linux-kernel - Re: [RFC PATCH] drm/panel: Make backlight attachment lazy

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Date:   Tue, 8 Dec 2020 13:47:56 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Sam Ravnborg <sam@...nborg.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] drm/panel: Make backlight attachment lazy

On Mon, Dec 07, 2020 at 10:44:46PM -0600, Bjorn Andersson wrote:
> Some bridge chips, such as the TI SN65DSI86 DSI/eDP bridge, provides
> means of generating a PWM signal for backlight control of the attached
> panel. The provided PWM chip is typically controlled by the
> pwm-backlight driver, which if tied to the panel will provide DPMS.
> 
> But with the current implementation the panel will refuse to probe
> because the bridge driver has yet to probe and register the PWM chip,
> and the bridge driver will refuse to probe because it's unable to find
> the panel.

What you're describing is basically a circular dependency. Can't we get
rid of that in some other way? Why exactly does the bridge driver refuse
to probe if the panel can't be found?

In other words, I see how the bridge would /use/ the panel in that it
forward a video stream to it. But how does the panel /use/ the bridge?

Thierry

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