netdev - [PATCH AUTOSEL 4.19 03/15] net: stmmac: allow CSR clock of 300MHz

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Date:   Wed, 22 Sep 2021 23:39:17 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Jesper Nilsson <jesper.nilsson@...s.com>,
        "David S . Miller" <davem@...emloft.net>,
        Sasha Levin <sashal@...nel.org>, peppe.cavallaro@...com,
        alexandre.torgue@...s.st.com, joabreu@...opsys.com,
        kuba@...nel.org, mcoquelin.stm32@...il.com, netdev@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org
Subject: [PATCH AUTOSEL 4.19 03/15] net: stmmac: allow CSR clock of 300MHz

From: Jesper Nilsson <jesper.nilsson@...s.com>

[ Upstream commit 08dad2f4d541fcfe5e7bfda72cc6314bbfd2802f ]

The Synopsys Ethernet IP uses the CSR clock as a base clock for MDC.
The divisor used is set in the MAC_MDIO_Address register field CR
(Clock Rate)

The divisor is there to change the CSR clock into a clock that falls
below the IEEE 802.3 specified max frequency of 2.5MHz.

If the CSR clock is 300MHz, the code falls back to using the reset
value in the MAC_MDIO_Address register, as described in the comment
above this code.

However, 300MHz is actually an allowed value and the proper divider
can be estimated quite easily (it's just 1Hz difference!)

A CSR frequency of 300MHz with the maximum clock rate value of 0x5
(STMMAC_CSR_250_300M, a divisor of 124) gives somewhere around
~2.42MHz which is below the IEEE 802.3 specified maximum.

For the ARTPEC-8 SoC, the CSR clock is this problematic 300MHz,
and unfortunately, the reset-value of the MAC_MDIO_Address CR field
is 0x0.

This leads to a clock rate of zero and a divisor of 42, and gives an
MDC frequency of ~7.14MHz.

Allow CSR clock of 300MHz by making the comparison inclusive.

Signed-off-by: Jesper Nilsson <jesper.nilsson@...s.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index af59761ddfa0..064e13bd2c8b 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -227,7 +227,7 @@ static void stmmac_clk_csr_set(struct stmmac_priv *priv)
 			priv->clk_csr = STMMAC_CSR_100_150M;
 		else if ((clk_rate >= CSR_F_150M) && (clk_rate < CSR_F_250M))
 			priv->clk_csr = STMMAC_CSR_150_250M;
-		else if ((clk_rate >= CSR_F_250M) && (clk_rate < CSR_F_300M))
+		else if ((clk_rate >= CSR_F_250M) && (clk_rate <= CSR_F_300M))
 			priv->clk_csr = STMMAC_CSR_250_300M;
 	}

-- 
2.30.2