On Wed, May 13, 2020 at 02:59:02PM -0700, Douglas Anderson wrote: > The ti-sn65dsi86 MIPI DSI to eDP bridge chip has a dedicated hardware > HPD (Hot Plug Detect) pin on it, but it's mostly useless for eDP > because of excessive debouncing in hardware. Specifically there is no > way to disable the debouncing and for eDP debouncing hurts you because > HPD is just used for knowing when the panel is ready, not for > detecting physical plug events. > > Currently the driver in Linux just assumes that nobody has HPD hooked > up. It relies on folks setting the "no-hpd" property in the panel > node to specify that HPD isn't hooked up and then the panel driver > using this to add some worst case delays when turning on the panel. > > Apparently it's also useful to specify "no-hpd" in the bridge node so > that the bridge driver can make sure it's doing the right thing > without peeking into the panel [1]. This would be used if anyone ever > found it useful to implement support for the HW HPD pin on the bridge. > Let's add this property to the bindings. > > NOTES: > - This is somewhat of a backward-incompatible change. All current > known users of ti-sn65dsi86 didn't have "no-hpd" specified in the > bridge node yet none of them had HPD hooked up. This worked because > the current Linux driver just assumed that HPD was never hooked up. > We could make it less incompatible by saying that for this bridge > it's assumed HPD isn't hooked up _unless_ a property is defined, but > "no-hpd" is much more standard and it's unlikely to matter unless > someone quickly goes and implements HPD in the driver. > - It is sensible to specify "no-hpd" at the bridge chip level and > specify "hpd-gpios" at the panel level. That would mean HPD is > hooked up to some other GPIO in the system, just not the hardware > HPD pin on the bridge chip. I would say 'no-hpd' belongs wherever HPD is broken. So it may still make sense in the panel. (Otherwise, it needs to be removed from panel-common.yaml and some panel bindings, right?) > [1] https://lore.kernel.org/r/20200417180819.GE5861@xxxxxxxxxxxxxxxxxxxxxxxxxx > > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx> > Reviewed-by: Stephen Boyd <swboyd@xxxxxxxxxxxx> > Reviewed-by: Linus Walleij <linus.walleij@xxxxxxxxxx> > Reviewed-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> > --- In any case, Reviewed-by: Rob Herring <robh@xxxxxxxxxx>
