CEC blocks idle on omap4

[Tony Lindgren]

* Hans Verkuil <hverkuil at xs4all.nl> [190325 15:52]:
> Hi Tony,
> 
> On 3/25/19 4:32 PM, Tony Lindgren wrote:
> > Hi Hans,
> > 
> > Looks like CONFIG_OMAP4_DSS_HDMI_CEC=y blocks SoC core retention
> > idle on omap4 if selected.
> > 
> > Should we maybe move hdmi4_cec_init() to hdmi_display_enable()
> > and hdmi4_cec_uninit() to hdmi_display_disable()?
> > 
> > Or add some enable/disable calls in addtion to the init and
> > uninit calls that can be called from hdmi_display_enable()
> > and hdmi_display_disable()?
> 
> For proper HDMI CEC behavior the CEC adapter has to remain active
> even if the HPD of the display is low. Some displays pull down the
> HPD when in standby, but CEC can still be used to wake them up.

OK

> And we see this more often as regulations for the maximum power
> consumption of displays are getting more and more strict.
> 
> So disabling CEC when the display is disabled is not an option.

OK

> Disabling CEC if the source is no longer transmitting isn't a good
> idea either since the display will typically still send periodic
> CEC commands to the source that it expects to reply to.

Hmm I wonder if we could test for HPD floating and then disable
cec? Then we would know nothing is connected, right?

> The reality is that HDMI CEC and HDMI video are really independent of
> one another. So I wonder if it isn't better to explain the downsides
> of enabling CEC for the omap4 in the CONFIG_OMAP4_DSS_HDMI_CEC
> description. And perhaps disable it by default?

>From distro kernel point of view we should find some way to
disable cec dynamically. Can we maybe do something via /sys or
some command line option for cec.ko?

Then if we find a way to check for floating HPD pin or something
we could support that too.

Regards,

Tony