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Subject[PATCH AUTOSEL for 4.15 041/189] net: stmmac: dwmac-meson8b: fix setting the RGMII TX clock on Meson8b
From: Martin Blumenstingl <>

[ Upstream commit 433c6cab9d298687c097f6ee82e49157044dc7c6 ]

Meson8b only supports MPLL2 as clock input. The rate of the MPLL2 clock
set by Odroid-C1's u-boot is close to (but not exactly) 500MHz. The
exact rate is 500002394Hz, which is calculated in
drivers/clk/meson/clk-mpll.c using the following formula:
DIV_ROUND_UP_ULL((u64)parent_rate * SDM_DEN, (SDM_DEN * n2) + sdm)
Odroid-C1's u-boot configures MPLL2 with the following values:
- SDM_DEN = 16384
- SDM = 1638
- N2 = 5

The 250MHz clock (m250_div) inside dwmac-meson8b driver is derived from
the MPLL2 clock. Due to MPLL2 running slightly faster than 500MHz the
common clock framework chooses a divider which is too big to generate
the 250MHz clock (a divider of 2 would be needed, but this is rounded up
to a divider of 3). This breaks the RTL8211F RGMII PHY on Odroid-C1
because it requires a (close to) 125MHz RGMII TX clock (on Gbit speeds,
the IP block internally divides that down to 25MHz on 100Mbit/s
connections and 2.5MHz on 10Mbit/s connections - we don't need any
special configuration for that).

Round the divider to the closest value to prevent this issue on Meson8b.
This means we'll now end up with a clock rate for the RGMII TX clock of
125001197Hz (= 125MHz plus 1197Hz), which is close-enough to 125MHz.
This has no effect on the Meson GX SoCs since there fclk_div2 is used as
input clock, which has a rate of 1000MHz (and thus is divisible cleanly
to 250MHz and 125MHz).

Fixes: 566e8251625304 ("net: stmmac: add a glue driver for the Amlogic Meson 8b / GXBB DWMAC")
Reported-by: Emiliano Ingrassia <>
Signed-off-by: Martin Blumenstingl <>
Reviewed-by: Jerome Brunet <>
Tested-by: Jerome Brunet <>
Signed-off-by: David S. Miller <>
Signed-off-by: Sasha Levin <>
drivers/net/ethernet/stmicro/stmmac/dwmac-meson8b.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac-meson8b.c b/drivers/net/ethernet/stmicro/stmmac/dwmac-meson8b.c
index 4404650b32c5..157e12e15f28 100644
--- a/drivers/net/ethernet/stmicro/stmmac/dwmac-meson8b.c
+++ b/drivers/net/ethernet/stmicro/stmmac/dwmac-meson8b.c
@@ -144,7 +144,9 @@ static int meson8b_init_clk(struct meson8b_dwmac *dwmac)
dwmac->m250_div.shift = PRG_ETH0_CLK_M250_DIV_SHIFT;
dwmac->m250_div.width = PRG_ETH0_CLK_M250_DIV_WIDTH;
dwmac->m250_div.hw.init = &init;
+ dwmac->m250_div.flags = CLK_DIVIDER_ONE_BASED |

dwmac->m250_div_clk = devm_clk_register(dev, &dwmac->m250_div.hw);
if (WARN_ON(IS_ERR(dwmac->m250_div_clk)))
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