[PATCH] drm/sun4i: hdmi: Improve compatibility with non-hotplug capable connectors

[Maxime Ripard]

On Mon, Nov 19, 2018 at 10:26:38AM +0000, Russell King - ARM Linux wrote:
> On Mon, Nov 19, 2018 at 09:19:34AM +0100, Maxime Ripard wrote:
> > Hi,
> > 
> > On Fri, Nov 16, 2018 at 07:18:29PM +0200, Priit Laes wrote:
> > > From: Priit Laes <priit.laes at paf.com>
> > > 
> > > Even though HDMI connector features hotplug detect pin (HPD), there are
> > > devices that which do not support it.
> > 
> > Which devices?
> > 
> > > For these devices fall back to additional check on I2C bus. Of
> > > course, there might be also devices that do not wire DDC pins too,
> > > so we don't really know whether cable has been connected.
> > 
> > Again, which devices?
> > 
> > > 
> > > Signed-off-by: Priit Laes <plaes at plaes.org>
> > > Signed-off-by: Priit Laes <priit.laes at paf.com>
> > 
> > You only need one :)
> > 
> > > ---
> > >  drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c | 12 ++++++++----
> > >  1 file changed, 8 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c b/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
> > > index 061d2e0d9011..bded09af1340 100644
> > > --- a/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
> > > +++ b/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
> > > @@ -238,14 +238,18 @@ sun4i_hdmi_connector_detect(struct drm_connector *connector, bool force)
> > >  	struct sun4i_hdmi *hdmi = drm_connector_to_sun4i_hdmi(connector);
> > >  	unsigned long reg;
> > >  
> > > -	if (readl_poll_timeout(hdmi->base + SUN4I_HDMI_HPD_REG, reg,
> > > +	if (!readl_poll_timeout(hdmi->base + SUN4I_HDMI_HPD_REG, reg,
> > >  			       reg & SUN4I_HDMI_HPD_HIGH,
> > >  			       0, 500000)) {
> > > -		cec_phys_addr_invalidate(hdmi->cec_adap);
> > > -		return connector_status_disconnected;
> > > +		return connector_status_connected;
> > >  	}
> > >  
> > > -	return connector_status_connected;
> > > +	if (!IS_ERR(hdmi->i2c) && drm_probe_ddc(hdmi->i2c))
> > > +		return connector_status_connected;
> > > +
> > > +	cec_phys_addr_invalidate(hdmi->cec_adap);
> > > +
> > > +	return connector_status_unknown;
> > 
> > You're doing basically two things in that patch, first adding the
> > fallback to the DDC probe if the hotplug mechanism couldn't detect the
> > display, and then returning a status unknown if both fail.
> > 
> > While I don't really have an opinion on the first one, it's mandatory
> > for every HDMI device to be able to retrieve the EDID through the
> > DDC. If a device was to disallow that, it would violate the HDMI, and
> > I'm not sure we want to start supporting those devices.
> 
> There is also the problem that HDMI uses the HPD signal to indicate
> that the source should re-read the EDID due to the EDID changing.
> In HDMI, you don't necessarily have a fixed-for-all-time EDID, but
> one which can change depending on what devices are in the HDMI path.
> 
> Consider, for example, an AV amplifier which needs to subsitute the
> audio capabilities when it is turned on, but when in standby needs
> to pass through the TVs audio capabilities.  It informs the source
> by momentarily deasserting the HDMI HPD signal, which is the HDMI
> way to inform the source that the EDID should be re-read.
> 
> If you're going to use "read EDID" as the hotplug method, I think
> you need to keep track of when it changes so that EDID updates are
> correctly handled.

I didn't think about that, thanks for bringing it up!

Maxime

-- 
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
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