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devscan: priority 3; docs - units and rules are libsystemd's (follow-up to #75) #77

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Merged
JohnAmadis merged 2 commits into from
Oct 26, 2026
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devscan: priority 3; docs - units and rules are libsystemd's (follow-up to #75) #77

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9fdbf4a
devscan: only the drivers are dmdevfs's; units and rules are libsyste…
JaxCalder Oct 25, 2026
f1f6224
devscan: priority 3 - the unmount waits for its remove unit
JaxCalder Oct 25, 2026
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6 changes: 4 additions & 2 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -363,10 +363,12 @@ The [devscan](services/devscan/README.md) service makes the calls: libsystemd
reports the mounts (dmfsi's `_mount_changed` dif, which dmvfs calls) as devices
of class `mount`, a rule starts `devscan add /dev <mount point>` for a mount and
`devscan remove /dev <mount point>` before an unmount, and both end when they
are done. So a mounted card or `/ext` just brings its drivers:
are done. So a mounted card or `/ext` just brings its configuration:

```
<mount point>/cfg/drivers/ driver configurations -> dmdevfs
<mount point>/cfg/drivers/ driver configurations -> dmdevfs (devscan)
<mount point>/cfg/services/ units -> libsystemd (by itself, on the mount)
<mount point>/cfg/rules/ device rules -> libsystemd (by itself, on the mount)
```

- **mounted** - the drivers are configured by the hot-plug thread. A mount
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5 changes: 4 additions & 1 deletion services/devscan/CMakeLists.txt
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Expand Up @@ -9,7 +9,10 @@ set(DMOD_AUTHOR_NAME "Patryk Kubiak")
# Its own stack: it only opens a file and calls an ioctl, but the thread that mounts (automount's) is far too small
# to do even that inline
set(DMOD_STACK_SIZE 2048)
set(DMOD_PRIORITY 0)
# Above the services (0): a mount's `remove` unit is waited for by the unmount, and on a busy system (the car HMI) a
# priority-0 process gets the CPU only in the gaps - it took the 20 s the unmount waits for it at most. It does
# a few file operations and ends.
set(DMOD_PRIORITY 3)
set(DMOD_DMR_PATH ${CMAKE_CURRENT_SOURCE_DIR}/devscan.dmr)

dmod_add_executable(${DMOD_MODULE_NAME} ${DMOD_MODULE_VERSION}
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4 changes: 4 additions & 0 deletions services/devscan/README.md
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Expand Up @@ -16,6 +16,10 @@ and does an ioctl on it - `DMDEVFS_IOCTL_ADD_CONFIG_DIR` /
[`include/dmdevfs.h`](../../include/dmdevfs.h) - then ends. dmdevfs queues the
work for its hot-plug thread. A mount point without `cfg/drivers` is left alone.

Only the drivers are dmdevfs's: the units and rules of a mount point
(`cfg/services`, `cfg/rules`) are loaded by libsystemd itself when the file
system is mounted.

## How it is started

Nothing here follows mounts. libsystemd reports every mount as a device of class
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