Re: [PATCH v3 0/5] Fix some race conditions that exists between fbmem and sysfb

From: Thomas Zimmermann
Date: Mon Apr 25 2022 - 05:17:50 EST


Am 25.04.22 um 10:54 schrieb Thomas Zimmermann:

Am 20.04.22 um 10:52 schrieb Javier Martinez Canillas:

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.

As I mentioned on IRC, I think this series should be merged for the reasons I give in the other comments.

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

Here's as far as I understand the problem:

 1) build DRM/fbdev and sysfb code into the kernel
 2) during boot, load the DRM/fbdev modules and have them acquire I/O ranges
 3) afterwards load sysfb and have it register platform devices for the generic framebuffers
 4) these devices now conflict with the already-registered DRM/fbdev devices

If that is the problem here, let's simply set a sysfb_disable flag in sysfb code when the first DRM/fbdev driver first loads. With the flag set, sysfb won't create any platform devices. We assume that there are now DRM/fbdev drivers for the framebuffers and sysfb won't be needed.

We can set the flag internally from drm_aperture_detach_drivers() [1] and do_remove_conflicting_framebuffers() [2].

And further thinking about it, it would be better to set such a flag after successfully registering a DRM/fbdev device. So we know that there's at least one working display in the system. We don't have to rely on generic framebuffers after that.

Best regards

Best regards


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).

Best regards,

Changes in v3:
- Rebase on top of latest drm-misc-next branch.

Changes in v2:
- Rebase on top of latest drm-misc-next and fix conflicts (Daniel Vetter).
- Add kernel-doc comments and include in other_interfaces.rst (Daniel Vetter).
- Explain in the commit message that fbmem has to unregister the device
   as fallback if a driver registered the device itself (Daniel Vetter).
- Also explain that fallback in a comment in the code (Daniel Vetter).
- Don't encode in fbmem the assumption that sysfb will always register
   platform devices (Daniel Vetter).
- Add a FIXME comment about drivers registering devices (Daniel Vetter).
- Drop RFC prefix since patches were already reviewed by Daniel Vetter.
- Add Daniel Reviewed-by tags to the patches.

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

  .../driver-api/firmware/other_interfaces.rst  |  6 ++
  drivers/firmware/sysfb.c                      | 77 +++++++++++++++++--
  drivers/firmware/sysfb_simplefb.c             | 16 ++--
  drivers/video/fbdev/core/fbmem.c              | 62 ++++++++++++---
  drivers/video/fbdev/efifb.c                   | 11 ---
  drivers/video/fbdev/simplefb.c                | 11 ---
  include/linux/fb.h                            |  1 -
  include/linux/sysfb.h                         | 29 +++++--
  8 files changed, 158 insertions(+), 55 deletions(-)

Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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