Date:   Tue, 12 Jul 2022 13:36:51 -0600
From:   Rob Herring <robh@...nel.org>
To:     Sean Anderson <sean.anderson@...o.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Madalin Bucur <madalin.bucur@....com>, netdev@...r.kernel.org,
        Russell King <linux@...linux.org.uk>,
        Paolo Abeni <pabeni@...hat.com>,
        linux-arm-kernel@...ts.infradead.org,
        Eric Dumazet <edumazet@...gle.com>,
        linux-kernel@...r.kernel.org,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        devicetree@...r.kernel.org
Subject: Re: [PATCH net-next v2 03/35] dt-bindings: net: fman: Add additional
 interface properties

On Thu, Jun 30, 2022 at 12:11:10PM -0400, Sean Anderson wrote:
> Hi Rob,
> 
> On 6/30/22 12:01 PM, Rob Herring wrote:
> > On Tue, Jun 28, 2022 at 06:13:32PM -0400, 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. This is generally OK, but
> >> restricts runtime reconfiguration. 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 XFI PCS/PMA be the default when
> >> phy-connection-type is xgmii. 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>
> >> ---
> >> 
> >> Changes in v2:
> >> - Better document how we select which PCS to use in the default case
> >> 
> >>  .../bindings/net/fsl,fman-dtsec.yaml          | 52 +++++++++++++++++--
> >>  .../devicetree/bindings/net/fsl-fman.txt      |  5 +-
> >>  2 files changed, 51 insertions(+), 6 deletions(-)
> >> 
> >> diff --git a/Documentation/devicetree/bindings/net/fsl,fman-dtsec.yaml b/Documentation/devicetree/bindings/net/fsl,fman-dtsec.yaml
> >> index 809df1589f20..ecb772258164 100644
> >> --- a/Documentation/devicetree/bindings/net/fsl,fman-dtsec.yaml
> >> +++ b/Documentation/devicetree/bindings/net/fsl,fman-dtsec.yaml
> >> @@ -85,9 +85,41 @@ properties:
> >>      $ref: /schemas/types.yaml#/definitions/phandle
> >>      description: A reference to the IEEE1588 timer
> >>  
> >> +  phys:
> >> +    description: A reference to the SerDes lane(s)
> >> +    maxItems: 1
> >> +
> >> +  phy-names:
> >> +    items:
> >> +      - const: serdes
> >> +
> >>    pcsphy-handle:
> >> -    $ref: /schemas/types.yaml#/definitions/phandle
> >> -    description: A reference to the PCS (typically found on the SerDes)
> >> +    $ref: /schemas/types.yaml#/definitions/phandle-array
> >> +    minItems: 1
> >> +    maxItems: 3
> > 
> > What determines how many entries?
> 
> It depends on what the particular MAC supports. From what I can tell, the following
> combinations are valid:
> 
> - Neither SGMII, QSGMII, or XFI
> - Just SGMII
> - Just QSGMII
> - SGMII and QSGMII
> - SGMII and XFI
> - All of SGMII, QSGMII, and XFI
> 
> All of these are used on different SoCs.

So there will be a different PCS device for SGMII, QSGMII, and XFI 
rather than 1 PCS device that supports those 3 interfaces?


> >> +    description: |
> >> +      A reference to the various PCSs (typically found on the SerDes). If
> >> +      pcs-names is absent, and phy-connection-type is "xgmii", then the first
> >> +      reference will be assumed to be for "xfi". Otherwise, if pcs-names is
> >> +      absent, then the first reference will be assumed to be for "sgmii".
> >> +
> >> +  pcs-names:
> >> +    $ref: /schemas/types.yaml#/definitions/string-array
> >> +    minItems: 1
> >> +    maxItems: 3
> >> +    contains:
> >> +      enum:
> >> +        - sgmii
> >> +        - qsgmii
> >> +        - xfi
> > 
> > This means '"foo", "xfi", "bar"' is valid. I think you want to 
> > s/contains/items/.
> > 
> >> +    description: The type of each PCS in pcsphy-handle.
> >> +
> > 
> >> +  rgmii:
> >> +    enum: [0, 1]
> >> +    description: 1 indicates RGMII is supported, and 0 indicates it is not.
> >> +
> >> +  mii:
> >> +    description: If present, indicates that MII is supported.
> > 
> > Types? Need vendor prefixes.
> 
> OK.
> 
> > Are these board specific or SoC specific? Properties are appropriate for 
> > the former. The latter case should be implied by the compatible string.
> 
> Unfortunately, there are not existing specific compatible strings for each
> device in each SoC. I suppose those could be added; however, this basically
> reflects how each device is hooked up. E.g. on one SoC a device would be
> connected to the RGMII pins, but not on another SoC. The MAC itself still
> has hardware support for RGMII, but such a configuration would not function.

A difference in instances on a given SoC would also be reason for 
properties rather than different compatible strings. However, we already 
have such properties. We have 'phy-connection-type' for which mode to 
use. Do you have some need to know all possible modes? I think there was 
something posted to allow 'phy-connection-type' to be an array of 
supported modes instead.

Rob

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