linux-kernel - Re: [PATCH -next] fbdev: offb: allow build when DRM

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Message-Id: <4b10b87d-f255-4839-8700-858d98ffb801@app.fastmail.com>
Date:   Wed, 23 Nov 2022 10:08:45 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Randy Dunlap" <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org
Cc:     linux-fbdev@...r.kernel.org,
        "Thomas Zimmermann" <tzimmermann@...e.de>,
        "Masahiro Yamada" <masahiroy@...nel.org>,
        dri-devel@...ts.freedesktop.org, "Daniel Vetter" <daniel@...ll.ch>,
        "Michal Suchanek" <msuchanek@...e.de>,
        linuxppc-dev@...ts.ozlabs.org, "Helge Deller" <deller@....de>
Subject: Re: [PATCH -next] fbdev: offb: allow build when DRM_OFDRM=m

On Wed, Nov 23, 2022, at 04:16, Randy Dunlap wrote:
> Fix build when CONFIG_FB_OF=y and CONFIG_DRM_OFDRM=m.
> When the latter symbol is =m, kconfig downgrades (limits) the 'select's
> under FB_OF to modular (=m). This causes undefined symbol references:
>
> powerpc64-linux-ld: drivers/video/fbdev/offb.o:(.data.rel.ro+0x58): 
> undefined reference to `cfb_fillrect'
> powerpc64-linux-ld: drivers/video/fbdev/offb.o:(.data.rel.ro+0x60): 
> undefined reference to `cfb_copyarea'
> powerpc64-linux-ld: drivers/video/fbdev/offb.o:(.data.rel.ro+0x68): 
> undefined reference to `cfb_imageblit'
>
> Fix this by allowing FB_OF any time that DRM_OFDRM != y so that the
> selected FB_CFB_* symbols will become =y instead of =m.
>
> In tristate logic (for DRM_OFDRM), this changes the dependency from
>     !DRM_OFDRM	== 2 - 1 == 1 => modular only (or disabled)
> to (boolean)
>     DRM_OFDRM != y == y, allowing the 'select's to cause the
> FB_CFB_* symbols to =y instead of =m.
>

Is it actually a useful configuration to have OFDRM=m and
FB_OF=y though? I would expect in that case that the OFDRM
driver never binds to a device because it's already owned
by FB_OF.

> diff -- a/drivers/video/fbdev/Kconfig b/drivers/video/fbdev/Kconfig
> --- a/drivers/video/fbdev/Kconfig
> +++ b/drivers/video/fbdev/Kconfig
> @@ -455,7 +455,7 @@ config FB_ATARI
>  config FB_OF
>  	bool "Open Firmware frame buffer device support"
>  	depends on (FB = y) && PPC && (!PPC_PSERIES || PCI)
> -	depends on !DRM_OFDRM
> +	depends on DRM_OFDRM != y
>  	select APERTURE_HELPERS

I would instead make this 'depends on DRM_OFDRM=n', which
completely eliminates configs that have both driver enabled.

A nicer change would be to make FB_OF a tristate symbol,
which makes it possible to load one of the two modules if
both are enabled =m, while only allowing one of them to
be =y if the other is completely disabled. It looks like
offb was originally written to be usable as a loadable module,
but Kconfig has prevented this since at least the start of
the git history.

     Arnd