linux-kernel - Re: [PATCH v5 17/20] drm/panel: panel-simple: Power the panel when reading the EDID

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Message-ID: <CAOw6vbLqDgqhXH8pUZqfhKCZk68SfzLf76Z4au3g83=Uw_8z2Q@mail.gmail.com>
Date:   Wed, 28 Apr 2021 12:44:45 -0400
From:   Sean Paul <seanpaul@...omium.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Sam Ravnborg <sam@...nborg.org>, Wolfram Sang <wsa@...nel.org>,
        robdclark@...omium.org,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        David Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Steev Klimaszewski <steev@...i.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Stanislav Lisovskiy <stanislav.lisovskiy@...el.com>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-i2c@...r.kernel.org,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 17/20] drm/panel: panel-simple: Power the panel when
 reading the EDID

On Fri, Apr 23, 2021 at 1:00 PM Douglas Anderson <dianders@...omium.org> wrote:
>
> I don't believe that it ever makes sense to read the EDID when a panel
> is not powered and the powering on of the panel is the job of
> prepare(). Let's make sure that this happens before we try to read the
> EDID. We use the pm_runtime functions directly rather than directly
> calling the normal prepare() function because the pm_runtime functions
> are definitely refcounted whereas it's less clear if the prepare() one
> is.
>
> NOTE: I'm not 100% sure how EDID reading was working for folks in the
> past, but I can only assume that it was failing on the initial attempt
> and then working only later. This patch, presumably, will fix that. If
> some panel out there really can read the EDID without powering up and
> it's a big advantage to preserve the old behavior we can add a
> per-panel flag. It appears that providing the DDC bus to the panel in
> the past was somewhat uncommon in any case.
>

Maybe some combination of drivers caching the EDID for panels while
they're already powered and overly broad pm runtime references?

At any rate, this makes sense to me,

Reviewed-by: Sean Paul <seanpaul@...omium.org>

> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> Reviewed-by: Bjorn Andersson <bjorn.andersson@...aro.org>
> ---
>
> (no changes since v1)
>
>  drivers/gpu/drm/panel/panel-simple.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c
> index 4de33c929a59..a12dfe8b8d90 100644
> --- a/drivers/gpu/drm/panel/panel-simple.c
> +++ b/drivers/gpu/drm/panel/panel-simple.c
> @@ -510,12 +510,18 @@ static int panel_simple_get_modes(struct drm_panel *panel,
>
>         /* probe EDID if a DDC bus is available */
>         if (p->ddc) {
> -               struct edid *edid = drm_get_edid(connector, p->ddc);
> +               struct edid *edid;
>
> +               pm_runtime_get_sync(panel->dev);
> +
> +               edid = drm_get_edid(connector, p->ddc);
>                 if (edid) {
>                         num += drm_add_edid_modes(connector, edid);
>                         kfree(edid);
>                 }
> +
> +               pm_runtime_mark_last_busy(panel->dev);
> +               pm_runtime_put_autosuspend(panel->dev);
>         }
>
>         /* add hard-coded panel modes */
> --
> 2.31.1.498.g6c1eba8ee3d-goog
>
> _______________________________________________
> dri-devel mailing list
> dri-devel@...ts.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

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