netdev - Re: [PATCH v4 3/4] dt-bindings: net: xilinx

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Date:   Mon, 21 Mar 2022 23:56:19 +0000
From:   Robert Hancock <robert.hancock@...ian.com>
To:     "robh@...nel.org" <robh@...nel.org>,
        "radheys@...inx.com" <radheys@...inx.com>
CC:     "andrew@...n.ch" <andrew@...n.ch>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>,
        "michals@...inx.com" <michals@...inx.com>,
        "greentime.hu@...ive.com" <greentime.hu@...ive.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "harinik@...inx.com" <harinik@...inx.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "andy.chiu@...ive.com" <andy.chiu@...ive.com>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCH v4 3/4] dt-bindings: net: xilinx_axienet: add pcs-handle
 attribute

On Mon, 2022-03-21 at 18:44 -0500, Rob Herring wrote:
> On Mon, Mar 21, 2022 at 03:42:52PM +0000, Radhey Shyam Pandey wrote:
> > > -----Original Message-----
> > > From: Andy Chiu <andy.chiu@...ive.com>
> > > Sent: Monday, March 21, 2022 8:55 PM
> > > To: Radhey Shyam Pandey <radheys@...inx.com>; robert.hancock@...ian.com;
> > > Michal Simek <michals@...inx.com>
> > > Cc: davem@...emloft.net; kuba@...nel.org; pabeni@...hat.com;
> > > robh+dt@...nel.org; linux@...linux.org.uk; andrew@...n.ch;
> > > netdev@...r.kernel.org; devicetree@...r.kernel.org; Andy Chiu
> > > <andy.chiu@...ive.com>; Greentime Hu <greentime.hu@...ive.com>
> > > Subject: [PATCH v4 3/4] dt-bindings: net: xilinx_axienet: add pcs-handle
> > > attribute
> > > 
> > > Document the new pcs-handle attribute to support connecting to an
> > > external
> > > PHY in SGMII or 1000Base-X modes through the internal PCS/PMA PHY.
> > > 
> > > Signed-off-by: Andy Chiu <andy.chiu@...ive.com>
> > > Reviewed-by: Greentime Hu <greentime.hu@...ive.com>
> > > ---
> > >  Documentation/devicetree/bindings/net/xilinx_axienet.txt | 8 +++++++-
> > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/Documentation/devicetree/bindings/net/xilinx_axienet.txt
> > > b/Documentation/devicetree/bindings/net/xilinx_axienet.txt
> > > index b8e4894bc634..ba720a2ea5fc 100644
> > > --- a/Documentation/devicetree/bindings/net/xilinx_axienet.txt
> > > +++ b/Documentation/devicetree/bindings/net/xilinx_axienet.txt
> > > @@ -26,7 +26,8 @@ Required properties:
> > >  		  specified, the TX/RX DMA interrupts should be on that node
> > >  		  instead, and only the Ethernet core interrupt is optionally
> > >  		  specified here.
> > > -- phy-handle	: Should point to the external phy device.
> > > +- phy-handle	: Should point to the external phy device if exists.
> > > Pointing
> > > +		  this to the PCS/PMA PHY is deprecated and should be
> > > avoided.
> > >  		  See ethernet.txt file in the same directory.
> > >  - xlnx,rxmem	: Set to allocated memory buffer for Rx/Tx in the
> > > hardware
> > > 
> > > @@ -68,6 +69,11 @@ Optional properties:
> > >  		  required through the core's MDIO interface (i.e. always,
> > >  		  unless the PHY is accessed through a different bus).
> > > 
> > > + - pcs-handle: 	  Phandle to the internal PCS/PMA PHY in SGMII or
> > > 1000Base-X
> > > +		  modes, where "pcs-handle" should be preferably used to
> > > point
> > > +		  to the PCS/PMA PHY, and "phy-handle" should point to an
> > > +		  external PHY if exists.
> > 
> > I would like to have Rob feedback on this pcs-handle DT property.
> > 
> > The use case is generic i.e. require separate handle to internal SGMII
> > and external Phy so would prefer this new DT convention is 
> > standardized or we discuss possible approaches on how to handle
> > both phys and not add it as vendor specific property in the first 
> > place.
> 
> IMO, you should use 'phys' for the internal PCS phy. That's aligned with 
> other uses like PCIe, SATA, etc. (there is phy h/w that will do PCS, 
> PCIe, SATA). 'phy-handle' is for the ethernet PHY.

That might be confusing in some cases - I'm thinking of the Cadence GEM
controllers on the Xilinx MPSoC devices, where there is a PCS internal to the
controller, as well as a generic PHY device which is used to control the actual
low-level I/O transceiver on the chip (which can work in SGMII mode but also in
USB, PCIe, SATA modes). Calling them both just a "PHY" makes that distinction
rather unclear. In the case of that driver I believe its PCS is just "known
about" by the MAC when it is present and not configured in the device tree like
this driver does, but possibly it could be DT configurable someday (since I
think it's potentially possible to use a different PCS implemented in the
programmable logic if one wanted).

The way this PCS is accessed, both it and the potential external PHY are both
"Ethernet PHYs" in that they support standard Ethernet PHY registers etc., it's
just that generally the external one is the one the outside Linux networking
infrastructure cares about, the PCS PHY is more of an implementation detail
internal to the driver.

> 
> Rob