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

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Apr 05, 2022 at 06:12:59PM +0200, Daniel Vetter wrote:
> On Tue, Apr 05, 2022 at 03:33:17PM +0200, Greg KH wrote:
> > On Tue, Apr 05, 2022 at 03:24:40PM +0200, Geert Uytterhoeven wrote:
> > > Hi Daniel,
> > > 
> > > On Tue, Apr 5, 2022 at 1:48 PM Daniel Vetter <daniel@ffwll.ch> wrote:
> > > > On Tue, 5 Apr 2022 at 11:52, Javier Martinez Canillas
> > > > <javierm@redhat.com> wrote:
> > > > > On 4/5/22 11:24, Daniel Vetter wrote:
> > > > > > On Tue, 5 Apr 2022 at 11:19, Javier Martinez Canillas
> > > > > >> This is how I think that work, please let me know if you see something
> > > > > >> wrong in my logic:
> > > > > >>
> > > > > >> 1) A PCI device of OF device is registered for the GPU, this attempt to
> > > > > >>    match a registered driver but no driver was registered that match yet.
> > > > > >>
> > > > > >> 2) The efifb driver is built-in, will be initialized according to the link
> > > > > >>    order of the objects under drivers/video and the fbdev driver is registered.
> > > > > >>
> > > > > >>    There is no platform device or PCI/OF device registered that matches.
> > > > > >>
> > > > > >> 3) The DRM driver is built-in, will be initialized according to the link
> > > > > >>    order of the objects under drivers/gpu and the DRM driver is registered.
> > > > > >>
> > > > > >>    This matches the device registered in (1) and the DRM driver probes.
> > > > > >>
> > > > > >> 4) The DRM driver .probe kicks out any conflicting DRM drivers and pdev
> > > > > >>    before registering the DRM device.
> > > > > >>
> > > > > >>    There are no conflicting drivers or platform device at this point.
> > > > > >>
> > > > > >> 5) Latter at some point the drivers/firmware/sysfb.c init function is
> > > > > >>    executed, and this registers a platform device for the generic fb.
> > > > > >>
> > > > > >>    This device matches the efifb driver registered in (2) and the fbdev
> > > > > >>    driver probes.
> > > > > >>
> > > > > >>    Since that happens *after* the DRM driver already matched, probed
> > > > > >>    and registered the DRM device, that is a bug and what the reverted
> > > > > >>    patch worked around.
> > > > > >>
> > > > > >> So we need to prevent (5) if (1) and (3) already happened. Having a flag
> > > > > >> set in the fbdev core somewhere when remove_conflicting_framebuffers()
> > > > > >> is called could be a solution indeed.
> > > > > >>
> > > > > >> That is, the fbdev core needs to know that a DRM driver already probed
> > > > > >> and make register_framebuffer() fail if info->flag & FBINFO_MISC_FIRMWARE
> > > > > >>
> > > > > >> I can attempt to write a patch for that.
> > > > > >
> > > > > > Ah yeah that could be an issue. I think the right fix is to replace
> > > > > > the platform dev unregister with a sysfb_unregister() function in
> > > > > > sysfb.c, which is synced with a common lock with the sysfb_init
> > > > > > function and a small boolean. I think I can type that up quickly for
> > > > > > v3.
> > > > >
> > > > > It's more complicated than that since sysfb is just *one* of the several
> > > > > places where platform devices can be registered for video devices.
> > > > >
> > > > > For instance, the vga16fb driver registers its own platform device in
> > > > > its module_init() function so that can also happen after the conflicting
> > > > > framebuffers (and associated devices) were removed by a DRM driver probe.
> > > > >
> > > > > I tried to minimize the issue for that particular driver with commit:
> > > > >
> > > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0499f419b76f
> > > > >
> > > > > But the point stands, it all boils down to the fact that you have two
> > > > > different subsystems registering video drivers and they don't know all
> > > > > about each other to take a proper decision.
> > > > >
> > > > > Right now the drm_aperture_remove_conflicting_framebuffers() call signals
> > > > > in one direction from DRM to fbdev but there isn't a communication in the
> > > > > other direction, from fbdev to DRM.
> > > > >
> > > > > I believe the correct fix would be for the fbdev core to keep a list of
> > > > > the apertures struct that are passed to remove_conflicting_framebuffers(),
> > > > > that way it will know what apertures are not available anymore and prevent
> > > > > to register any fbdev framebuffer that conflicts with one already present.
> > > >
> > > > Hm that still feels like reinventing a driver model, badly.
> > > >
> > > > I think there's two cleaner solutions:
> > > > - move all the firmware driver platform_dev into sysfb.c, and then
> > > > just bind the special cases against that (e.g. offb, vga16fb and all
> > > > these). Then we'd have one sysfb_try_unregister(struct device *dev)
> > > > interface that fbmem.c uses.
> > > > - let fbmem.c call into each of these firmware device providers, which
> > > > means some loops most likely (like we can't call into vga16fb), so
> > > > probably need to move that into fbmem.c and it all gets a bit messy.
> > > >
> > > > > Let me know if you think that makes sense and I can attempt to write a fix.
> > > >
> > > > I still think unregistering the platform_dev properly makes the most
> > > 
> > > That doesn't sound very driver-model-aware to me. The device is what
> > > the driver binds to; it does not cease to exist.
> > 
> > I agree, that sounds odd.
> > 
> > The device should always stick around (as the bus creates it), it's up
> > to the driver to bind to the device as needed.
> 
> The device actually disappears when the real driver takes over.
> 
> The firmware fb is a special thing which only really exists as long as the
> firmware is in charge of the display hardware. As soon as a real driver
> takes over, it stops being a thing.
> 
> And since a driver without a device is a bit a funny thing, we have been
> pushing towards a model where the firmware code sets up a platform_device
> for this fw interface, and the fw driver (efifb, simplefb and others like
> that) bind against it. And then we started to throw out that
> platform_device (which unbinds the fw driver and prevents it from ever
> rebinding), except in the wrong layer so there's a few races.
> 
> Should we throw out all that code and replace it with something else? What
> would that be like?

Ah, no, sorry, I didn't know that at all.

That sounds semi-sane, just fix the races by moving the layer elsewhere?