On Tue, Apr 05, 2022 at 07:29:22PM +0200, Daniel Vetter wrote: > On Tue, 5 Apr 2022 at 18:45, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > > > > 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@xxxxxxxx> wrote: > > > > > > On Tue, 5 Apr 2022 at 11:52, Javier Martinez Canillas > > > > > > <javierm@xxxxxxxxxx> 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? > > Yeah essentially move it all into drivers/firmware/sysfb.c, for all > drivers, both the registering and the nuking, and warp that into a > local mutex. Currently parts is in there, parts is in fbmem.c, parts > in some of the drivers like vga16fb, and some drivers (iirc only offb) > still don't even have any platform_dev underneath their driver. So > ideally the drivers would all just have their platform_driver probe > functions, and that's it. It does mean though that some of that stuff > needs to be moved to sysfb.c or into the relevant fw code that sets > stuff up. > > It'll take some, so really just a direction check before we move > further. You should get cc'ed on the patches (like with the sysfb > stuff) anyway. Sounds roughly right? That's fine with me, thanks.
