linux-kernel - Re: [PATCH v5 3/4] fbdev: Split frame buffer support in FB and FB

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Date:   Wed, 19 Jul 2023 09:57:03 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H. Peter Anvin" <hpa@...or.com>, Helge Deller <deller@....de>,
        Ingo Molnar <mingo@...hat.com>,
        Jingoo Han <jingoohan1@...il.com>, Lee Jones <lee@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH v5 3/4] fbdev: Split frame buffer support in FB and
 FB_CORE symbols

"Arnd Bergmann" <arnd@...db.de> writes:

> On Fri, Jul 14, 2023, at 19:16, 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).
>>
>> Nothing from fb_backlight.o and fbmon.o is used by the DRM fbdev emulation
>> layer so these two objects can be compiled out when CONFIG_FB is disabled.
>
> I gave this a spin in my randconfig build setup and found one small
> mistake:
>

Thanks for testing!

>> diff --git a/drivers/video/fbdev/core/Makefile 
>> b/drivers/video/fbdev/core/Makefile
>> index 9150bafd9e89..2cd213716c12 100644
>> --- a/drivers/video/fbdev/core/Makefile
>> +++ b/drivers/video/fbdev/core/Makefile
>> @@ -1,10 +1,10 @@
>>  # SPDX-License-Identifier: GPL-2.0
>>  obj-$(CONFIG_FB_NOTIFY)           += fb_notify.o
>> -obj-$(CONFIG_FB)                  += fb.o
>> -fb-y                              := fb_backlight.o \
>> -                                     fb_info.o \
>> -                                     fbmem.o fbmon.o fbcmap.o \
>> +obj-$(CONFIG_FB_CORE)             += fb.o
>> +fb-y                              := fb_info.o \
>> +                                     fbmem.o fbcmap.o \
>>                                       modedb.o fbcvt.o fb_cmdline.o 
>> fb_io_fops.o
>> +fb-$(CONFIG_FB)                   += fb_backlight.o fbmon.o
>
> With CONFIG_FB_CORE=y and CONFIG_FB=m, Kbuild does not include
> the fb_backlight.o and fbmon.o files in fb.ko because they are not
> set to =y, causing link failures for fbdev drivers later:
>
> ERROR: modpost: "of_get_fb_videomode" [drivers/video/fbdev/clps711x-fb.ko] undefined!
> ERROR: modpost: "fb_videomode_from_videomode" [drivers/video/fbdev/atmel_lcdfb.ko] undefined!
> ERROR: modpost: "of_get_fb_videomode" [drivers/video/fbdev/imxfb.ko] undefined!
> ERROR: modpost: "fb_destroy_modedb" [drivers/video/fbdev/udlfb.ko] undefined!
> ERROR: modpost: "fb_edid_to_monspecs" [drivers/video/fbdev/udlfb.ko] undefined!
> ERROR: modpost: "fb_destroy_modedb" [drivers/video/fbdev/smscufx.ko] undefined!
> ERROR: modpost: "fb_edid_to_monspecs" [drivers/video/fbdev/smscufx.ko] undefined!
> ERROR: modpost: "fb_destroy_modedb" [drivers/video/fbdev/uvesafb.ko] undefined!
> ERROR: modpost: "fb_validate_mode" [drivers/video/fbdev/uvesafb.ko] undefined!
> ERROR: modpost: "fb_get_mode" [drivers/video/fbdev/uvesafb.ko] undefined!
>

Right, sorry for missing that combination.

>
> Folding this fixup into the patch makes it work:
>
> diff --git a/drivers/video/fbdev/core/Makefile b/drivers/video/fbdev/core/Makefile
> index 2cd213716c12f..84ddc5d308b58 100644
> --- a/drivers/video/fbdev/core/Makefile
> +++ b/drivers/video/fbdev/core/Makefile
> @@ -4,7 +4,9 @@ obj-$(CONFIG_FB_CORE)             += fb.o
>  fb-y                              := fb_info.o \
>                                       fbmem.o fbcmap.o \
>                                       modedb.o fbcvt.o fb_cmdline.o fb_io_fops.o
> -fb-$(CONFIG_FB)                   += fb_backlight.o fbmon.o
> +ifdef CONFIG_FB
> +fb-y		                   += fb_backlight.o fbmon.o
> +endif
>  fb-$(CONFIG_FB_DEFERRED_IO)       += fb_defio.o
>  fb-$(CONFIG_FB_DEVICE)            += fb_chrdev.o \
>                                       fb_procfs.o \
>

Perfect, I'll squash your fix in v6. Thanks!

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat