[PATCH v2 09/10] drm: bridge: dw_hdmi: Update EDID during hotplug processing

[Jonas Karlman]

On 2024-09-20 09:04, neil.armstrong at linaro.org wrote:
> On 19/09/2024 22:34, Jonas Karlman wrote:
>> Hi Neil,
>>
>> On 2024-09-13 10:02, Neil Armstrong wrote:
>>> On 08/09/2024 15:28, Jonas Karlman wrote:
>>>> Update successfully read EDID during hotplug processing to ensure the
>>>> connector diplay_info is always up-to-date.
>>>>
>>>> Signed-off-by: Jonas Karlman <jonas at kwiboo.se>
>>>> ---
>>>> v2: No change
>>>> ---
>>>>    drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 12 ++++++++++++
>>>>    1 file changed, 12 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
>>>> index c19307120909..7bd9f895f03f 100644
>>>> --- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
>>>> +++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
>>>> @@ -2457,6 +2457,18 @@ dw_hdmi_connector_detect(struct drm_connector *connector, bool force)
>>>>    
>>>>    	status = dw_hdmi_detect(hdmi);
>>>>    
>>>> +	/* Update EDID during hotplug processing (force=false) */
>>>> +	if (status == connector_status_connected && !force) {
>>>> +		const struct drm_edid *drm_edid;
>>>> +
>>>> +		drm_edid = dw_hdmi_edid_read(hdmi, connector);
>>>> +		if (drm_edid)
>>>> +			drm_edid_connector_update(connector, drm_edid);
>>>> +		cec_notifier_set_phys_addr(hdmi->cec_notifier,
>>>> +			connector->display_info.source_physical_address);
>>>> +		drm_edid_free(drm_edid);
>>>> +	}
>>>> +
>>>>    	if (status == connector_status_disconnected)
>>>>    		cec_notifier_phys_addr_invalidate(hdmi->cec_notifier);
>>>>    
>>>
>>> I wonder why we should read edid at each dw_hdmi_connector_detect() call,
>>> AFAIK it should only be when we have HPD pulses
>>
>> That is what this change intends to help do.
>>
>> As stated in the short comment EDID is only updated at HPD processing,
>> i.e. when force=false. To be on the safe side EDID is also only updated
>> here when connected and EDID could be read.
>>
>> drm_helper_probe_detect() is called with force=true in the
>> fill_modes/get_modes call path that is triggered by userspace
>> or the kernel kms client.
>>
>> After a HPD interrupt the call to drm_helper_hpd_irq_event() will call
>> check_connector_changed() that in turn calls drm_helper_probe_detect()
>> with force=false to check/detect if connector status has changed. It is
>> in this call chain the EDID may be read and updated in this detect ops.
>>
>> Reading EDID here at HPD processing may not be fully needed, however it
>> help kernel keep the internal EDID state in better sync with sink when
>> userspace does not act on the HOTPLUG=1 uevent.
> 
> 
> I understand but if somehow a dw-hdmi integration fails to have HDP working
> properly, EDID will be read continuously which is really not what we want.

I do not fully understand when or how that could happen.

The dw_hdmi_detect() -> phy.ops->read_hpd() call chain only return
connected status when HPD is high, for what I can see from all current
integrations.

The default dw_hdmi_phy_read_hpd() used by all but meson should only
return connected status when the HPD is high:

	return hdmi_readb(hdmi, HDMI_PHY_STAT0) & HDMI_PHY_HPD ?
		connector_status_connected : connector_status_disconnected;

DRM_CONNECTOR_POLL_HPD is also used for all integrations so there should
not be any polling happening that would trigger multiple detect calls.

I guess this could/should be improved to also check for the polled type
to not cause multiple unnecessary EDID reads, in case a future
integration need to poll connector status.

> 
> HDMI 1.4b specifies in Section 8.5 and Appendix A:
> ============><==========================================
> An HDMI Sink shall not assert high voltage level on its Hot Plug Detect pin when the E-EDID
> is not available for reading.
> An HDMI Sink shall indicate any change to the contents of the E-EDID by driving a low
> voltage level pulse on the Hot Plug Detect pin. This pulse shall be at least 100 msec.
> ============><==========================================
> 
> So this is OK with the first sentence, and should also work with the second one because
> right after the pulse we will read the EDID again, but I think we should have a much
> more robust way to detect those 100ms pulses, no ?

Using a work queue to debounce reacting on HPD event for >100 ms when
HPD goes from high to low and a few ms when it goes from low to high
could probably further prevent unnecessary detect calls, and also help
avoid a possible unnecessary disable/enable cycle.

I have not seen anything other in core that handles the EDID refresh in
any special way, but I may have missed something?

Will try to use a debounce work queue to delay any calls to
helper_hpd_irq_event() and drm_bridge_hpd_notify() and see if that could
improve the HPD handling.

Regards,
Jonas

> 
> Maxime, do you have an opinion on this ?
> 
> Neil
> 
>>
>> Regards,
>> Jonas
>>
>>>
>>> Neil
>>
>