[PATCH 0/4] drm/meson: Module removal fixes

Marc Zyngier maz at kernel.org
Mon Nov 16 15:07:40 EST 2020

Hi all,

Having recently moved over to a top-of-the-tree u-boot on one of my
VIM3L systems in order to benefit from unrelated improvements
(automatic PCIe detection, EFI...), I faced the issue that my kernel
would hang like this:

[  OK  ] Finished Helper to synchronize boot up for ifupdown.
[  OK  ] Started Rule-based Manager for Device Events and Files.
[    7.114516] VDDCPU: supplied by regulator-dummy
[  OK  ] Found device /dev/ttyAML0.
[    7.146862] meson-drm ff900000.vpu: Queued 2 outputs on vpu
[    7.169630] fb0: switching to meson-drm-fb from simple
[    7.169944] Console: switching to colour dummy device 80x25
[    7.179250] meson-drm ff900000.vpu: CVBS Output connector not available

and that's it.

After some poking around, I figured out that it is in the
meson-dw-hdmi module that the CPU was hanging...

Reverting to the kernel DT instead of u-boot's papered over it
somehow, but it turned out that removing the module (modprobe -r)
resulted in a firework. And for every issue I was fixing, another
followed. Much fun for a rainy Monday in the basement!

I ended up with the following 4 patches, which solve all my problems:
I can now boot with the u-boot provided DT, and the hdmi and DRM
drivers can be removed and re-inserted at will.

The first patch is a straightforward use-after-free, causing a NULL
pointer dereference. Moving things around fixes it.

The second patch shows that I have no clue about the DRM subsystem
whatsoever. I mimicked what my Rockchip systems are doing, and the two
warnings disappeared. It can't completely be wrong (famous last

The third patch fixes a *very* common issue with regulators (I've
fixed at least 3 drivers with a similar issue). I guess the devm
subsystem needs to grow a new helper at some point.

The last patch finally fixes the issue I was seeing: the HDMI driver
hangs when accessing a register with clocks disabled, which they are
on module removal. It also fixes my u-boot booting for similar
reasons, I guess.

I went as far as reaching out for a HDMI cable and verifying that I
was getting a working display. Total dedication.

Feedback much appreciated.


Marc Zyngier (4):
  drm/meson: Free RDMA resources after tearing down DRM
  drm/meson: Unbind all connectors on module removal
  drm/meson: dw-hdmi: Register a callback to disable the regulator
  drm/meson: dw-hdmi: Ensure that clocks are enabled before touching the
    TOP registers

 drivers/gpu/drm/meson/meson_drv.c     | 12 +++++++-----
 drivers/gpu/drm/meson/meson_dw_hdmi.c | 13 +++++++++++--
 2 files changed, 18 insertions(+), 7 deletions(-)


More information about the linux-amlogic mailing list