linux-kernel - Re: [PATCH v3 5/6] dt-bindings: drm/bridge: ti-sn65dsi86: Document no-hpd

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Message-ID: <CACRpkdZkRKgRo2-pxiyoz-3W_aoR+qb+AA-4+ZaPtt2Ykecs0w@mail.gmail.com>
Date:   Tue, 28 Apr 2020 14:25:42 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Dave Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Rob Herring <robh+dt@...nel.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Sandeep Panda <spanda@...eaurora.org>,
        Stephen Boyd <swboyd@...omium.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Jeffrey Hugo <jeffrey.l.hugo@...il.com>,
        "open list:DRM PANEL DRIVERS" <dri-devel@...ts.freedesktop.org>,
        MSM <linux-arm-msm@...r.kernel.org>,
        Rob Clark <robdclark@...omium.org>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Jonas Karlman <jonas@...boo.se>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 5/6] dt-bindings: drm/bridge: ti-sn65dsi86: Document no-hpd

On Thu, Apr 23, 2020 at 6:26 PM Douglas Anderson <dianders@...omium.org> 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.
>
> [1] https://lore.kernel.org/r/20200417180819.GE5861@pendragon.ideasonboard.com
>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> Reviewed-by: Stephen Boyd <swboyd@...omium.org>

Makes sense to me so:
Reviewed-by: Linus Walleij <linus.walleij@...aro.org>

> +  no-hpd:
> +    type: boolean
> +    description: Set if the HPD line on the bridge isn't hooked up to anything.

I would perhaps tag on:
... or is otherwise unusable?

Yours,
Linus Walleij