linux-kernel - Re: [PATCH v2 5/6] dt-bindings: drm/bridge: ti-sn65dsi86: Document no-hpd

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Message-ID: <158755120316.159702.16847202705854698366@swboyd.mtv.corp.google.com>
Date:   Wed, 22 Apr 2020 03:26:43 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Douglas Anderson <dianders@...omium.org>,
        Laurent.pinchart@...asonboard.com, a.hajda@...sung.com,
        airlied@...ux.ie, bgolaszewski@...libre.com, daniel@...ll.ch,
        linus.walleij@...aro.org, narmstrong@...libre.com,
        robh+dt@...nel.org, spanda@...eaurora.org
Cc:     jonas@...boo.se, jeffrey.l.hugo@...il.com,
        linux-gpio@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
        jernej.skrabec@...l.net, bjorn.andersson@...aro.org,
        robdclark@...omium.org, Douglas Anderson <dianders@...omium.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/6] dt-bindings: drm/bridge: ti-sn65dsi86: Document no-hpd

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@...omium.org>
> ---

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