linux-kernel - Re: [RFC PATCH 0/4] Allow to use DRM fbdev emulation layer with CONFIG

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Message-ID: <b6ba8f6b-4f0c-53cc-e384-ecea3af78410@redhat.com>
Date:   Wed, 1 Sep 2021 11:08:10 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        linux-kernel@...r.kernel.org, "H . Peter Anvin" <hpa@...or.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        x86@...nel.org, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-fbdev@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Maxime Ripard <mripard@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Peter Robinson <pbrobinson@...il.com>,
        Hans de Goede <hdegoede@...hat.com>,
        dri-devel@...ts.freedesktop.org, Ingo Molnar <mingo@...hat.com>,
        David Airlie <airlied@...ux.ie>
Subject: Re: [RFC PATCH 0/4] Allow to use DRM fbdev emulation layer with
 CONFIG_FB disabled

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