Re: [PATCH v2 18/19] Revert "fbdev: Prevent probing generic drivers if a FB is already registered"

From: Daniel Vetter <daniel@ffwll.ch>
To: Javier Martinez Canillas <javierm@redhat.com>,
	DRI Development <dri-devel@lists.freedesktop.org>,
	Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
	linux-fbdev@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	Zack Rusin <zackr@vmware.com>,
	Hans de Goede <hdegoede@redhat.com>,
	Ilya Trukhanov <lahvuun@gmail.com>,
	Daniel Vetter <daniel.vetter@intel.com>,
	Peter Jones <pjones@redhat.com>
Subject: Re: [PATCH v2 18/19] Revert "fbdev: Prevent probing generic drivers if a FB is already registered"
Date: Tue, 5 Apr 2022 10:40:37 +0200	[thread overview]
Message-ID: <YkwAhSt9HlbxcuZo@phenom.ffwll.local> (raw)
In-Reply-To: <Ykv/k/WoVemoCJJA@phenom.ffwll.local>

On Tue, Apr 05, 2022 at 10:36:35AM +0200, Daniel Vetter wrote:
> On Wed, Feb 09, 2022 at 01:19:26AM +0100, Javier Martinez Canillas wrote:
> > On 2/8/22 22:08, Daniel Vetter wrote:
> > > This reverts commit fb561bf9abde49f7e00fdbf9ed2ccf2d86cac8ee.
> > > 
> > > With
> > > 
> > > commit 27599aacbaefcbf2af7b06b0029459bbf682000d
> > > Author: Thomas Zimmermann <tzimmermann@suse.de>
> > > Date:   Tue Jan 25 10:12:18 2022 +0100
> > > 
> > >     fbdev: Hot-unplug firmware fb devices on forced removal
> > > 
> > > this should be fixed properly and we can remove this somewhat hackish
> > > check here (e.g. this won't catch drm drivers if fbdev emulation isn't
> > > enabled).
> > >
> > 
> > Unfortunately this hack can't be reverted yet. Thomas' patch solves the issue
> > of platform devices matched with fbdev drivers to be properly unregistered if
> > a DRM driver attempts to remove all the conflicting framebuffers.
> > 
> > But the problem that fb561bf9abde ("fbdev: Prevent probing generic drivers if
> > a FB is already registered") worked around is different. It happens when the
> > DRM driver is probed before the {efi,simple}fb and other fbdev drivers, the
> > kicking out of conflicting framebuffers already happened and these drivers
> > will be allowed to probe even when a DRM driver is already present.
> > 
> > We need a clearer way to prevent it, but can't revert fb561bf9abde until that.
> 
> Yeah that entire area is a mess still, ideally we'd have something else
> creating the platform devices, and efifb/offb and all these would just
> bind against them.
> 
> Hm one idea that just crossed my mind: Could we have a flag in fb_info for
> fw drivers, and check this in framebuffer_register? Then at least all the
> logic would be in the fbdev core.

Ok coffee just kicked in, how exactly does your scenario work?

This code I'm reverting here is in the platform_dev->probe function.
Thomas' patch removes the platform_dev. How exactly can you still probe
against a platform dev if that platform dev is gone?

Iow, now that I reponder your case after a few weeks I'm no longer sure
things work like you claim.

There is the issue that offb still bidns without a platform_dev, but
that's not affected by this patch here.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch