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From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Subject: Re: Precise vblank timestamping for VC4 kms.
Date: Thu, 23 Jun 2016 16:18:59 +0300	[thread overview]
Message-ID: <20160623131859.GH4329@intel.com> (raw)
In-Reply-To: <1466662670-9884-1-git-send-email-mario.kleiner.de@gmail.com>

On Thu, Jun 23, 2016 at 08:17:49AM +0200, Mario Kleiner wrote:
> The following patch implements precise vblank timestamping
> for RaspberryPi's VC4, at least for standard progressive
> scan display modes.
> 
> It has been tested on the HDMI output with half a dozen different
> video modes using special hardware measurement equipment to compare
> generated time stamps against reality. According to the tests it
> works well in its current form.
> 
> Due to hw limitations of the VC4, timestamps can't be scanline
> accurate when taken within vblank, as explained in the patch,
> but at least they will never be off by more than 1 vblank
> duration, and are typically still accurate to ~0.1 msecs
> for the common case when the timestamping is triggered from
> vblank interrupt.
> 
> The patch exposed some problems with how the drm core handles
> calculation of vblank timestamping constants for interlaced
> video modes in drm_calc_timestamping_constants(). Seems it cuts
> the expected frame duration framedur_ns into half for interlaced
> modes two times, so it ends up expecting a field duration half
> of what it should be and then miscalculates vblank counter increments
> as soon as vblank timestamping is supported and the core tries to
> derive vblank counts from it. To work around this bug, for the
> moment the vblank timestamping will disable itself for interlaced
> modes and only work for regular progressive scan.

The code should be correct for i915. The framedur_ns is actually the
duration of a single field (since we get a vblank interrupt for each
field). But it's going to be wrong for anyone that populates the
crtc_ timings with CRTC_INTERLACE_HALVE_V. So to fix it, you'd need
the driver to tell drm_calc_timestamping_constants() whether the
vertical timings were already halved or not.

-- 
Ville Syrjälä
Intel OTC
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      parent reply	other threads:[~2016-06-23 13:19 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-06-23  6:17 Precise vblank timestamping for VC4 kms Mario Kleiner
2016-06-23  6:17 ` [PATCH] drm/vc4: Implement precise vblank timestamping Mario Kleiner
2016-07-08  2:31   ` Eric Anholt
2016-07-08 18:44   ` [PATCH 1/2] drm/vc4: Bind the HVS before we bind the individual CRTCs Eric Anholt
2016-07-08 18:44     ` Eric Anholt
2016-07-08 18:44     ` [PATCH 2/2] drm/vc4: Squash commit for Mario's precise vblank timestamping Eric Anholt
2016-07-08 18:44       ` Eric Anholt
2016-07-09 17:09       ` Mario Kleiner
2016-07-09 17:09         ` Mario Kleiner
2016-07-10 16:05         ` Eric Anholt
2016-07-10 16:05           ` Eric Anholt
2016-06-23  7:28 ` Precise vblank timestamping for VC4 kms Daniel Vetter
2016-06-27 10:31   ` Mario Kleiner
2016-06-23 13:18 ` Ville Syrjälä [this message]

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