes with other drivers as well. At least
>>>>> the calls for sending fake vblanks should be removed from all affected
>>>>> drivers.
>>>
>>> Hm xen is really special, in that it has a flip complete event, but not a
>>> vblank. I think forcing drivers to overwrite stuff in that case makes
>>> sense.
>>>
>>>> I'm not sure if setting no_vblank based on dev->num_crtcs is the correct thing.
>>>> From the original commit and associated description for no_vblank:
>>>>
>>>> In some cases CRTCs are active but are not able to generating events, at
>>>> least not at every frame at it's expected to.
>>>> This is typically the case when the CRTC is feeding a writeback connector...
>>>
>>> Yeah, but Thomas' series here wants to extend that. And I think if we roll
>>> this out the common case will be "no hw vblank", and the writeback special
>>
>> Default values should usually be 0 for zalloc and static initializers.
>> Should we rename no_vblank to has_vblank then?
> 
> Hm, imo feels like hw without vblank is still the uncommon case. I'd leave
> this as-is, but also no objections if you feel like repainting :-)

Not a bit. ;)

> 
>>> case is going to be the exception to the exception. Yup, patch 1 that
>>> updates the docs doesn't reflect that, which is why I'm bringing up more
>>> suggestions here around code & semantics of all these pieces to make them
>>> do the most reasonable thing for most of the drivers.
>>>
>>>> Reflects the ability of a CRTC to send VBLANK events....
>>>>
>>>>
>>>> The proposed handling of no_vblank feels a little dirty, although
>>>> nothing better comes to mind.
>>>> Nevertheless code seems perfectly reasonable, so if it were me I'd merge it.
>>>
>>> The idea with setting it very early is that drivers can overwrite it very
>>> easily. Feels slightly dirty, so I guess we could also set it somewhere in
>>> the atomic_helper_check function (similar to how we set the various
>>> crtc->*_changed flags, but we're not entirely consistent on these either).
>>>
>>> For the overall thing what feels irky to me is making this no_vblank
>>> default logic (however we end up computing it in the end, whether like
>>> this or what I suggested) specific to simple pipe helpers feels kinda
>>> wrong. Simple pipe tends to have a higher ratio of drivers for hw without
>>> vblank support, but by far not the only ones. Having that special case
>>> feels confusing to me (and likely will trip up some people, vblank and
>>> event handling is already a huge source of confusion in drm).
>>
>> Making it a default for simple KMS was only the start. I intended to
>> cover all drivers at some point. I just didn't want to go through all
>> drivers at once.
>>
>> I guess for the patchset's v3 I'll audit all drivers for the use of
>> no_blank and drm_crtc_send_vblank_event(); and convert the possible
>> candidates.
> 
> Yeah it's a pain, thanks for volunteering. Just figured the half-step here
> is too much in the uncanney valley. If we're going to polish this, let's
> do it right (and we have plenty enough drivers to make sure what we pick
> will be a solid choice I think).

I went through the drivers and updated them. It's the ones covered here
plus some more virtual HW. My search heuristic was to look for drivers
that call drm_crtc_send_vblank_event() but do not call
drm_vblank_init(). Please see v3 of this patchset. It should be pending
on this ML.

Best regards
Thomas

> -Daniel
> 
>>
>> Best regards
>> Thomas
>>
>>>
>>> One idea behind drm_dev_has_vblank() is also that we could formalize a bit
>>> all that, at least for the usual case - xen and maybe others being some
>>> exceptions as usual (hence definitely not something the core code should
>>> handle).
>>>
>>> Cheers, Daniel
>>>
>>
>> -- 
>> Thomas Zimmermann
>> Graphics Driver Developer
>> SUSE Software Solutions Germany GmbH
>> Maxfeldstr. 5, 90409 Nürnberg, Germany
>> (HRB 36809, AG Nürnberg)
>> Geschäftsführer: Felix Imendörffer
>>
> 
> 
> 
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer


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