Re: Aquantia PHY in OCSGMII mode?

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Wed, Aug 27, 2025 at 10:32:27AM +0200, Alexander Wilhelm wrote:
> Am Wed, Aug 27, 2025 at 09:08:24AM +0100 schrieb Russell King (Oracle):
> > On Wed, Aug 27, 2025 at 07:57:28AM +0200, Alexander Wilhelm wrote:
> > > Hi Vladimir,
> > > 
> > > One of our hardware engineers has looked into the issue with the 100M link and
> > > found the following: the Aquantia AQR115 always uses 2500BASE-X (GMII) on the
> > > host side. For both 1G and 100M operation, it enables pause rate adaptation.
> > > However, our MAC only applies rate adaptation for 1G links. For 100M, it uses a
> > > 10x symbol replication instead.
> > 
> > This sounds like a misunderstanding, specifically:
> > 
> > "our MAC only applies rate adaptation for 1G links. For 100M, it uses
> > 10x symbol replication instead."
> > 
> > It is the PHY that does rate adaption, so the MAC doesn't need to
> > support other speeds. Therefore, if the PHY is using a 2.5Gbps link
> > to the MAC with rate adaption for 100M, then the MAC needs to operate
> > at that 2.5Gbps speed.
> > 
> > You don't program the MAC differently depending on the media side
> > speed, unlike when rate adaption is not being used.
> 
> You're right. The flow control with rate adaptation is controlled by PHY. The
> MAC should remain on the 2.5Gbps speed. Therefore I wonder why it uses 10x
> symbol repetition.

As I say, someone is misunderstanding something. The PHY controls what
happens at the different media speeds.

I wonder whether the hardware engineer is thinking that the PHY is
configured for SGMII mode at 100Mbps - whic his controlled by vendor
1 register 0x31b. If the 3 LSBs are 4, then it's using "OCSGMII"
otherwise if 3, then it'll be as the hardware engineer states.

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