[Bug 47007] HDMI monitor polling causing 100ms rendering stalls

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=47007

--- Comment #8 from Alex Deucher <agd5f at yahoo.com> 2012-03-07 12:03:07 PST ---
(In reply to comment #7)
> Poll helper is running every ten seconds because VGA connector "asks it to",
> given how it has DRM_CONNECTOR_POLL_CONNECT set.

Right.  The VGA connector has to poll because analog VGA does not support
hotplug detect.

> 
> Since the poll helper runs, and HDMI connector has DRM_CONNECTOR_POLL_HPD set
> due to hpd.hpd being not RADEON_HPD_NONE, it calls it's detect method
> (radeon_dvi_detect).
> 
> This in turns discards an re-fetches the full EDID every time it runs.
> 
> What is the bug here?
> 
> 1. That HDMI connector has DRM_CONNECTOR_POLL_HPD set when hpd.hpd is not
> RADEON_HPD_NONE?

That's correct.  If the connector supports an hotplug pin (hpd.hpd !=
RADEON_HPD_NONE), there is no need for the driver to poll every 10 seconds
because the hw will generate an interrupt on a plug or unplug event.  When an
interrupt arrives, the the poll handler runs to see if the monitor is still
there and to send an event to userspace (same poll handler that is run by the
driver for non-HPD capable connectors).

> 2. That discard and full fetch of EDID is done when is not needed?
> 
> Or both? Or something else? Hard to say for me. HPD sounds like Hotplug detect?

Yes, HPD stands for hotplug detect.

Does your monitor have multiple inputs?  Some monitors have an option to poll
their inputs at regular intervals.  When they do that, it often causes a
disconnect/connect signal on the HPD pin which generates an interrupt and the
driver's detect logic runs.  Try and disable the monitor's polling logic if you
can and see if that helps.

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