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    SubjectRe: [PATCH v2 5/6] dt-bindings: drm/bridge: ti-sn65dsi86: Document no-hpd
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    Quoting Douglas Anderson (2020-04-20 22:06:21)
    > 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 implement support for the HW HPD pin on the bridge.

    useful to implement?

    > 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.
    >
    > [1] https://lore.kernel.org/r/20200417180819.GE5861@pendragon.ideasonboard.com
    >
    > Signed-off-by: Douglas Anderson <dianders@chromium.org>
    > ---

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

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