linux-kernel - Re: [PATCH v2 3/6] drm/panel-simple: Support hpd-gpios for delaying prepare()

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Message-ID: <158758994028.230545.10042873479857418029@swboyd.mtv.corp.google.com>
Date:   Wed, 22 Apr 2020 14:12:20 -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>,
        Sam Ravnborg <sam@...nborg.org>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/6] drm/panel-simple: Support hpd-gpios for delaying prepare()

Quoting Douglas Anderson (2020-04-20 22:06:19)
> People use panel-simple when they have panels that are builtin to
> their device.  In these cases the HPD (Hot Plug Detect) signal isn't
> really used for hotplugging devices but instead is used for power
> sequencing.  Panel timing diagrams (especially for eDP panels) usually
> have the HPD signal in them and it acts as an indicator that the panel
> is ready for us to talk to it.
> 
> Sometimes the HPD signal is hooked up to a normal GPIO on a system.
> In this case we need to poll it in the correct place to know that the
> panel is ready for us.  In some system designs the right place for
> this is panel-simple.
> 
> When adding this support, we'll account for the case that there might
> be a circular dependency between panel-simple and the provider of the
> GPIO.  The case this was designed for was for the "ti-sn65dsi86"
> bridge chip.  If HPD is hooked up to one of the GPIOs provided by the
> bridge chip then in our probe function we'll always get back
> -EPROBE_DEFER.  Let's handle this by allowing this GPIO to show up
> late if we saw -EPROBE_DEFER during probe.

May be worth mentioning that if there isn't an hpd-gpios property then
we only try once during probe and then after that the prepare callback
doesn't try again because the gpio_get_optional() APIs are used. I had
to think about that for a minute.

> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---

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