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

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Message-ID: <6d2cdd37-f2d7-4537-a7e7-8e525c4378dd@linux.dev>
Date: Tue, 19 Nov 2024 11:42:15 -0500
From: Sean Anderson <sean.anderson@...ux.dev>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Andrew Lunn <andrew@...n.ch>, Suraj Gupta <suraj.gupta2@....com>,
 andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com,
 kuba@...nel.org, pabeni@...hat.com, michal.simek@....com,
 radhey.shyam.pandey@....com, horms@...nel.org, netdev@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 git@....com, harini.katakam@....com
Subject: Re: [PATCH net-next 2/2] net: axienet: Add support for AXI 2.5G MAC

On 11/19/24 10:49, Russell King (Oracle) wrote:
> On Tue, Nov 19, 2024 at 10:26:52AM -0500, Sean Anderson wrote:
>> On 11/18/24 20:35, Andrew Lunn wrote:
>> > 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.
>> > 
>> > 
>> > And can the PCS change between these modes? It is pretty typical to
>> > use SGMII for 10/100/1G and then swap to 2500BaseX for 2.5G.
>> 
>> Not AFAIK. There's only a bit for switching between 1000Base-X and
>> SGMII. 2500Base-X is selected at synthesis time, and AIUI the serdes
>> settings are different.
> 
> Okay. First it was a PCS limitation. Then it was a MAC limitation. Now
> it's a synthesis limitation.
> 
> I'm coming to the conclusion that those I'm communicating with don't
> actually know, and are just throwing random thoughts out there.
> 
> Please do the research, and come back to me with a real and complete
> answer, not some hand-wavey "it's a limitation of X, no it's a
> limitation of Y, no it's a limitation of Z" which looks like no one
> really knows the correct answer.
> 
> Just because the PCS doesn't have a bit that selects 2500base-X is
> meaningless. 2500base-X is generally implemented by upclocking
> 1000base-X by 2.5x. Marvell does this at their Serdes, there is
> no configuration at the MAC/PCS for 2.5G speeds.
> 
> The same is true of 10GBASE-R vs 5GBASE-R in Marvell - 5GBASE-R is
> just the serdes clocking the MAC/PCS at half the rate that 10GBASE-R
> would run at.
> 
> I suspect this Xilinx hardware is just the same - clock the transmit
> path it at 62.5MHz, and you get 1G speeds. Clock it at 156.25MHz,
> and you get 2.5G speeds.

Hey, I'm just a l^Huser.

In the synthesis settings for the PCS, you can select

  - 1G
    - 1000BASEX
    - SGMII
    - BOTH
  - 2 5G
    - 2500 BASEX
    - 2.5G SGMII

(all of the above being exclusive choices)

In the synthesis settings for the MAC, you can select

  - 1 Gbps
    - Tri speed
    - 1000 Mbps
  - 2.5 Gbps
    - 2500 Mbps

(ditto)

I can't comment on what happens when you over/underclock the MAC or PCS.

--Sean