PCI/pwrctl: Ensure that pwrctl drivers are probed before PCI client drivers

As per the kernel device driver model, a pwrctl device is the supplier for
the PCI device, but the device link that enforces the supplier-consumer
relationship was previously created by the pwrctl driver. Therefore, the
driver model didn't prevent probing PCI client drivers before probing the
corresponding pwrctl drivers. This may lead to a race condition if the PCI
device was already powered on by the bootloader (before the pwrctl driver).

If the bootloader did not power on the PCI device, this wouldn't create any
problem as the pwrctl driver will be the one powering on the device, so the
PCI client driver always gets probed afterward. But if the device was
already powered on, then the device will be seen by the PCI core and the
PCI client driver may get probed before its pwrctl driver. This creates a
race condition as the pwrctl driver may change the device power state while
the device is being accessed by the client driver.

One such issue was already reported on the Qcom X13s platform with the WLAN
device and fixed with a hack in the WCN pwrseq driver by a9aaf1ff
("power: sequencing: request the WLAN enable GPIO as-is").

A cleaner way to fix the above mentioned race condition is to ensure that
the pwrctl drivers are always probed before the client drivers.

If the PCI device is associated with a pwrctl platform device with a power
supply, add a device link between the PCI device and the pwrctl device
before device_attach() in pci_bus_add_device().

Note that there is no need to explicitly remove the device link as that
will be taken care of by the driver core when the PCI device gets removed.

Fixes: 4565d265 ("PCI/pwrctl: Add PCI power control core code")
Fixes: 8fb18619 ("PCI/pwrctl: Create platform devices for child OF nodes of the port node")
Link: https://lore.kernel.org/r/20241025-pci-pwrctl-rework-v2-3-568756156cbe@linaro.org


Tested-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Tested-by: Krishna chaitanya chundru <quic_krichai@quicinc.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
[bhelgaas: squash fix from
https://lore.kernel.org/r/20241120062459.6371-1-manivannan.sadhasivam@linaro.org


for SPARCv9 issue reported by Jonathan Currier <dullfire@yahoo.com>]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
[kwilczynski: wrap code to 80 columns]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Cc: stable+noautosel@kernel.org		# Depends on power supply check