linux-kernel - Re: [PATCH v4 23/27] drm/panel: panel-simple: Power the panel when reading the EDID

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Date:   Fri, 23 Apr 2021 10:16:12 -0500
From:   Bjorn Andersson <bjorn.andersson@...aro.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>,
        Stephen Boyd <swboyd@...omium.org>, robdclark@...omium.org,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Stanislav Lisovskiy <stanislav.lisovskiy@...el.com>,
        Steev Klimaszewski <steev@...i.org>,
        linux-arm-msm@...r.kernel.org, Linus W <linus.walleij@...aro.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Thierry Reding <thierry.reding@...il.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 23/27] drm/panel: panel-simple: Power the panel when
 reading the EDID

On Fri 16 Apr 17:39 CDT 2021, Douglas Anderson 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.
> 

Reviewed-by: Bjorn Andersson <bjorn.andersson@...aro.org>

Regards,
Bjorn

> Signed-off-by: Douglas Anderson <dianders@...omium.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 c91e8aa108f7..40382c1be692 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 */
> -- 
> 2.31.1.368.gbe11c130af-goog
>