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@xxxxxxxxxxxx> > --- Reviewed-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
