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PERST# is active low according to the PCIe specification.

However, the existing pcie-dw-rockchip.c driver does:

  gpiod_set_value(..., 0); msleep(100); gpiod_set_value(..., 1);

when asserting + deasserting PERST#.

This is of course wrong, but because all the device trees for this
compatible string have also incorrectly marked this GPIO as ACTIVE_HIGH:

  $ git grep -B 10 reset-gpios arch/arm64/boot/dts/rockchip/rk3568*
  $ git grep -B 10 reset-gpios arch/arm64/boot/dts/rockchip/rk3588*

The actual toggling of PERST# is correct, and we cannot change it anyway,
since that would break device tree compatibility.

However, this driver does request the GPIO to be initialized as
GPIOD_OUT_HIGH, which does cause a silly sequence where PERST# gets
toggled back and forth for no good reason.

Fix this by requesting the GPIO to be initialized as GPIOD_OUT_LOW (which
for this driver means PERST# asserted).

This will avoid an unnecessary signal change where PERST# gets deasserted
(by devm_gpiod_get_optional()) and then gets asserted (by
rockchip_pcie_start_link()) just a few instructions later.

Before patch, debug prints on EP side, when booting RC:

  [  845.606810] pci: PERST# asserted by host!
  [  852.483985] pci: PERST# de-asserted by host!
  [  852.503041] pci: PERST# asserted by host!
  [  852.610318] pci: PERST# de-asserted by host!

After patch, debug prints on EP side, when booting RC:

  [  125.107921] pci: PERST# asserted by host!
  [  132.111429] pci: PERST# de-asserted by host!

This extra, very short, PERST# assertion + deassertion has been reported to
cause issues with certain WLAN controllers, e.g. RTL8822CE.

Fixes: 0e898eb8 ("PCI: rockchip-dwc: Add Rockchip RK356X host controller driver")
Link: https://lore.kernel.org/linux-pci/20240417164227.398901-1-cassel@kernel.org

Tested-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Jianfeng Liu <liujianfeng1994@gmail.com>
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc: stable@vger.kernel.org	# v5.15+