[PATCH 2/6] arm64: dts: rockchip: Fix the 1Ghz ethernet on Qnap TS433

[Heiko Stuebner <heiko@sntech.de>]

While I want to remember that the dwmac on the TS433 was working at some
point, it seems I had my network always connected to the 2.5G nic after
that "point". And testing now revealed that the gmac does not actually
manages to transfer data.

Currently the gmac is set to rgmii-id with no rx-/tx-delay values set
which makes the driver use default values. Setting the delays to 0
also does not provide a working interface.

The vendor kernel is running with phy-mode set to rgmii and delays of
    tx_delay = 0x3c, rx_delay = 0x2f

As Andrew points out often, those delay values "are magic" and rgmii-id
should definitly be used "with small values" for delays, if really needed.

The Rockchip vendor-kernel actually contains additional code in the dwmac
driver to use the loopback function of a phy to find a window of usable
delay values. Code can be found for example on [0] and the process is
described in a document called "Rockchip GMAC RGMII Delayline Guide"
which has made its way onto the internet in a lot of places [1].

So I used this process, with the interface set to rgmii-id to get values
for this mode, which are in face lower than the ones for rgmii with
    tx_delay = 0x21, rx_delay = 0x15
and results in a working interface on the dwmac.

[0] https://github.com/armbian/linux-rockchip/blob/rk-6.1-rkr6.1/drivers/net/ethernet/stmicro/stmmac/dwmac-rk-tool.c
[1] https://gitlab.com/firefly-linux/docs/-/blob/rk356x/firefly/Common/GMAC/Rockchip_Developer_Guide_Linux_GMAC_RGMII_Delayline_EN.pdf

Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
---
 arch/arm64/boot/dts/rockchip/rk3568-qnap-ts433.dts | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm64/boot/dts/rockchip/rk3568-qnap-ts433.dts b/arch/arm64/boot/dts/rockchip/rk3568-qnap-ts433.dts
index 5656554ca284..e8af92a011d6 100644
--- a/arch/arm64/boot/dts/rockchip/rk3568-qnap-ts433.dts
+++ b/arch/arm64/boot/dts/rockchip/rk3568-qnap-ts433.dts
@@ -257,6 +257,8 @@ &gmac0_tx_bus2
 		     &gmac0_rx_bus2
 		     &gmac0_rgmii_clk
 		     &gmac0_rgmii_bus>;
+	rx_delay = <0x15>;
+	tx_delay = <0x21>;
 	status = "okay";
 };
 
-- 
2.47.2