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

From: "Michel Dänzer" <michel@daenzer.net>
To: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: "daniel.vetter@ffwll.ch" <daniel.vetter@ffwll.ch>,
	"Olsak, Marek" <Marek.Olsak@amd.com>,
	"amd-gfx@lists.freedesktop.org" <amd-gfx@lists.freedesktop.org>,
	"manasi.d.navare@intel.com" <manasi.d.navare@intel.com>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	"Deucher, Alexander" <Alexander.Deucher@amd.com>,
	"Kazlauskas, Nicholas" <Nicholas.Kazlauskas@amd.com>
Subject: Re: [PATCH v4 3/4] drm: Document variable refresh properties
Date: Mon, 29 Oct 2018 17:37:49 +0100	[thread overview]
Message-ID: <481ae686-adda-5857-5538-2b0af2e25427@daenzer.net> (raw)
In-Reply-To: <20181026175924.GH9144@intel.com>

On 2018-10-26 7:59 p.m., Ville Syrjälä wrote:
> On Fri, Oct 26, 2018 at 05:34:15PM +0000, Kazlauskas, Nicholas wrote:
>> On 10/26/18 10:53 AM, Ville Syrjälä wrote:
>>>
>>> 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.
>>
>> No changes to vblank timestamping handling should be necessary to
>> accommodate variable refresh rate.
> 
> The problem is that the timestamp is supposed to correspond to the first
> active pixel. And since we don't know how long the front porch will be
> we can't realistically report the true value. So I guess just assuming
> min front porch length is as good as anything else?

That (and documenting that the timestamp corresponds to the earliest
possible first active pixel, not necessarily the actual one, with VRR)
might be good enough for the actual vblank event timestamps.

However, I'm not so sure about the timestamps of page flip completion
events. Those could be very misleading if the flip completes towards the
timeout, which could result in bad behaviour of applications which use
them for animation timing.

Maybe the timestamp could be updated appropriately (yes, I'm hand-waving
:) in drm_crtc_send_vblank_event?

-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer
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