linux-kernel - Re: [PATCH v2] net: mvneta: Fix Serdes configuration for 2.5Gbps modes

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Date:   Fri, 12 Jun 2020 09:47:10 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Sascha Hauer <s.hauer@...gutronix.de>
Cc:     netdev@...r.kernel.org,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kernel@...gutronix.de
Subject: Re: [PATCH v2] net: mvneta: Fix Serdes configuration for 2.5Gbps
 modes

On Fri, Jun 12, 2020 at 10:38:47AM +0200, Sascha Hauer wrote:
> The Marvell MVNETA Ethernet controller supports a 2.5Gbps SGMII mode
> called DRSGMII. Depending on the Port MAC Control Register0 PortType
> setting this seems to be either an overclocked SGMII mode or 2500BaseX.
> 
> This patch adds the necessary Serdes Configuration setting for the
> 2.5Gbps modes. There is no phy interface mode define for overclocked
> SGMII, so only 2500BaseX is handled for now.
> 
> As phy_interface_mode_is_8023z() returns true for both
> PHY_INTERFACE_MODE_1000BASEX and PHY_INTERFACE_MODE_2500BASEX we
> explicitly test for 1000BaseX instead of using
> phy_interface_mode_is_8023z() to differentiate the different
> possibilities.
> 
> Fixes: da58a931f248f ("net: mvneta: Add support for 2500Mbps SGMII")
> Signed-off-by: Sascha Hauer <s.hauer@...gutronix.de>

2500base-X is used today on Armada 388 and Armada 3720 platforms and
works - it is known to interoperate with Marvell PP2.2 hardware, as
well was various SFPs such as the Huawei MA5671A at 2.5Gbps.  The way
it is handled on these platforms is via the COMPHY, requesting that
the serdes is upclocked from 1.25Gbps to 3.125Gbps.

This "DRSGMII" mode is not mentioned in the functional specs for either
the Armada 388 or Armada 3720, the value you poke into the register is
not mentioned either.  As I've already requested, some information on
exactly what this "DRSGMII" is would be very useful, it can't be
"double-rate SGMII" because that would give you 2Gbps instead of 1Gbps.

So, I suspect this breaks the platforms that are known to work.

We need a proper description of what DRSGMII is before we can consider
taking any patches for it.

So, sorry, but NAK.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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