Re: [PATCH net-next 4/7] net: pcs: xpcs: adapt Wangxun NICs for SGMII mode

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Tue, Jul 25, 2023 at 10:58:25AM +0100, Russell King (Oracle) wrote:
> > The information obtained from the IC designer is that "PHY/MAC side SGMII"
> > is configured by experimentation. For these different kinds of NICs:
> > 1) fiber + SFP-RJ45 module: PHY side SGMII
> > 2) copper (pcs + external PHY): MAC side SGMII
> 
> This makes no sense. a PHY on a RJ45 SFP module is just the same as a
> PHY integrated into a board with the MAC.


MAC ---- PCS <----- sgmii -----> PHY (whether on a board or SFP)

Control word flow:
             <------------------ link, speed, duplex
	     ------------------> acknowledge (value = 0x4001)

Sometimes, it's possible to connect two MACs/PCSs together:

MAC ---- PCS <----- sgmii -----> PCS ---- MAC

and in this case, one PCS would need to be configured in "MAC" mode
and the other would need to be configured in "PHY" mode because SGMII
is fundamentally asymmetric.

Here is the definition of the control word sent by either end:

Bit	MAC->PHY	PHY->MAC
15	0: Reserved	Link status, 1 = link up
14	1: Acknowledge	Reserved for AN acknowledge
13	0: Reserved	0: Reserved
12	0: Reserved	Duplex mode 1 = full, 0 = half
11:10	0: Reserved	Speed 11 = Reserved 10=1G, 01=100M, 00=10M
9:1	0: Reserved	0: Reserved
0	1		1

So my guess would be that "PHY side SGMII" means the device generates
the "PHY->MAC" format word whereas "MAC side SGMII" generates the
"MAC->PHY" format word - and it's the latter that you want to be using
both for Copper SFPs, which are no different from PHYs integrated onto
the board connected with SGMII.

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