PCI: rockchip-ep: Handle PERST# signal in EP mode

PCI: rockchip-ep: Handle PERST# signal in EP mode

Currently, the Rockchip PCIe endpoint controller driver does not handle
the PERST# signal, which prevents detecting when link training should
actually be started or if the host resets the device. This however can
be supported using the controller reset_gpios property set as an input
GPIO for endpoint mode.

Modify the Rockchip PCI endpoint controller driver to get the reset_gpio
and its associated interrupt which is serviced using a threaded IRQ with
the function rockchip_pcie_ep_perst_irq_thread() as handler.

This handler function notifies a link down event corresponding to the RC
side asserting the PERST# signal using pci_epc_linkdown() when the gpio
is high. Once the gpio value goes down, corresponding to the RC
de-asserting the PERST# signal, link training is started. The polarity
of the gpio interrupt trigger is changed from high to low after the RC
asserted PERST#, and conversely changed from low to high after the RC
de-asserts PERST#.

Also, given that the host mode controller and the endpoint mode
controller use two different property names for the same PERST# signal
(ep_gpios property and reset_gpios property respectively), for clarity,
rename the ep_gpio field of struct rockchip_pcie to perst_gpio.

Link: https://lore.kernel.org/r/20241017015849.190271-14-dlemoal@kernel.org
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
[kwilczynski: make log messages consistent, add missing include]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>