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

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Date: Fri, 28 Jul 2023 18:11:51 +0800
From: Jiawen Wu <jiawenwu@...stnetic.com>
To: "'Russell King \(Oracle\)'" <linux@...linux.org.uk>
Cc: <netdev@...r.kernel.org>,
	<andrew@...n.ch>,
	<hkallweit1@...il.com>,
	<Jose.Abreu@...opsys.com>,
	<mengyuanlou@...-swift.com>
Subject: RE: [PATCH net-next 4/7] net: pcs: xpcs: adapt Wangxun NICs for SGMII mode

On Tuesday, July 25, 2023 6:08 PM, Russell King (Oracle) wrote:
> 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.

There is a question about I2C MII read ops. I see that PHY in SFP-RJ45 module
is read by i2c_mii_read_default_c22(), but it limits the msgs[0].len=1.

A description in  the SFP-RJ45 datasheet shows:
The registers are accessible through the 2-wire serial CMOS EEPROM protocol
of the ATMEL AT24C01A or equivalent. The address of the PHY is 1010110x,
where x is the R/W bit. Each register's address is 000yyyyy, where yyyyy is the
binary equivalent of the register number. Write and read operations must send
or receive 16 bits of data, so the "multi-page" access protocol must be used.

So the PHY register address should be written twice: first high 8 bits, second low
8 bits. to read the register value.

Is there a problem with which driver?