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 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?

> 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.

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


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