[Intel-gfx] [PATCH 2/2] drm/i915: Support for HDMI complaince HPD

[Chris Wilson]

On Wed, Aug 13, 2014 at 11:34:02AM +0530, Sharma, Shashank wrote:
> > I know. That is orthogonal to the tweaks I was suggesting. Also if you
> > feel you need to add details to your rationale, then your changelog is
> > incomplete.
> > -Chris
> 
> Thanks Chris,
> Please find my comments inline to your previous mail, with suggestions.
> 
> On 8/12/2014 6:17 PM, Chris Wilson wrote:
> >On Tue, Aug 12, 2014 at 06:08:21PM +0530, shashank.sharma at intel.com wrote:
> >>From: Shashank Sharma <shashank.sharma at intel.com>
> >>
> >>During the HDMI complaince tests, most of the HDMI analyzers
> >>issue a soft HPD, to automate the tests. This process keeps
> >>the HDMI cable connected, and DDC chhanel alive.
> >>
> >>HDMI detect() logic relies on EDID readability, to decide if
> >>its a HDMI connect or disconnect event. But in this case, the
> >>EDID is always readable, as HDMI cable is physically connected
> >>and DDC channel is UP, so detect() always reports a HDMI connect
> >>even if its intended to be a HDMI disconnect call.
> >>
> >>So if HDMI compliance is enabled, we should rely on the live status
> >>register, than EDID availability. This patch adds:
> >>1. One kernel command line parameter for i915 module, which indicates
> >>    if we want to support HDMI compliance, for this platform.
> >
> >I would rather have this as an output property. In fact, I would like
> >the hotplug detection method exposed (and even selectable, but other
> >than this compliance testing, I can't think of a scenario where the
> >kernel shouldn't be able to figure things out for itself).
> Considering the history of the case, can you please elaborate this
> suggestion ? I dont think I am getting it right.

Instead of (or in addition to) adding a kernel parameter, you add an
output property so that it can be adjusted on the fly for individual
monitors.

> >
> >>2. A new function to read EDID, which gets called only in case of
> >>    HDMI compliance support is required. This function reads EDID only
> >>    if live status register is up. The normal code flow doesn't get effected
> >>    if kernel command line parameter is not enabled.
> >>3. After various experiments on VLV2 board, with various HDMI monitors
> >>    we have seen that, with few monitors, the live status register gets
> >>    set after a slight delay, and then stays reliably. To support such
> >>    monitors, there is a busy-loop added, with a max delay upto 50ms,
> >>    with a status check after every 10ms. Please see the comment in
> >>    intel_hdmi_get_edid.
> >
> >Wouldn't a tidier solution be to delay the hpd by 50-100ms after the
> >hotplug interrupt? That may overcome the issue with the live status for
> >all connectors...
> >-Chris
> >
> There would be few problems in this case:
> 1. We dont want this scenario to come into picture for DP, as DP HPD
>    pulse can be as small as 2ms.

If the live status is asserted and deasserted within 2ms, do you care?
Or perhaps you are talking about something else entirely.

> 2. Not all the HDMI monitors show this problem, but a significant
>    subset of popular monitors do.
> 3. In HDCP compliance testing, we send a HPD pulse train of UP and
>    Down, where down pulse can be as small as 100ms. If we increase the
>    delay by 100ms, we will definitely miss the HPD down pulse.

And? If the monitor is only plugged in for less than 0.1s do I really
want to waste 1-2s of user time reconfiguring the desktop and
applications before undoing all the changes?

There is no point in compliance testing if it does not actually test the
code going to be used.

> 4. The method what we are using is a busy waiting check, where we delay
>    the pulse for 50ms, but take a sample of live_status per 10ms, so if
>    the live status is up with a delay of 20ms, we needn't to waste
>    another 30.

Yes. You block using 100% of the cpu in an uninterruptable context for a
significant period of time. DO NOT DO THIS.

> 5. We want this code routine only to be executed for commercial (like
>    android) platforms, whereas others get the routine code.

In other words, you want to ignore years of real world compatibity testing
and larger user bases.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre