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The ASoC core has for the longest time increased the module reference
counts, even before the transition to the component model. This is
probably fine on most platforms, but it introduces a deadlock case on
Intel devices with the Skylake and SOF drivers which cannot be removed
due to their reference counts being modified by the core.
In these 2 cases, the PCI or ACPI driver .probe creates a platform
device to let the machine driver .probe register the audio
card. Conversely the PCI or ACPI driver .remove will unregister the
platform device which results in the card being removed by the machine
driver .remove.
With ascii art, this can be represented as
modprobe
snd_soc_skl/
soc-pci-dev/sof-acpci-dev ----------> pci/acpi probe
^ |
| ---------------|
| | |
| V V
increase register register machine
refcount component platform_device
^ |
| |
| V
component <---- register card <---- probe
probe
The issue is that by playing with the component's module reference
counts during the card registration, it's no longer possible to remove
the module which controls the component. This can be shown, e.g. with
the following error:
root@plb-XPS-13-9350:~# lsmod | grep snd_soc_skl
snd_soc_skl 110592 1
root@plb-XPS-13-9350:~# rmmod snd_soc_skl
rmmod: ERROR: Module snd_soc_skl is in use
Increasing the reference count during the component probe is not
useful. If the PCI/ACPI module is removed, the card will be removed
anyway.
To avoid breaking existing platforms and allowing Intel platforms to
safely deal with module load/unload cases, this patch introduces a
flag which needs to be set during the component initialization. This
is a strictly opt-in capability that should only be used when the
handling of the component module does not require a reference count
increase to prevent removal during use.
Note that this solution is not directly applicable to the legacy
Atom/SST driver, which uses a different device hierarchy. There are
however additional refcount issues which prevent the ACPI driver from
being removed. This is a different issue which would need a different
patch.
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.