[systemd-devel] Fwd: Proper way for systemd service to wait mdev gvt device initialization

[Alex Ivanov]

Could Intel fix that?

-------- Пересылаемое сообщение --------
20.05.2019, 15:18, "Andrei Borzenkov" <arvidjaar at gmail.com>:

On Mon, May 20, 2019 at 10:08 AM Mantas Mikulėnas <grawity at gmail.com> wrote:
>  On Sun, May 19, 2019 at 9:50 PM Alex Ivanov <gnidorah at ya.ru> wrote:
>>  Hello.
>>  What is the proper way to do that? I have a unit that creates gvt device in the system
>>
>>  ExecStart = "sh -c 'echo a297db4a-f4c2-11e6-90f6-d3b88d6c9525 > /sys/bus/pci/devices/0000:00:02.0/mdev_supported_types/i915-GVTg_V5_8/create'";
>>  ExecStop = "sh -c 'echo 1 > /sys/bus/pci/devices/0000:00:02.0/a297db4a-f4c2-11e6-90f6-d3b88d6c9525/remove'";
>
>  Personally, I would use an udev rule:
>
>  ACTION=="add", SUBSYSTEM=="pci", ENV{PCI_SLOT_NAME}=="0000:00:02.0", ATTR{mdev_supported_types/i915-GVTg_V5_8/create}="a297db4a-f4c2-11e6-90f6-d3b88d6c9525"

There is a race condition here, driver creates
.../mdev_supported_types after it has registered device so udev may
process event before directory is available.

>  Though on the other hand, a service is a good choice if you want to `systemctl stop` it later on.
>
>  ACTION=="add", SUBSYSTEM=="pci", ENV{PCI_SLOT_NAME}=="0000:00:02.0", ENV{SYSTEMD_WANTS}+="create-gvt.service"
>
>>  Ideally I would to like to start this service when 0000:00:02.0 device appears in the system, but the problem is that /sys/bus/pci/devices/0000:00:02.0/mdev_supported_types/ tree is populated later, so my service will fail.
>>
>>  So the question what is the proper way to fix that.
>
>  If the driver doesn't populate its sysfs entries in time, maybe it at least generates 'change' uevents? (udevadm --monitor)

I would tentatively say this is driver bug. This directory is created
during initial device setup, not in response to some event later. From
https://github.com/torvalds/linux/blob/master/Documentation/driver-model/device.txt:

--><--
As explained in Documentation/kobject.txt, device attributes must be
created before the KOBJ_ADD uevent is generated.
--><--

Note that some drivers even disable KOBJ_ADD notification during
device_register() and trigger it manually later, after sysfs layout is
complete. I cannot evaluate whether this directory can be created and
populated before device_register().

>  If there are no uevents either, well, there's nothing you can do from systemd's side. (Other than making a script that loops repeatedly checking "is it there yet? is it there yet?")

Should really be fixed on kernel side.
-------- Конец пересылаемого сообщения --------