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

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Date:   Fri, 10 Nov 2023 10:33:28 +0100
From:   Maxime Chevallier <maxime.chevallier@...tlin.com>
To:     Jie Luo <quic_luoj@...cinc.com>
Cc:     <andrew@...n.ch>, <hkallweit1@...il.com>, <linux@...linux.org.uk>,
        <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
        <pabeni@...hat.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] net: phy: at803x: add QCA8084 ethernet phy
 support

On Fri, 10 Nov 2023 17:17:58 +0800
Jie Luo <quic_luoj@...cinc.com> wrote:

> On 11/10/2023 4:53 PM, Jie Luo wrote:
> > 
> > 
> > On 11/9/2023 5:16 PM, Maxime Chevallier wrote:  
> >> Hello,
> >>
> >> On Thu, 9 Nov 2023 16:32:36 +0800
> >> Jie Luo <quic_luoj@...cinc.com> 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 ?
> >>
> >> Thanks,
> >>
> >> Maxime  
> > 
> > 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...

Also, net-next is still currently closed.