PCI: Allow built-in drivers to use async initial probing

The PCI core has historically not allowed built-in drivers to opt in to
async initial probing:  Drivers may set "PROBE_PREFER_ASYNCHRONOUS", but
initial probing always happens synchronously.  That's because the PCI core
uses device_attach() instead of device_initial_probe().

Should a driver return -EPROBE_DEFER on initial probe, reprobing later on
does honor the PROBE_PREFER_ASYNCHRONOUS setting.  Modular drivers are
also allowed to probe asynchronously, which is inconsistent.

The choice of device_attach() is likely not deliberate:  It was introduced
in 2013 with commit 58d9a38f6fac ("PCI: Skip attaching driver in
device_add()"), but asynchronous probing was added two years later with
commit 765230b5f084 ("driver-core: add asynchronous probing support for
drivers").

According to the kernel-doc of "enum probe_type", "the end goal is to
switch the kernel to use asynchronous probing by default".  To this end,
use device_initial_probe() to allow asynchronous initial probing.  The
function returns void, making the return value check unnecessary.

Initial PCI probing often takes on the order of seconds even on laptops,
so this may speed up booting significantly.

A small number of PCI drivers already opt in to asynchronous probing.
Their maintainers (who are all cc'ed) should watch out for issues, now
that asynchronous probing is not just allowed for deferred and modular
probing, but also initial probing:

  hl_pci_driver        drivers/accel/habanalabs/common/habanalabs_drv.c
  cxl_pci_driver       drivers/cxl/pci.c
  quicki2c_driver      drivers/hid/intel-thc-hid/intel-quicki2c/pci-quicki2c.c
  quickspi_driver      drivers/hid/intel-thc-hid/intel-quickspi/pci-quickspi.c
  i801_driver          drivers/i2c/busses/i2c-i801.c
  mei_me_driver        drivers/misc/mei/pci-me.c
  mei_vsc_drv          drivers/misc/mei/platform-vsc.c
  sdhci_driver         drivers/mmc/host/sdhci-pci-core.c
  nvme_driver          drivers/nvme/host/pci.c
  ehci_pci_driver      drivers/usb/host/ehci-pci.c
  hvfb_pci_stub_driver drivers/video/fbdev/hyperv_fb.c

All other driver maintainers may test asynchronous probing by specifying
the command line parameter "driver_async_probe=drv_name1,drv_name2,...",
and on success setting "probe_type = PROBE_PREFER_ASYNCHRONOUS" in the
pci_driver struct.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
[bhelgaas: updated commit log per https://lore.kernel.org/r/aHYUh7WoDlhHckxd@wunner.de]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/53abe6f5ac7c631f95f5d061aa748b192eda0379.1751614426.git.lukas@wunner.de

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