linux-kernel - Re: [PATCH v6 2/2] drm/bridge: hx8837: add a Himax HX8837 display controller driver

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Date:   Sat, 31 Oct 2020 21:56:17 +0100
From:   Sam Ravnborg <sam@...nborg.org>
To:     Lubomir Rintel <lkundrak@...sk>
Cc:     Andrzej Hajda <a.hajda@...sung.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Rob Herring <robh+dt@...nel.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Rob Herring <robh@...nel.org>
Subject: Re: [PATCH v6 2/2] drm/bridge: hx8837: add a Himax HX8837 display
 controller driver

Hi Lubomir.

> > 
> > > +	select BACKLIGHT_CLASS_DEVICE
> > Please use a depends - using select on a symbol with a prompt is always
> > wrong. Yeah, I know you then need to enable backlight to see this
> > driver. Sorry, but this is the best we can do now.
> > Many other drivers can cope with depends here.
> 
> This results in a dependency loop:
> 
>   drivers/video/fbdev/Kconfig:12:error: recursive dependency detected!
>   drivers/video/fbdev/Kconfig:12: symbol FB is selected by DRM_KMS_FB_HELPER
>   drivers/gpu/drm/Kconfig:80:     symbol DRM_KMS_FB_HELPER depends on DRM_KMS_HELPER
>   drivers/gpu/drm/Kconfig:74:     symbol DRM_KMS_HELPER is selected by DRM_HIMAX_HX8837
>   drivers/gpu/drm/bridge/Kconfig:51:      symbol DRM_HIMAX_HX8837 depends on BACKLIGHT_CLASS_DEVICE
>   drivers/video/backlight/Kconfig:143:    symbol BACKLIGHT_CLASS_DEVICE is selected by FB_BACKLIGHT
>   drivers/video/fbdev/Kconfig:187:        symbol FB_BACKLIGHT depends on FB
> 
> Unfortunately I have no idea how to resolve it at the moment.
> 
> I suppose I can look further into it if necessary. Or is it okay if I
> leave it at select BACKLIGHT_CLASS_DEVICE for now?

Sigh, leave it as a select then :-(
The "sigh" is not directed at you but the mess this
BACKLIGHT_CLASS_DEVICE is and the limitations of Kconfig.

	Sam