netdev - Re: [PATCH net-next 2/2] net: axienet: Add support for AXI 2.5G MAC

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Message-ID: <ZzyQQV4qM_fTrpMf@shell.armlinux.org.uk>
Date: Tue, 19 Nov 2024 13:18:57 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: "Gupta, Suraj" <Suraj.Gupta2@....com>
Cc: Sean Anderson <sean.anderson@...ux.dev>,
	"andrew+netdev@...n.ch" <andrew+netdev@...n.ch>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	"kuba@...nel.org" <kuba@...nel.org>,
	"pabeni@...hat.com" <pabeni@...hat.com>,
	"Simek, Michal" <michal.simek@....com>,
	"Pandey, Radhey Shyam" <radhey.shyam.pandey@....com>,
	"horms@...nel.org" <horms@...nel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"git (AMD-Xilinx)" <git@....com>,
	"Katakam, Harini" <harini.katakam@....com>
Subject: Re: [PATCH net-next 2/2] net: axienet: Add support for AXI 2.5G MAC

On Tue, Nov 19, 2024 at 10:28:48AM +0000, Gupta, Suraj wrote:
> > -----Original Message-----
> > From: Russell King <linux@...linux.org.uk>
> > 
> > On Mon, Nov 18, 2024 at 11:00:22AM -0500, Sean Anderson wrote:
> > > On 11/18/24 10:56, Russell King (Oracle) wrote:
> > > > On Mon, Nov 18, 2024 at 01:48:22PM +0530, Suraj Gupta wrote:
> > > >> Add AXI 2.5G MAC support, which is an incremental speed upgrade of
> > > >> AXI 1G MAC and supports 2.5G speed only. "max-speed" DT property is
> > > >> used in driver to distinguish 1G and 2.5G MACs of AXI 1G/2.5G IP.
> > > >> If max-speed property is missing, 1G is assumed to support backward
> > > >> compatibility.
> > > >>
> > > >> Co-developed-by: Harini Katakam <harini.katakam@....com>
> > > >> Signed-off-by: Harini Katakam <harini.katakam@....com>
> > > >> Signed-off-by: Suraj Gupta <suraj.gupta2@....com>
> > > >> ---
> > > >
> > > > ...
> > > >
> > > >> -  lp->phylink_config.mac_capabilities = MAC_SYM_PAUSE |
> > MAC_ASYM_PAUSE |
> > > >> -          MAC_10FD | MAC_100FD | MAC_1000FD;
> > > >> +  lp->phylink_config.mac_capabilities = MAC_SYM_PAUSE |
> > > >> + MAC_ASYM_PAUSE;
> > > >> +
> > > >> +  /* Set MAC capabilities based on MAC type */  if (lp->max_speed
> > > >> + == SPEED_1000)
> > > >> +          lp->phylink_config.mac_capabilities |= MAC_10FD |
> > > >> + MAC_100FD | MAC_1000FD;  else
> > > >> +          lp->phylink_config.mac_capabilities |= MAC_2500FD;
> > > >
> > > > The MAC can only operate at (10M, 100M, 1G) _or_ 2.5G ?
> > >
> > > It's a PCS limitation. It either does (1000Base-X and/or SGMII) OR
> > > (2500Base-X). The MAC itself doesn't have this limitation AFAIK.
> > 
> > That means the patch is definitely wrong, and the proposed DT change is also
> > wrong.
> > 
> > If it's a limitation of the PCS, that limitation should be applied via the PCS's
> > .pcs_validate() method, not at the MAC level.
> > 
> As mentioned in IP PG (https://docs.amd.com/r/en-US/pg051-tri-mode-eth-mac/Ethernet-Overview#:~:text=Typical%20Ethernet%20Architecture-,MAC,-For%2010/100), it's limitation in MAC also.

I'm not reading it as a limitation of the MAC.

The limitation stated there is that internal mode (GMII) is only
supported for 2.5Gbps speeds. At 2.5Gbps speeds, the clock rate is
increased from 125MHz to 312.5MHz (which makes it non-compliant
with 802.3-2008, because that version doesn't define 2.5Gbps speeds.)

So long as the clock rate and interface can be safely switched, I
don't see any reason to restrict the MAC itself to be either
10/100/1G _or_ 2.5G.

Note that 2.5G will only become available if it is supported by one
of the supported interface modes (e.g. 2500base-X). If the supported
interface modes do not include a mode that supports >1G, then 2.5G
won't be available even if MAC_2500FD is set in mac_capabilities.

-- 
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