Hello,
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.
Applied to controller/rockchip, thank you!
[1/1] PCI: dw-rockchip: Fix initial PERST# GPIO value https://git.kernel.org/pci/pci/c/b00c483a1075
Krzysztof