linux-kernel - Re: [PATCH v6 3/3] dt-bindings: drm/bridge: ti-sn65dsi86: Document no-hpd

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Date:   Tue, 26 May 2020 16:22:22 -0600
From:   Rob Herring <robh@...nel.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     linus.walleij@...aro.org, bgolaszewski@...libre.com,
        airlied@...ux.ie, daniel@...ll.ch, narmstrong@...libre.com,
        a.hajda@...sung.com, Laurent.pinchart@...asonboard.com,
        spanda@...eaurora.org, bjorn.andersson@...aro.org,
        dri-devel@...ts.freedesktop.org, swboyd@...omium.org,
        devicetree@...r.kernel.org, jeffrey.l.hugo@...il.com,
        jernej.skrabec@...l.net, linux-arm-msm@...r.kernel.org,
        robdclark@...omium.org, jonas@...boo.se,
        linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 3/3] dt-bindings: drm/bridge: ti-sn65dsi86: Document
 no-hpd

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@pendragon.ideasonboard.com
> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> Reviewed-by: Stephen Boyd <swboyd@...omium.org>
> Reviewed-by: Linus Walleij <linus.walleij@...aro.org>
> Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> ---

In any case,

Reviewed-by: Rob Herring <robh@...nel.org>