Default async probing for DT based systems

[Saravana Kannan]

Hi,

TL;DR: I want to improve boot times by enabling async probing by
default for DT based systems. Can you give it a shot please?

Why do you think it might work when async probing has been a pain before?
==========================================================
Since fw_devlink=on is the default behavior and fw_devlink understands
approximately 24 DT bindings, it should capture most of the device
dependencies and ensure proper probe ordering. For example, on a Pixel
6, fw_devlink figures out 325 dependencies that it uses to enforce
probe ordering.

You can find all the dependencies under /sys/class/devlink.

Fine! What do you want me to try?
==========================
With the patches that have landed in 5.19-rc1, we can now enable
default async probing for all built-in drivers by adding the following
to the kernel command line:

driver_async_probe=*

I'd really appreciate it if folks can try adding driver_async_probe=*
to their kernel command line and report any boot time
improvements/regression or boot/probe failures it causes.

(* insert cat begging pic *)
Can I haz some reports pleez?!

Tips:
====
One easy way to check for devices probe failures is to compare the
output of this command with/without the commandline changes:

# find /sys/devices -name driver

Btw, if you need to force sync probing for a few drivers so that your
rootfs mounts properly, you can set the flag in the driver, or you can
list them as exceptions following the * in the command line option.
For example:
driver_async_probe=*,my_flash_driver,my_sdio_driver

Report these too.

Known issues:
===========
fw_devlink doesn't yet support phy-handle and mdio-parent DT bindings.
They have some tricky issues I need to resolve first and I need to add
both of them together -- I'm working on it. If there are other DT
bindings that are missing support, do let me know. Most of them are
easy to add.

The driver_async_probe=* won't have any useful impact for modules
without adding <module_name>.async_probe to the commandline or
async_probe as a module option when loading it. This is because
modules intentionally force sync before they exit. There's a patch for
a global flag for modules too [1], but it can never be set by default
for userspace reasons[2].

[1] - https://lore.kernel.org/lkml/20220604010101.719508-1-saravanak@xxxxxxxxxx/
[2] - https://lore.kernel.org/lkml/CA+55aFxV40V2WvNtJY3EC0F-B9wPk8CV2o1TTTyoF4CoWH7rhQ@xxxxxxxxxxxxxx/

Thanks,
Saravana