linux-kernel - Re: [PATCH v3 3/6] drm/panel-simple: Support hpd-gpios for delaying prepare()

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Date:   Tue, 28 Apr 2020 14:21:22 +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>,
        Sam Ravnborg <sam@...nborg.org>,
        Thierry Reding <thierry.reding@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 3/6] drm/panel-simple: Support hpd-gpios for delaying prepare()

On Thu, Apr 23, 2020 at 6:26 PM Douglas Anderson <dianders@...omium.org> wrote:

> 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.  NOTE: since the
> gpio_get_optional() is used, if the "hpd-gpios" isn't there our
> variable will just be NULL and we won't do anything in prepare().
>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> Reviewed-by: Stephen Boyd <swboyd@...omium.org>
> ---
>
> Changes in v3:
> - Remind how gpio_get_optional() works in the commit message.

Reviewed-by: Linus Walleij <linus.walleij@...aro.org>

I have a small inkling to protest against calling this driver "panel-simple"
as we tend to stockpile things like this.

I suppose panel-panacea.c is a better name at this point :/

Yours,
Linus Walleij