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

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Date:   Sun, 9 Jan 2022 15:20:07 -0500
From:   "Kris Karas (Bug reporting)" <bugs-a21@...nlit-rail.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Javier Martinez Canillas <javierm@...hat.com>
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

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.

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.

Kris