Re: [PATCH v2 1/3] net: phy: at803x: add QCA8084 ethernet phy support

[Vladimir Oltean]

On Fri, Nov 10, 2023 at 05:56:09PM +0800, Jie Luo wrote:
> > > > > > > What I understand from this is that this PHY can be used either as a
> > > > > > > switch, in which case port 4 would be connected to the host interface
> > > > > > > at up to 2.5G, or as a quad-phy, but since it uses QUSGMII the link
> > > > > > > speed would be limited to 1G per-port, is that correct ?
> > > > > > 
> > > > > > When the PHY works on the interface mode QUSGMII for quad-phy, all 4
> > > > > > PHYs can support to the max link speed 2.5G, actually the PHY can
> > > > > > support to max link speed 2.5G for all supported interface modes
> > > > > > including qusgmii and sgmii.
> > > > > 
> > > > > I'm a bit confused then, as the USGMII spec says that Quad USGMII really
> > > > > is for quad 10/100/1000 speeds, using 10b/8b encoding.
> > > > > 
> > > > > Aren't you using the USXGMII mode instead, which can convey 4 x 2.5Gbps
> > > > >    with 66b/64b encoding ?
> > > > 
> > > > Hi Maxime,
> > > > Yes, for quad PHY mode, it is using 66b/64 encoding.
> > > > 
> > > > it seems that PHY_INTERFACE_MODE_USXGMII is for single port,
> > > > so i take the interface name PHY_INTERFACE_MODE_QUSGMII for
> > > > quad PHYs here.
> > > > 
> > > > can we apply PHY_INTERFACE_MODE_USXGMII to quad PHYs in this
> > > > case(qca8084 quad PHY mode)?
> > > > 
> > > > Thanks,
> > > > Jie.
> > > 
> > > one more thing, if we use the PHY_INTERFACE_MODE_USXGMII for
> > > the quad PHY here, the MAC serdes can't distinguish the actual
> > > mode PHY_INTERFACE_MODE_USXGMII and 10G-QXGMII(qca8084 quad phy mode),
> > > the MAC serdes has the different configurations for usxgmii(10g single
> > > port) and qxsgmii(quad PHY).
> > 
> > Yes you do need a way to know which mode to use, what I'm wondering is
> > that the usxgmii spec actually defines something like 9 different modes
> > ( 1/2/4/8 ports, with a total bandwidth ranging from 2.5Gbps to 20 Gbps
> > ), should we define a phy mode for all of these variants, or should we
> > have another way of getting the mode variant (like, saying I want to
> > use usxgmii, in 4 ports mode, with the serdes at 10.3125Gbps).
> > 
> > That being said, QUSGMII already exists to define a specific variant of
> > USGMII, so maybe adding 10G-QXGMII is fine...
> 
> Yes, Maxime, I agree with this solution, the name 10G-QXGMII is exactly
> the working mode of qca8084 quad phy mode.

FWIW, NXP has these 2 patches in its trees. One day I was going to make
an attempt at upstreaming them, but the Aquantia PHY driver is a bit of
a sticky topic since I could find no way to distinguish between the
single-port and multi-port variants of USXGMII in this PHY IP, and the
AQR412 uses a multi-port mode which is now treated as USXGMII by Linux.

Anyway, if you do make progress with this ahead of me, please be aware
that these patches exist, and I would appreciate if you tried to keep
the name as "10g-qxgmii" so that I don't have to modify the (downstream)
device trees again :)