Message-ID: <DB8PR04MB698591AAC029ADE9F7FFF69BEC230@DB8PR04MB6985.eurprd04.prod.outlook.com>
Date:   Fri, 3 Jan 2020 12:03:44 +0000
From:   "Madalin Bucur (OSS)" <madalin.bucur@....nxp.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        "Madalin Bucur (OSS)" <madalin.bucur@....nxp.com>
CC:     "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "hkallweit1@...il.com" <hkallweit1@...il.com>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>
Subject: RE: [PATCH 1/6] net: phy: add interface modes for XFI, SFI

> -----Original Message-----
> From: Russell King - ARM Linux admin <linux@...linux.org.uk>
> Sent: Friday, January 3, 2020 11:42 AM
> To: Madalin Bucur (OSS) <madalin.bucur@....nxp.com>
> Cc: devicetree@...r.kernel.org; davem@...emloft.net;
> netdev@...r.kernel.org; andrew@...n.ch; f.fainelli@...il.com;
> hkallweit1@...il.com; shawnguo@...nel.org
> Subject: Re: [PATCH 1/6] net: phy: add interface modes for XFI, SFI
> 
> On Fri, Jan 03, 2020 at 09:27:18AM +0000, Russell King - ARM Linux admin
> wrote:
> > Merely specifying "xfi" does not tell you what you need to do to achieve
> > XFI compliance at the point defined in INF8077i.  Plus, XFI can also be
> > protocols _other_ than 10GBASE-R.
> >
> > Claiming that "XFI" properly defines the interface is utter rubbish. It
> > does not. XFI defines the electrical characteristics *only* and not
> > the underlying protocol. It is not limited to 10GBASE-R, but includes
> > other protocols as well.
> 
> Let me quote from INF-8077i, which is the XFP MSA, the document
> responsible for defining XFI:
> 
> 3.1 INTRODUCTION
>    XFI signaling is based on high speed low voltage logic, with nominal
> 100
>    Ohms differential impedance and AC coupled in the module. XFI was de-
>    veloped with the primary goal of low power and low Electro-Magnetic-In-
> 
>    terference (EMI). To satisfy this requirement the nominal differential
> signal   levels are 500 mV p-p with edge speed control to reduce EMI.
> 
> 3.2 XFI APPLICATIONS DEFINITION
>    The application reference model for XFI, which connects a high speed
>    ASIC/SERDES to the XFP module is shown in Figure 4. The XFI interface
>    is designed to support SONET OC-192, IEEE.Std-802.3ae, 10GFC and
>    G.709(OTU-2) applications. The SERDES is required to meet the applica-
>    tion requirements for jitter generation and transfer when interfaced
> with a
>    compliant XFP module through an XFP compliant channel. Modules or
> 
>    hosts designed only for 10 Gigabit Ethernet or 10GFC are not required
> to
>    meet more stringent Telecom jitter requirements. XFI supported data
>    rates are listed in Table 5. XFP compliant module are not required to
> sup-
>    port all the rates listed in Table 5 in simultaneously.
> 
>    Standard                            Description           Nominal Bit
> Rate     Units
>    OC-192/SDH-64                         SONET                     9.95
> Gigabaud
>    IEEE std-802.3ae             10 Gb/s Ethernet LAN PHY           10.31
> Gigabaud
>    INCITS/T11 Project 1413-D             10GFC                     10.52
> Gigabaud
>    ITU G.709(OTU-2)                 OC-192 Over FEC                10.70
> Gigabaud
>    Emerging                     10Gb/s Ethernet Over G.709         11.09
> Gigabaud
> 
> So here, we can clearly see that it's possible to run SONET, 10GBASE-R,
> 10G Fiberchannel, OC-192, and G.709 over XFI, so XFI does not describe
> _just_ ethernet. If we're going to be configuring a serdes to output
> XFI, we need to know a lot more than just "XFI".
> 
>    XFI Compliance points are defined as the following:
> 
>    •   A: SerDes transmitter output at ASIC/SerDes package pin on a DUT
>        board 3.6 and A.1
>    •   B: Host system SerDes output across the host board and connector
>        at the Host Compliance Test Card 3.7.1 and A.2
>    •   B': XFP transmitter input across the Module Compliance Test Board
>        3.8.1 and A.3.
>    •   C: Host system input at the Host Compliance Test Card input 3.7.2
>        and A.2
>    •   C': XFP module output across the Module Compliance Test Board
>        3.8.2 and A.3.
> 
>    •   D: ASIC/SerDes input package pin on the DUT board 3.6.2 and A.1.
> 
>    ASIC/SerDes compliance points are informative.
> 
> So the electrical points that count are B, B', C and C'. A and D
> are merely "informative".  Hence, compliance with XFI is required
> to take the entire platform into account, not just the output of
> the serdes/asic.  That means the performance of the PCB needs to
> be described in DT if you want to achieve compliance with XFI.
> phy_interface_t can't do that.
> 
> So, let me re-iterate: neither XFI nor SFI are suitable for
> phy_interface_t. XFI defines merely a group of possible protocols
> and an electrical specification. It doesn't tell you which of those
> protocols you should be using.

