CEC blocks idle on omap4

[Hans Verkuil]

On 3/25/19 4:55 PM, Laurent Pinchart wrote:
> Hi Hans,
> 
> On Mon, Mar 25, 2019 at 04:51:57PM +0100, Hans Verkuil wrote:
>> 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.
>>
>> 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.
>>
>> 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.
> 
> What's the periodicity of those commands ? Can the system be put to
> sleep and get woken up when there's CEC activity ?

You don't control that. The sink can transmit a CEC message at any
time and the omap4 CEC adapter has to be active to correctly react.

> 
>> 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?
> 
> This should be controllable by userspace. From a product point of view,
> it should be possible to put the system in a deep sleep state where CEC
> isn't available, or in a low sleep state where CEC works as expected.
> 

Userspace can always disable CEC. The hdmi_cec_adap_enable() callback in
hdmi4_cec.c is called whenever the CEC adapter is enabled or disabled.

I'm not actually sure why hdmi4_cec_init() would block anything since it
just registers the CEC device. It does not enable it until userspace
explicitly enables it with e.g. 'cec-ctl --playback'.

hdmi4_cec_init() does configure a CEC clock, but that can be moved to
hdmi_cec_adap_enable() if necessary.

Note that I am not sure what is meant with 'SoC core retention idle',
so perhaps I just misunderstand the problem.

Regards,

	Hans