linux-kernel - Re: [PATCH] x86: sysfb: fool-proof CONFIG_X86

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Date:	Thu, 19 Dec 2013 16:54:37 +0100
From:	David Herrmann <dh.herrmann@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Takashi Iwai <tiwai@...e.de>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
	stable <stable@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86: sysfb: fool-proof CONFIG_X86_SYSFB

Hi

On Thu, Dec 19, 2013 at 4:40 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * David Herrmann <dh.herrmann@...il.com> wrote:
>
>> Turns out, people do not read help-texts of new config-options and
>> enable them nonetheless. [...]
>
> Yeah, I too don't read them either, whenever an option name seems
> obvious to enable, so this is really something that happens frequently
> ;-)
>
>> [...] So several reports came in with X86_SYSFB=y and FB_SIMPLE=n,
>> which in almost all situations prevents firmware-fbs from being
>> probed.
>>
>> X86_SYSFB clearly states that it turns legacy vesa/efi framebuffers
>> into a format compatible to simplefb (and does nothing else..). So
>> to avoid further complaints about missing gfx-support during boot,
>> simply depend on FB_SIMPLE now.
>>
>> As FB_SIMPLE is disabled by default and usually only enabled on
>> selected ARM architectures, x86 users should thus never see the
>> X86_SYSFB config-option. And if they do, everything is fine as
>> simplefb will be available.
>>
>> Note that most of the sysfb code is enabled independently of
>> X86_SYSFB. The config option only selects a compatibility mode for
>> simplefb. It was introduced to ease the transition to SimpleDRM and
>> disabling fbdev. As this is still ongoing, there's no need for
>> non-developers to care for X86_SYSFB so just change the help-text
>> recommendation to "n".
>>
>> Cc: <stable@...r.kernel.org> # 3.11+
>> Signed-off-by: David Herrmann <dh.herrmann@...il.com>
>> ---
>>  arch/x86/Kconfig | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index e903c71..9317ede 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -2298,6 +2298,7 @@ source "drivers/rapidio/Kconfig"
>>
>>  config X86_SYSFB
>>       bool "Mark VGA/VBE/EFI FB as generic system framebuffer"
>> +     depends on (FB_SIMPLE = y)
>
> Could that be written as:
>
>         depends on FB_SIMPLE
>
> Or is there some complication with modular builds?

simplefb is actually "bool" so yeah, I can remove the "= y".

Note that *if* it ever is built as module, it will not get loaded
during boot, thus not showing any boot-messages. But that's true for
vesafb/efifb, too, so we're fine.

>>       help
>>         Firmwares often provide initial graphics framebuffers so the BIOS,
>>         bootloader or kernel can show basic video-output during boot for
>> @@ -2320,7 +2321,7 @@ config X86_SYSFB
>>         and others enabled as fallback if a system framebuffer is
>>         incompatible with simplefb.
>>
>> -       If unsure, say Y.
>> +       If unsure, say N.
>
> We might in fact leave this bit alone and suggest 'Y' - with the
> robustification fixes it's not possible anymore to crash the boot or
> to create an unintentionally headless system, right?

Nope, we will not cause a headless system, but the handover to other
fbdev/DRM drivers (other than the common nouveau/radeon/i915) may
still fail without the other patch (x86: sysfb: remove sysfb when
probing real hw).

I will gladly keep the Y, if no-one disagrees. Given that the other
patch is rather complex (and a no-op with X86_SYSFB=n) I wasn't quite
sure. But as SYSFB is set to 'n' now if FB_SIMPLE wasn't explicitly
selected, I think we're fine both ways.

Thanks
David
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