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From: Thomas Zimmermann <tzimmermann@suse.de>
To: Daniel Vetter <daniel@ffwll.ch>, Emil Velikov <emil.l.velikov@gmail.com>
Cc: david@lechnology.com, oleksandr_andrushchenko@epam.com,
	Dave Airlie <airlied@linux.ie>, Sam Ravnborg <sam@ravnborg.org>,
	ML dri-devel <dri-devel@lists.freedesktop.org>,
	"open list:VIRTIO GPU DRIVER"
	<virtualization@lists.linux-foundation.org>,
	Hans de Goede <hdegoede@redhat.com>,
	Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	xen-devel@lists.xenproject.org,
	Emil Velikov <emil.velikov@collabora.com>,
	Sean Paul <sean@poorly.run>, Gerd Hoffmann <kraxel@redhat.com>
Subject: Re: [Xen-devel] [PATCH v2 4/4] drm/simple-kms: Let DRM core send VBLANK events by default
Date: Fri, 17 Jan 2020 08:17:10 +0100
Message-ID: <fa8a22b5-9c19-7896-5b0d-06b038ed4862@suse.de> (raw)
In-Reply-To: <20200116235951.GD8400@dvetter-linux.ger.corp.intel.com>

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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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Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-15 12:52 [Xen-devel] [PATCH v2 0/4] Use no_vblank property for drivers without VBLANK Thomas Zimmermann
2020-01-15 12:52 ` [Xen-devel] [PATCH v2 1/4] drm: Document struct drm_crtc_state.no_vblank for faking VBLANK events Thomas Zimmermann
2020-01-15 12:52 ` [Xen-devel] [PATCH v2 2/4] drm/ast: Set struct drm_crtc_state.no_vblank in atomic_check() Thomas Zimmermann
2020-01-15 12:52 ` [Xen-devel] [PATCH v2 3/4] drm/cirrus: Let DRM core send VBLANK events Thomas Zimmermann
2020-01-15 12:52 ` [Xen-devel] [PATCH v2 4/4] drm/simple-kms: Let DRM core send VBLANK events by default Thomas Zimmermann
2020-01-16  6:41   ` Daniel Vetter
2020-01-16  7:37     ` Thomas Zimmermann
2020-01-16 17:22       ` Emil Velikov
2020-01-16 23:59         ` Daniel Vetter
2020-01-17  7:17           ` Thomas Zimmermann [this message]
2020-01-22  8:11             ` Daniel Vetter
2020-01-22  8:20               ` Thomas Zimmermann
2020-01-15 13:04 ` [Xen-devel] [PATCH v2 0/4] Use no_vblank property for drivers without VBLANK Hans de Goede
  -- strict thread matches above, loose matches on Subject: below --
2020-01-13 10:38 Thomas Zimmermann
2020-01-13 10:38 ` [Xen-devel] [PATCH v2 4/4] drm/simple-kms: Let DRM core send VBLANK events by default Thomas Zimmermann

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