linux-kernel - Re: [PATCH 0/4] drm/meson: Module removal fixes

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Date:   Tue, 17 Nov 2020 09:49:49 +0100
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Marc Zyngier <maz@...nel.org>, Kevin Hilman <khilman@...libre.com>
Cc:     Jerome Brunet <jbrunet@...libre.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        kernel-team@...roid.com, dri-devel@...ts.freedesktop.org,
        linux-amlogic@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/4] drm/meson: Module removal fixes

Hi Marc,

On 16/11/2020 21:07, Marc Zyngier wrote:
> 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...

I'll be interested in having your kernel config, I never had such report
since I enabled HDMI support in U-Boot a few years ago.

> 
> 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
> words...).
> 
> 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.

Anyway, thanks for fixing this !

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

This is very appreciated :-)

> 
> Feedback much appreciated.
> 
> 	M.
> 
> 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(-)
>