Re: [PATCH 3/3] host/rootfs: switch to systemd-udevd

[Demi Marie Obenour <demiobenour@gmail.com>]

[-- Attachment #1.1.1: Type: text/plain, Size: 4554 bytes --]

On 9/19/25 15:32, Demi Marie Obenour wrote:
> On 9/19/25 10:12, Alyssa Ross wrote:
>> I'm pleasantly surprised by how straightforward this is (mostly)!
>>
>> Demi Marie Obenour <demiobenour@gmail.com> writes:
>>
>>> The dependency on /dev/dri/card0 being present is eliminated, and
>>> whatever devices the user has are now picked up by the compositor.  New
>>> dependencies are added to ensure that udev coldplug has finished before
>>> any non-trivial services are started.  systemd-udev-trigger.service runs
>>> 'udevadm trigger' and has Before=sysinit.target, so anything that is not
>>> an early boot service can assume 'udevadm trigger' has run.
>>
>> If software is expected to integrate with udev to discover new devices
>> at runtime, why do we need to introduce new dependencies?
> 
> I believe 'udevadm trigger' is needed to process devices that are
> already plugged when systemd-udevd starts.  Otherwise, they will
> never appear.
> 
>>> systemd-udevd doesn't set PATH to anything useful, presumably because
>>> under NixOS this is handled some other way.  Therefore, explicitly set
>>> it to /usr/bin in the scripts systemd-udevd calls.
>>
>> It empties it, or it just passes it through?  The former would be very
>> strange to me, but why isn't adding -p /usr/bin to the s6-linux-init
>> invocation enough?
> 
> I will do some more debugging here, but I distinctly remember this being
> the case.
> 
>>> ---
>>>  host/rootfs/Makefile                               | 17 ++--
>>>  host/rootfs/default.nix                            | 94 ++++++----------------
>>>  host/rootfs/etc/init                               |  2 +-
>>>  host/rootfs/etc/mdev.conf                          |  7 --
>>>  host/rootfs/etc/mdev/listen                        |  2 +-
>>>  host/rootfs/etc/mdev/net/add                       |  1 +
>>
>> We probably ought to rename /etc/mdev… is there an idiomatic place to
>> put these sorts of things with udev?
> 
> /usr/lib/spectrum is the idiomatic place for this stuff.
> 
>>>  host/rootfs/etc/s6-rc/mdevd-coldplug/dependencies  |  4 -
>>>  host/rootfs/etc/s6-rc/mdevd-coldplug/up            |  4 -
>>>  host/rootfs/etc/s6-rc/mdevd/run                    |  5 --
>>>  host/rootfs/etc/s6-rc/ok-all/contents              |  2 +-
>>>  .../dependencies.d/systemd-udevd                   |  0
>>>  .../type                                           |  0
>>>  .../type.license                                   |  0
>>>  host/rootfs/etc/s6-rc/systemd-udevd-coldplug/up    |  3 +
>>>  host/rootfs/etc/s6-rc/systemd-udevd/flag-essential |  0
>>
>> Is it really that essential in comparison to other system services?  Why
>> would we need to keep udevd around but not all other services?
> systemd-udevd.service doesn't have RefuseManualStop=,
> so I will drop this.
> 
>>>  .../s6-rc/{mdevd => systemd-udevd}/notification-fd |  0
>>>  .../notification-fd.license                        |  0
>>>  host/rootfs/etc/s6-rc/systemd-udevd/run            | 10 +++
>>>  .../rootfs/etc/s6-rc/{mdevd => systemd-udevd}/type |  0
>>>  .../s6-rc/{mdevd => systemd-udevd}/type.license    |  0
>>>  host/rootfs/etc/s6-rc/vmm-env/contents             |  1 +
>>>  host/rootfs/etc/s6-rc/weston/dependencies          |  4 -
>>>  .../weston/dependencies.d/systemd-udevd-coldplug   |  0
>>>  host/rootfs/etc/udev/rules.d/99-spectrum.rules     |  5 ++
>>>  24 files changed, 56 insertions(+), 105 deletions(-)
>>>
>>> diff --git a/host/rootfs/etc/udev/rules.d/99-spectrum.rules b/host/rootfs/etc/udev/rules.d/99-spectrum.rules
>>> new file mode 100644
>>> index 0000000000000000000000000000000000000000..199397bc26874a261c9e1ea1778207fdb0d8ad39
>>> --- /dev/null
>>> +++ b/host/rootfs/etc/udev/rules.d/99-spectrum.rules
>>> @@ -0,0 +1,5 @@
>>> +# SPDX-License-Identifier: EUPL-1.2+
>>> +# SPDX-FileCopyrightText: 2025 Demi Marie Obenour <demiobenour@gmail.com>
>>> +ACTION!="remove", KERNEL=="kvm", RUN+="/etc/mdev/listen kvm"
>>> +ACTION!="remove", ENV{PCI_CLASS}=="2????", RUN+="/etc/mdev/net/add"
>>> +ACTION!="remove", ENV{MODALIAS}=="?*", RUN+="/usr/bin/modprobe -q $env{MODALIAS}"
>>
>> Can't we rely on the default rule in 80-drivers.rules to load modules
>> based on modalias?
> 
> I will see if this is necessary.  It might be that this was broken due
> to wrong permissions or something like that.

I tested this and it does not work.  I suspect that
this has a dependency on the service manager.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

[-- Attachment #1.1.2: OpenPGP public key --]
[-- Type: application/pgp-keys, Size: 7253 bytes --]

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 833 bytes --]