drm, fbdev emulation and drm_dev_unplug()

[Noralf Trønnes]

Hi,

I'm looking into what happens if fbdev has open file handles when
drm_dev_unplug() is called. udl is the only user of drm_dev_unplug(),
but AFAICT it will keel over if unplugged with a /dev/fb handle open
since fbdev is torn down during drm_driver->unload.

My first attempt was to rely on the drm_device ref counting and tear
down fbdev in drm_driver->release. This requires that a ref is taken on
drm_device in &fb_ops.fb_open and dropped in &fb_ops.fb_release.
The problem is that .fb_release is called under console_lock(), which
means that unregister_framebuffer() will deadlock (assuming last ref on
drm_device here). Trying to do trickery with console_lock to get this
through is bound to be painful, so a straightforward solution is to fork
of fbdev teardown to a worker.
Since we are already in drm_driver->release, this won't work.

My next idea is to refcount struct drm_fb_helper, take one ref on
drm_device and handle the lifetime of fbdev that way. Now it's possible to
do teardown in a worker and when that's done, drop the ref on drm_device.
The plan was to make this ref counting optional for drivers.

Is ref counting drm_fb_helper a good/bad idea, or is there another
solution to this?

Noralf.