[RFC PATCH 0/5] Fix some race conditions that exists between fbmem and sysfb

From: Javier Martinez Canillas
Date: Wed Apr 06 2022 - 13:55:27 EST


The patches in this series are mostly changes suggested by Daniel Vetter
to fix some race conditions that exists between the fbdev core (fbmem)
and sysfb with regard to device registration and removal.

For example, it is currently possible for sysfb to register a platform
device after a real DRM driver was registered and requested to remove the
conflicting framebuffers.

A symptom of this issue, was worked around with by commit fb561bf9abde
("fbdev: Prevent probing generic drivers if a FB is already registered")
but that's really a hack and should be reverted.

This series attempt to fix it more properly and reverted the mentioned
hack. This will also unblock a pending patch to not make num_registered_fb
visible to drivers anymore, since that's really internal to fbdev core.

Patch #1 is just a trivial preparatory change.

Patch #2 add sysfb_disable() and sysfb_try_unregister() helpers for fbmem
to use them.

Patch #3 changes how is dealt with conflicting framebuffers unregistering,
rather than having a variable to determine if a lock should be take, it
just drops the lock before unregistering the platform device.

Patch #4 fixes the mentioned race conditions and finally patch #5 is the
revert patch that was posted by Daniel before but he dropped from his set.

The patches were tested on a rpi4 using different video configurations:
(simpledrm -> vc4 both builtin, only vc4 builtin, only simpledrm builtin
and simpledrm builtin with vc4 built as a module).

I'm sending as an RFC since there are many changes to the locking scheme
and that is always tricky to get right. Please let me know what you think.

Best regards,

Daniel Vetter (1):
Revert "fbdev: Prevent probing generic drivers if a FB is already

Javier Martinez Canillas (4):
firmware: sysfb: Make sysfb_create_simplefb() return a pdev pointer
firmware: sysfb: Add helpers to unregister a pdev and disable
fbdev: Restart conflicting fb removal loop when unregistering devices
fbdev: Fix some race conditions between fbmem and sysfb

drivers/firmware/sysfb.c | 51 ++++++++++++++++++++++++++-----
drivers/firmware/sysfb_simplefb.c | 24 +++++++++------
drivers/video/fbdev/core/fbmem.c | 38 ++++++++++++++++++-----
drivers/video/fbdev/efifb.c | 11 -------
drivers/video/fbdev/simplefb.c | 11 -------
include/linux/fb.h | 1 -
include/linux/sysfb.h | 27 +++++++++++++---
7 files changed, 111 insertions(+), 52 deletions(-)