[PATCH v3] PCI: dw-rockchip: Fix initial PERST# GPIO value

[Niklas Cassel <cassel@kernel.org>]

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: 0e898eb8df4e ("PCI: rockchip-dwc: Add Rockchip RK356X host controller driver")
Tested-by: Jianfeng Liu <liujianfeng1994@gmail.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc: stable@vger.kernel.org	# 5.15+
---
Changes since v2:
-Picked up tag from Heiko.
-Change subject (Bjorn).
-s/PERST/PERST#/ (Bjorn).

 drivers/pci/controller/dwc/pcie-dw-rockchip.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/pci/controller/dwc/pcie-dw-rockchip.c b/drivers/pci/controller/dwc/pcie-dw-rockchip.c
index d6842141d384..a909e42b4273 100644
--- a/drivers/pci/controller/dwc/pcie-dw-rockchip.c
+++ b/drivers/pci/controller/dwc/pcie-dw-rockchip.c
@@ -240,7 +240,7 @@ static int rockchip_pcie_resource_get(struct platform_device *pdev,
 		return PTR_ERR(rockchip->apb_base);
 
 	rockchip->rst_gpio = devm_gpiod_get_optional(&pdev->dev, "reset",
-						     GPIOD_OUT_HIGH);
+						     GPIOD_OUT_LOW);
 	if (IS_ERR(rockchip->rst_gpio))
 		return PTR_ERR(rockchip->rst_gpio);
 
-- 
2.44.0