linux-kernel - Re: [PATCH v2] fbdev: Prevent probing generic drivers if a FB is already registered

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Message-ID: <7cbadb2a-b6e9-f264-9d95-b76c7071af27@redhat.com>
Date:   Tue, 16 Nov 2021 11:01:38 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Sam Ravnborg <sam@...nborg.org>,
        Peter Jones <pjones@...hat.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Ilya Trukhanov <lahvuun@...il.com>,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        Borislav Petkov <bp@...e.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2] fbdev: Prevent probing generic drivers if a FB is
 already registered

Hello Geert,

On 11/16/21 10:43, Geert Uytterhoeven wrote:

[snip]

>>
>> So this is already a fragile solution and $SUBJECT doesn't make things worse
>> IMO. Since not having something like this can lead to issues as reported by:
>>
>> https://lore.kernel.org/all/20211110200253.rfudkt3edbd3nsyj@lahvuun/
>>
>> We could probably do some smarter here by providing a function that checks
>> if the registered fbdev drivers matches the aperture base. But I'm unsure
>> if that's worth it. After all, fbdev drivers are likely to be disabled by
>> most distros soon now that we have the simpledrm driver.
> 
> Checking the aperture base is what was done in all other cases of
> preventing generic (fbdev) drivers from stepping on specific drivers'
> toes...
> 

Ok, I can re-spin the patch checking if the aperture ranges overlap. There's
an apertures_overlap() function in drivers/video/fbdev/core/fbmem.c that can
be exported for fbdev drivers to use.

Another option is to just say that DRM drivers should be built as a module if
the {efi,simple}fb driver are built-in.

Best regards,
-- 
Javier Martinez Canillas
Linux Engineering
Red Hat