The disconnect is you are focused on phy_interface_t and I'm looking at
the device tree as there's where one starts (actually at the device tree
binding document). Your concern is with configuring the HW to use a certain
PCS setting, thus 10GBASE-R, while I'm concerned with the fact the device
tree must not configure software but describe HW. So let's do some archeology
on the matter, to try to understand where this is coming from.

A device tree entry that described the electrical interface between the chip
harboring an Ethernet MAC bloc and another chip that served the purpose of an
Ethernet PHY was needed. In the past this parameter was "phy-connection-type".
We find it detailed in kernel v3.0 in 
Documentation/devicetree/bindings/net/fsl-tsec-phy.txt:52

  - phy-connection-type : a string naming the controller/PHY interface type,
    i.e., "mii" (default), "rmii", "gmii", "rgmii", "rgmii-id", "sgmii",
    "tbi", or "rtbi".  This property is only really needed if the connection
    is of type "rgmii-id", as all other connection types are detected by
    hardware.

Later, in kernel version v4.0 we find it described in 
Documentation/devicetree/bindings/net/ethernet.txt:16

- phy-mode: string, operation mode of the PHY interface; supported values are
  "mii", "gmii", "sgmii", "qsgmii", "tbi", "rev-mii", "rmii", "rgmii", "rgmii-id",
  "rgmii-rxid", "rgmii-txid", "rtbi", "smii", "xgmii"; this is now a de-facto
  standard property;
- phy-connection-type: the same as "phy-mode" property but described in ePAPR;

Now (v5.5-rc3) we find it moved to
Documentation/devicetree/bindings/net/ethernet-controller.yaml:57:

  phy-connection-type:
    description:
      Operation mode of the PHY interface
    enum:
      # There is not a standard bus between the MAC and the PHY,
      # something proprietary is being used to embed the PHY in the
      # MAC.
      - internal
      - mii
      - gmii
      - sgmii
      - qsgmii
      - tbi
      - rev-mii
      - rmii

      # RX and TX delays are added by the MAC when required
      - rgmii

      # RGMII with internal RX and TX delays provided by the PHY,
      # the MAC should not add the RX or TX delays in this case
      - rgmii-id

      # RGMII with internal RX delay provided by the PHY, the MAC
      # should not add an RX delay in this case
      - rgmii-rxid

      # RGMII with internal TX delay provided by the PHY, the MAC
      # should not add an TX delay in this case
      - rgmii-txid
      - rtbi
      - smii
      - xgmii 
      - trgmii
      - 1000base-x
      - 2500base-x
      - rxaui
      - xaui

      # 10GBASE-KR, XFI, SFI
      - 10gbase-kr
      - usxgmii

  phy-mode:
    $ref: "#/properties/phy-connection-type"

At each step, it was changed a bit. It started by describing the actual MII
connection (RGMII, SGMII, XGMII). Later is was changed to denote "operation
mode" of the interface. There is no reference here to PCS configuration (it
could not be as the device tree does not configure but describes the HW). I
see no reference about this device tree entry describing the protocol only
(I'm referring to your second reply on this here). If the device tree binding
does not describe the protocol only, but when it's parsed in software, into
the phy_interface_t it describes only the protocol and not the actual interface
type("mode"), then we have a disconnect here. This type is described as:

/* Interface Mode definitions */
typedef enum {
        PHY_INTERFACE_MODE_NA,
        PHY_INTERFACE_MODE_INTERNAL,
        PHY_INTERFACE_MODE_MII,
        PHY_INTERFACE_MODE_GMII,
        PHY_INTERFACE_MODE_SGMII,
        PHY_INTERFACE_MODE_TBI,
        PHY_INTERFACE_MODE_REVMII,
        PHY_INTERFACE_MODE_RMII,
        PHY_INTERFACE_MODE_RGMII,
        PHY_INTERFACE_MODE_RGMII_ID,
        PHY_INTERFACE_MODE_RGMII_RXID,
        PHY_INTERFACE_MODE_RGMII_TXID,
        PHY_INTERFACE_MODE_RTBI,
        PHY_INTERFACE_MODE_SMII,
        PHY_INTERFACE_MODE_XGMII,
...
} phy_interface_t;


So we can notice that is in sync with the device tree binding document.
Please note the RGMII, RGMII_ID, RGMII_RXID, RGMII_TXID. The only
difference there is in the delays on the electrical connections between
the chips. Take a step back, look at the list of existing entries, at
the history of this and see if it maps to one story or another.

Regards,
Madalin

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