Date:   Sat, 18 Jun 2022 11:55:38 -0400
From:   Sean Anderson <sean.anderson@...o.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Madalin Bucur <madalin.bucur@....com>, netdev@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Paolo Abeni <pabeni@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Eric Dumazet <edumazet@...gle.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH net-next 02/28] dt-bindings: net: fman: Add additional
 interface properties

Hi Krzysztof,

On 6/17/22 9:16 PM, Krzysztof Kozlowski wrote:
> On 17/06/2022 13:32, Sean Anderson wrote:
>> At the moment, MEMACs are configured almost completely based on the
>> phy-connection-type. That is, if the phy interface is RGMII, it assumed
>> that RGMII is supported. For some interfaces, it is assumed that the
>> RCW/bootloader has set up the SerDes properly. The actual link state is
>> never reported.
>>
>> To address these shortcomings, the driver will need additional
>> information. First, it needs to know how to access the PCS/PMAs (in
>> order to configure them and get the link status). The SGMII PCS/PMA is
>> the only currently-described PCS/PMA. Add the XFI and QSGMII PCS/PMAs as
>> well. The XFI (and 1GBase-KR) PCS/PMA is a c45 "phy" which sits on the
>> same MDIO bus as SGMII PCS/PMA. By default they will have conflicting
>> addresses, but they are also not enabled at the same time by default.
>> Therefore, we can let the default address for the XFI PCS/PMA be the
>> same as for SGMII. This will allow for backwards-compatibility.
>>
>> QSGMII, however, cannot work with the current binding. This is because
>> the QSGMII PCS/PMAs are only present on one MAC's MDIO bus. At the
>> moment this is worked around by having every MAC write to the PCS/PMA
>> addresses (without checking if they are present). This only works if
>> each MAC has the same configuration, and only if we don't need to know
>> the status. Because the QSGMII PCS/PMA will typically be located on a
>> different MDIO bus than the MAC's SGMII PCS/PMA, there is no fallback
>> for the QSGMII PCS/PMA.
>>
>> MEMACs (across all SoCs) support the following protocols:
>>
>> - MII
>> - RGMII
>> - SGMII, 1000Base-X, and 1000Base-KX
>> - 2500Base-X (aka 2.5G SGMII)
>> - QSGMII
>> - 10GBase-R (aka XFI) and 10GBase-KR
>> - XAUI and HiGig
>>
>> Each line documents a set of orthogonal protocols (e.g. XAUI is
>> supported if and only if HiGig is supported). Additionally,
>>
>> - XAUI implies support for 10GBase-R
>> - 10GBase-R is supported if and only if RGMII is not supported
>> - 2500Base-X implies support for 1000Base-X
>> - MII implies support for RGMII
>>
>> To switch between different protocols, we must reconfigure the SerDes.
>> This is done by using the standard phys property. We can also use it to
>> validate whether different protocols are supported (e.g. using
>> phy_validate). This will work for serial protocols, but not RGMII or
>> MII. Additionally, we still need to be compatible when there is no
>> SerDes.
>>
>> While we can detect 10G support by examining the port speed (as set by
>> fsl,fman-10g-port), we cannot determine support for any of the other
>> protocols based on the existing binding. In fact, the binding works
>> against us in some respects, because pcsphy-handle is required even if
>> there is no possible PCS/PMA for that MAC. To allow for backwards-
>> compatibility, we use a boolean-style property for RGMII (instead of
>> presence/absence-style). When the property for RGMII is missing, we will
>> assume that it is supported. The exception is MII, since no existing
>> device trees use it (as far as I could tell).
>>
>> Unfortunately, QSGMII support will be broken for old device trees. There
>> is nothing we can do about this because of the PCS/PMA situation (as
>> described above).
>>
>> Signed-off-by: Sean Anderson <sean.anderson@...o.com>
> 
> Thanks for the patch but you add too many new properties. The file
> should be converted to YAML/DT schema first.

Perhaps. However, conversion to yaml is a non-trivial task, especially for
a complicated binding such as this one. I am more than happy to rework this
patch to be based on a yaml conversion, but I do not have the bandwidth to
do so myself.

If you have any comments on the binding changes themselves, that would be
much appreciated.

--Sean

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