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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Tue, 28 Apr 2020 14:21:22 +0200
Message-ID: <CACRpkdZEhqaiStFPdg3VOamKnCMjMsj+MMXimqmHW6eSGah+nQ@mail.gmail.com>
Subject: Re: [PATCH v3 3/6] drm/panel-simple: Support hpd-gpios for delaying prepare()
To:     Douglas Anderson <dianders@chromium.org>
Cc:     Bartosz Golaszewski <bgolaszewski@baylibre.com>,
        Dave Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Rob Herring <robh+dt@kernel.org>,
        Neil Armstrong <narmstrong@baylibre.com>,
        Andrzej Hajda <a.hajda@samsung.com>,
        Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
        Sandeep Panda <spanda@codeaurora.org>,
        Stephen Boyd <swboyd@chromium.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        Jeffrey Hugo <jeffrey.l.hugo@gmail.com>,
        "open list:DRM PANEL DRIVERS" <dri-devel@lists.freedesktop.org>,
        MSM <linux-arm-msm@vger.kernel.org>,
        Rob Clark <robdclark@chromium.org>,
        Jernej Skrabec <jernej.skrabec@siol.net>,
        Jonas Karlman <jonas@kwiboo.se>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Sam Ravnborg <sam@ravnborg.org>,
        Thierry Reding <thierry.reding@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Thu, Apr 23, 2020 at 6:26 PM Douglas Anderson <dianders@chromium.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@chromium.org>
> Reviewed-by: Stephen Boyd <swboyd@chromium.org>
> ---
>
> Changes in v3:
> - Remind how gpio_get_optional() works in the commit message.

Reviewed-by: Linus Walleij <linus.walleij@linaro.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