[PATCH v2 1/1] ARM: dts: meson8b: drop eth_txd0_1 and eth_txd1_1 from eth_rgmii_pins

[Martin Blumenstingl <martin.blumenstingl@googlemail.com>]

According to the Odroid-C1+ schematics the Ethernet TXD1 signal is
routed to GPIOH_5 and the TXD0 signal is routed to GPIOH_6.

The public S805 datasheet shows that TXD0 can be routed to DIF_2_P and
TXD1 can be routed to DIF_2_N instead. GPIO mux register 6 bit 4
configures DIF_2_P as ETH_TXD0, register 6 bit 5 configures DIF_2_N as
ETH_TXD1.
GPIO mux register 7 bit 21 configures GPIOH_5 as ETH_TXD1, register 7
bit 20 configures GPIOH_6 as ETH_TXD0.

The pin groups eth_txd0_0 (GPIOH_6) and eth_txd0_1 (DIF_2_P) are both
configured as Ethernet TXD0 data line in meson8b.dtsi. Also eth_txd1_0
(GPIOH_5) and eth_txd1_1 (DIF_2_N) are configured as TXD1 data line.

The vendor u-boot sources for Odroid-C1 use the following Ethernet
pinmux configuration:
  SET_CBUS_REG_MASK(PERIPHS_PIN_MUX_6, 0x3f4f);
  SET_CBUS_REG_MASK(PERIPHS_PIN_MUX_7, 0xf00000);
This translates to the following pin groups in the mainline kernel:
- register 6 bit  0: eth_rxd1 (DIF_0_P)
- register 6 bit  1: eth_rxd0 (DIF_0_N)
- register 6 bit  2: eth_rx_dv (DIF_1_P)
- register 6 bit  3: eth_rx_clk (DIF_1_N)
- register 6 bit  6: eth_tx_en (DIF_3_P)
- register 6 bit  8: eth_ref_clk (DIF_3_N)
- register 6 bit  9: eth_mdc (DIF_4_P)
- register 6 bit 10: eth_mdio_en (DIF_4_N)
- register 6 bit 11: eth_tx_clk (GPIOH_9)
- register 6 bit 12: eth_txd2 (GPIOH_8)
- register 6 bit 13: eth_txd3 (GPIOH_7)
- register 7 bit 20: eth_txd0_0 (GPIOH_6)
- register 7 bit 21: eth_txd1_0 (GPIOH_5)
- register 7 bit 22: currently unknown, might be eth_rxd2 or eth_rxd3
- register 7 bit 23: currently unknown, might be eth_rxd2 or eth_rxd3

Dropping eth_txd0_1 and eth_txd1_1 improves the receive performance.
This is probably due to the eth_txd0 and eth_txd1 signal being routed to
the wrong pins, so data can only be transferred on eth_txd2 and
eth_txd3. However, I have no scope to confirm this assumption.

iperf3 statistics before this change:
- transmitting from Odroid-C1: 741 Mbits/sec (0 retries)
- receiving on Odroid-C1: 199 Mbits/sec (1713 retries)

iperf3 statistics after this change:
- transmitting from Odroid-C1: 667 Mbits/sec (0 retries)
- receiving on Odroid-C1: 750 Mbits/sec (0 retries)

Fixes: b96446541d8390 ("ARM: dts: meson8b: extend ethernet controller description")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Cc: Emiliano Ingrassia <ingrassia@epigenesys.com>
Cc: Linus Lüssing <linus.luessing@c0d3.blue>
---
 arch/arm/boot/dts/meson8b.dtsi | 2 --
 1 file changed, 2 deletions(-)

diff --git a/arch/arm/boot/dts/meson8b.dtsi b/arch/arm/boot/dts/meson8b.dtsi
index 22d775460767..ecca36a8678d 100644
--- a/arch/arm/boot/dts/meson8b.dtsi
+++ b/arch/arm/boot/dts/meson8b.dtsi
@@ -270,9 +270,7 @@
 				groups = "eth_tx_clk",
 					 "eth_tx_en",
 					 "eth_txd1_0",
-					 "eth_txd1_1",
 					 "eth_txd0_0",
-					 "eth_txd0_1",
 					 "eth_rx_clk",
 					 "eth_rx_dv",
 					 "eth_rxd1",
-- 
2.20.1


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