Re: [RFC PATCH 0/4] Allow to use DRM fbdev emulation layer with CONFIG_FB disabled

From: Javier Martinez Canillas <javierm@redhat.com>
To: Thomas Zimmermann <tzimmermann@suse.de>,
	linux-kernel@vger.kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	x86@kernel.org, Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-fbdev@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	Maxime Ripard <mripard@kernel.org>,
	Borislav Petkov <bp@alien8.de>,
	Peter Robinson <pbrobinson@gmail.com>,
	Hans de Goede <hdegoede@redhat.com>,
	dri-devel@lists.freedesktop.org, Ingo Molnar <mingo@redhat.com>,
	David Airlie <airlied@linux.ie>
Subject: Re: [RFC PATCH 0/4] Allow to use DRM fbdev emulation layer with CONFIG_FB disabled
Date: Wed, 1 Sep 2021 11:08:10 +0200	[thread overview]
Message-ID: <b6ba8f6b-4f0c-53cc-e384-ecea3af78410@redhat.com> (raw)
In-Reply-To: <YS4iIR689bAZ4QT9@phenom.ffwll.local>

On 8/31/21 2:35 PM, Daniel Vetter wrote:
> On Sat, Aug 28, 2021 at 12:02:21AM +0200, Javier Martinez Canillas wrote:

[snip]

>>
>> We talked about a drmcon with Peter Robinson as well but then decided that a
>> way to disable CONFIG_FB but still having the DRM fbdev emulation could be a
>> intermediary step, hence these RFC patches.
>>
>> But yes, I agree that a drmcon would be the proper approach for this, to not
>> need any fbdev support at all. We will just keep the explicit disable for the
>> fbdev drivers then in the meantime.
> 
> I think the only intermediate step would be to disable the fbdev uapi
> (char node and anything in sysfs), while still registering against the
> fbcon layer so you have a console.
>

Right, $subject disabled the sysfs interface but left the fbdev chardev. I can
try to do a v2 that also disables that interface but just keep the fbcon part.

> But looking at the things syzbot finds the really problematic code is all
> in the fbcon and console layer in general, and /dev/fb0 seems pretty
> solid.
>

Yes, but still would be an improvement in the sense that no legacy fbdev uAPI
will be exposed and so user-space would only depend on the DRM/KMS interface.

> I think for a substantial improvement here in robustness what you really
> want is
> - kmscon in userspace
> - disable FB layer
> - ideally also disable console/vt layer in the kernel

Earlier in the thread it was mentioned that an in-kernel drmcon could be used
instead. My worry with kmscon is that moving something as critical as console
output to user-space might make harder to troubleshoot early booting issues.

And also that will require user-space changes. An in-kernel drmcon could be a
drop-in replacement though.

> - have a minimal emergency/boot-up log thing in drm, patches for that
>   floated around a few times
>

Interesting. Do you have any pointers for this? My search-fu failed me when
trying to find these patches.

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