linux-kernel - Re: [PATCH 1/2] video: vga16fb: Fix logic that checks for the display standard

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Message-ID: <3355ac8b-fd59-068c-e40e-bf8c54e5abde@redhat.com>
Date:   Mon, 10 Jan 2022 09:22:53 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     "Kris Karas (Bug reporting)" <bugs-a21@...nlit-rail.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>
Subject: Re: [PATCH 1/2] video: vga16fb: Fix logic that checks for the display
 standard

Hello Geert and Kara,

On 1/9/22 21:20, Kris Karas (Bug reporting) wrote:
> Groetje, Geert,
> 
> Geert Uytterhoeven wrote:
>>
>>> -       par->isVGA = screen_info.orig_video_isVGA;
>>> +       par->isVGA = screen_info.orig_video_isVGA == VIDEO_TYPE_VGAC;
>> All non-x86 architectures (except for 2 MIPS platforms) treat
>> orig_video_isVGA as a boolean flag, and just assign 1 to it.
>> Hence this change would break them.
> 
> I see a bit of a conflict with using orig_video_isVGA as a boolean. All 
> the modern architecture-agnostic driver code, such as sysfb, 
> sysfb_simplefb, and efifb, all use and expect orig_video_isVGA to be an 
> integer.  On the other hand, the VGA driver for XEN first sets 
> orig_video_isVGA  = 1 (boolean), and then VIDEO_TYPE_VLFB or 
> VIDEO_TYPE_EFI (integer).  Overloading the definition for 
> orig_video_isVGA to be both boolean and integer - within the same file - 
> seems like a recipe for bugs to me.
>

Agreed with Kara on this. I believe the non-x86 arches should be fixed
to set it to the correct value.
 
> That said, I think that wrapping the par->isVGA code, above, within a 
> check for CONFIG_X86 seems safe and expedient.  But I would be much 
> happier if the non-x86 architectures would set it to a proper integer 
> value (even if fake) that coincidentally satisfies boolean "true", say 
> VIDEO_TYPE_VGAC; that way, there would be no confusion as to data type 
> in all the more recent architecture-agnostic framebuffer code.
> 

Yes, I'll post a v2 to do that but hopefully the CONFIG_X86 guard could
be dropped in the future once all the architectures are fixed.

> Kris
> 

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