[PATCH v2 3/6] drm/panel-simple: Support hpd-gpios for delaying prepare()

[Stephen Boyd]

Quoting Douglas Anderson (2020-04-20 22:06:19)
> People use panel-simple when they have panels that are builtin to
> their device.  In these cases the HPD (Hot Plug Detect) signal isn't
> really used for hotplugging devices but instead is used for power
> sequencing.  Panel timing diagrams (especially for eDP panels) usually
> have the HPD signal in them and it acts as an indicator that the panel
> is ready for us to talk to it.
> 
> Sometimes the HPD signal is hooked up to a normal GPIO on a system.
> In this case we need to poll it in the correct place to know that the
> panel is ready for us.  In some system designs the right place for
> this is panel-simple.
> 
> When adding this support, we'll account for the case that there might
> be a circular dependency between panel-simple and the provider of the
> GPIO.  The case this was designed for was for the "ti-sn65dsi86"
> bridge chip.  If HPD is hooked up to one of the GPIOs provided by the
> bridge chip then in our probe function we'll always get back
> -EPROBE_DEFER.  Let's handle this by allowing this GPIO to show up
> late if we saw -EPROBE_DEFER during probe.

May be worth mentioning that if there isn't an hpd-gpios property then
we only try once during probe and then after that the prepare callback
doesn't try again because the gpio_get_optional() APIs are used. I had
to think about that for a minute.

> 
> Signed-off-by: Douglas Anderson <dianders at chromium.org>
> ---

Reviewed-by: Stephen Boyd <swboyd at chromium.org>