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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Sun, 2 Jul 2023 11:07:53 +0200
Message-ID: <CAMuHMdWLRBePox24kBHBMhzKWZX2ncXCD_8JBZrWcbwo0eBZ1Q@mail.gmail.com>
Subject: Re: [PATCH v2 1/2] fbdev: Split frame buffer support in FB and
 FB_CORE symbols
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Javier Martinez Canillas <javierm@redhat.com>,
        linux-kernel@vger.kernel.org,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Borislav Petkov <bp@alien8.de>,
        Daniel Vetter <daniel@ffwll.ch>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Helge Deller <deller@gmx.de>,
        Ingo Molnar <mingo@redhat.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Sam Ravnborg <sam@ravnborg.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org,
        x86@kernel.org
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Hi Arnd,

On Sun, Jul 2, 2023 at 12:25 AM Arnd Bergmann <arnd@arndb.de> wrote:
> On Sat, Jul 1, 2023, at 23:44, Javier Martinez Canillas wrote:
> > Currently the CONFIG_FB option has to be enabled even if no legacy fbdev
> > drivers are needed (e.g: only to have support for framebuffer consoles).
> >
> > The DRM subsystem has a fbdev emulation layer, but depends on CONFIG_FB
> > and so it can only be enabled if that dependency is enabled as well.
> >
> > That means fbdev drivers have to be explicitly disabled if users want to
> > enable CONFIG_FB, only to use fbcon and/or the DRM fbdev emulation layer.
> >
> > This patch introduces a non-visible CONFIG_FB_CORE symbol that could be
> > enabled just to have core support needed for CONFIG_DRM_FBDEV_EMULATION,
> > allowing CONFIG_FB to be disabled (and automatically disabling all the
> > fbdev drivers).
> >
> > Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>

> > @@ -59,7 +69,7 @@ config FIRMWARE_EDID
> >
> >  config FB_DEVICE
> >       bool "Provide legacy /dev/fb* device"
> > -     depends on FB
> > +     select FB_CORE
> >       default y
> >       help
> >         Say Y here if you want the legacy /dev/fb* device file and
>
> These are now the only user visible sub-options when CONFIG_FB is
> disabled. I missed FIRMWARE_EDID earlier, but this also looks like
> it can clearly be left as depending on FB since nothing else calls
> fb_firmware_edid. In fact, it looks like all of fbmon.c could be
> left out since none of its exported symbols are needed for DRM.
>
> That would leave CONFIG_FB_DEVICE as the only user visible option
> for DRM-only configs, which is slightly odd for the menuconfig,
> so I still wonder if that could be done differently.
>
> Is there actually a point in configurations for kernels with FB=y,
> DRM=n and FB_DEVICE=n? If we don't expect that to be a useful
> configuration, an easier way would be to have CONFIG_FB turn it
> on implicitly and instead have a user-visible Kconfig option
> below CONFIG_DRM_FBDEV_EMULATION that allows controlling the
> creation of /dev/fb*.

Such a combination would allow the user to still have a text console
on a legacy fbdev, while not having to worry about possible security
ramifications of providing fbdev userspace access.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds