[PATCH 1/3] net: mvpp2: Replace PHY_INTERFACE_MODE_SGMII_2500 with SGMII and speed

[Marek Behún]

On Sun, 19 Mar 2023 18:06:45 +0100
Marek Vasut <marek.vasut+renesas at mailbox.org> wrote:

> Replace PHY_INTERFACE_MODE_SGMII_2500 with PHY_INTERFACE_MODE_SGMII and
> phydev->speed check where applicable. The PHY_INTERFACE_MODE_SGMII_2500
> does not exist in Linux, so remove it from U-Boot too.
> 
> Signed-off-by: Marek Vasut <marek.vasut+renesas at mailbox.org>

SGMII cannot operate on 2500mbps. As I explained to the author who added
PHY_INTERFACE_MODE_SGMII_2500 to U-Boot (and the conversation didn't lead
anywhere then), what I think happened here is that for some reason
Marvell sometimes calls 2500base-x mode SGMII_2500. Meaning that they
incorrectly interchange the names 2500base-x and sgmii_2500 as if they
were the same thing. This is probably due to how 1000base-x and SGMII
are similar and they do the same with those.

The thing is, 1000base-x and SGMII are different protocols / interface
modes. The difference is that they have different autonegotiation. SGMII
allows also for speed autonegotiation, whilst 1000base-x only for flow
control. The bits in the clause 37 link ability advertisement word have
different meaning for 1000base-x than for sgmii.

When SGMII negotiates lower speed, the actual serdes frequency does not
change. The lower speed of 100mbps or 10mbps is achieved by repeating
the word 10 or 100 times.

The 2500base-x mode works the same way as 1000base-x mode, but at 2.5x
the frequency. Calling it sgmii-2500 is wrong because it would mean that
it also allows for lower speeds by repeating the words. In reality it
is possible, on many controllers you can set SGMII mode at 2.5x normal
frequency and then instead of 1000/100/10mbps speeds achieve
2500/250/25mbps. But this is not standardized anyway.

Marek, your patch does a good thing getting rid of the sgmii-2500
constant, but you are still leaving the possibility of 2500mbps speed
in SGMII mode...

Marek