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

[Marek Vasut]

On 3/19/23 20:45, Marek Behún wrote:
> 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...

Thanks for the detailed explanation, I sent a V2 where I remove the 
SGMII 2500 . I hope I got the differences right in the V2 commit message.