PCI: rockchip: Use GPIOD_OUT_LOW flag while requesting ep_gpio

Rockchip platforms use 'GPIO_ACTIVE_HIGH' flag in the devicetree definition
for ep_gpio. This means, whatever the logical value set by the driver for
the ep_gpio, physical line will output the same logic level.

For instance,

  gpiod_set_value_cansleep(rockchip->ep_gpio, 0); --> Level low
  gpiod_set_value_cansleep(rockchip->ep_gpio, 1); --> Level high

But while requesting the ep_gpio, GPIOD_OUT_HIGH flag is currently used.
Now, this also causes the physical line to output 'high' creating trouble
for endpoint devices during host reboot.

When host reboot happens, the ep_gpio will initially output 'low' due to
the GPIO getting reset to its POR value. Then during host controller probe,
it will output 'high' due to GPIOD_OUT_HIGH flag. Then during
rockchip_pcie_host_init_port(), it will first output 'low' and then 'high'
indicating the completion of controller initialization.

On the endpoint side, each output 'low' of ep_gpio is accounted for PERST#
assert and 'high' for PERST# deassert. With the above mentioned flow during
host reboot, endpoint will witness below state changes for PERST#:

  (1) PERST# assert - GPIO POR state
  (2) PERST# deassert - GPIOD_OUT_HIGH while requesting GPIO
  (3) PERST# assert - rockchip_pcie_host_init_port()
  (4) PERST# deassert - rockchip_pcie_host_init_port()

Now the time interval between (2) and (3) is very short as both happen
during the driver probe(), and this results in a race in the endpoint.
Because, before completing the PERST# deassertion in (2), endpoint got
another PERST# assert in (3).

A proper way to fix this issue is to change the GPIOD_OUT_HIGH flag in (2)
to GPIOD_OUT_LOW. Because the usual convention is to request the GPIO with
a state corresponding to its 'initial/default' value and let the driver
change the state of the GPIO when required.

As per that, the ep_gpio should be requested with GPIOD_OUT_LOW as it
corresponds to the POR value of '0' (PERST# assert in the endpoint). Then
the driver can change the state of the ep_gpio later in
rockchip_pcie_host_init_port() as per the initialization sequence.

This fixes the firmware crash issue in Qcom based modems connected to
Rockpro64 based board.

Fixes: e77f847df5 ("PCI: rockchip: Add Rockchip PCIe controller support")
Closes: https://lore.kernel.org/mhi/20240402045647.GG2933@thinkpad/
Link: https://lore.kernel.org/linux-pci/20240416-pci-rockchip-perst-fix-v1-1-4800b1d4d954@linaro.org
Reported-by: Slark Xiao <slark_xiao@163.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Cc: stable@vger.kernel.org	# v4.9

drivers/pci/controller/pcie-rockchip.c

@@ -121,7 +121,7 @@ int rockchip_pcie_parse_dt(struct rockchip_pcie *rockchip)
 
 	if (rockchip->is_rc) {
 		rockchip->ep_gpio = devm_gpiod_get_optional(dev, "ep",
-							    GPIOD_OUT_HIGH);
+							    GPIOD_OUT_LOW);
 		if (IS_ERR(rockchip->ep_gpio))
 			return dev_err_probe(dev, PTR_ERR(rockchip->ep_gpio),
 					     "failed to get ep GPIO\n");