linux-kernel - Re: [PATCH] fbdev: Have CONFIG_FB

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Date: Fri, 03 May 2024 21:45:21 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Florian Fainelli" <florian.fainelli@...adcom.com>,
 linux-kernel@...r.kernel.org
Cc: "Daniel Vetter" <daniel@...ll.ch>, "Helge Deller" <deller@....de>,
 "Thomas Zimmermann" <tzimmermann@...e.de>,
 "Javier Martinez Canillas" <javierm@...hat.com>,
 "Sam Ravnborg" <sam@...nborg.org>,
 "open list:FRAMEBUFFER LAYER" <linux-fbdev@...r.kernel.org>,
 "open list:FRAMEBUFFER LAYER" <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH] fbdev: Have CONFIG_FB_NOTIFY be tristate

On Fri, May 3, 2024, at 21:28, Florian Fainelli wrote:
> Android devices in recovery mode make use of a framebuffer device to
> provide an user interface. In a GKI configuration that has CONFIG_FB=m,
> but CONFIG_FB_NOTIFY=y, loading the fb.ko module will fail with:
>
> fb: Unknown symbol fb_notifier_call_chain (err -2)
>
> Have CONFIG_FB_NOTIFY be tristate, just like CONFIG_FB such that both
> can be loaded as module with fb_notify.ko first, and fb.ko second.
>
> Signed-off-by: Florian Fainelli <florian.fainelli@...adcom.com>

I see two problems here, so I don't think this is the right
approach:

 1. I don't understand your description: Since fb_notifier_call_chain()
    is an exported symbol, it should work for both FB_NOTIFY=y and
    FB_NOTIFY=m, unless something in Android drops the exported
    symbols for some reason.

 2. This breaks if any of the four callers of fb_register_client()
    are built-in while CONFIG_FB_NOTIFY is set to =m:

$ git grep -w fb_register_client
arch/arm/mach-pxa/am200epd.c:   fb_register_client(&am200_fb_notif);
drivers/leds/trigger/ledtrig-backlight.c:       ret = fb_register_client(&n->notifier);
drivers/video/backlight/backlight.c:    return fb_register_client(&bd->fb_notif);
drivers/video/backlight/lcd.c:  return fb_register_client(&ld->fb_notif);

Somewhat related but not directly addressing your patch, I wonder
if Android itself could migrate to using FB_CORE=m FB=n FB_NOTIFY=n
instead and use simpledrm for the console in place of the legacy
fbdev layer.

    Arnd