netdev - Re: QUSGMII control word

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Date: Mon, 5 Jun 2023 11:45:12 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Maxime Chevallier <maxime.chevallier@...tlin.com>
Cc: Andrew Lunn <andrew@...n.ch>, netdev@...r.kernel.org
Subject: Re: QUSGMII control word

On Mon, Jun 05, 2023 at 08:13:34AM +0200, Maxime Chevallier wrote:
> Hello Russell,
> 
> On Fri, 2 Jun 2023 13:18:03 +0100
> "Russell King (Oracle)" <linux@...linux.org.uk> wrote:
> 
> > Hi Maxime,
> > 
> > Looking at your commit which introduced QUSGMII -
> > 5e61fe157a27 ("net: phy: Introduce QUSGMII PHY mode"), are you sure
> > your decoding of the control word is correct?
> > 
> > I've found some information online which suggests that QUSGMII uses a
> > slightly different format to the control word from SGMII. Most of the
> > bits are the same, but the speed bits occupy the three bits from 11:9,
> > and 10M, 100M and 1G are encoded using bits 10:9, whereas in SGMII
> > they are bits 11:10. In other words, in QUSGMII they are shifted one
> > bit down. In your commit, you used the SGMII decoder for QUSGMII,
> > which would mean we'd be picking out the wrong bits for decoding the
> > speed.
> > 
> > QUSGMII also introduces EEE information into bits 8 and 7 whereas
> > these are reserved in SGMII.
> > 
> > Please could you take a look, because I think we need a different
> > decoder for the QUSGMII speed bits.
> 
> I've taken a look at it, back when I sent that patch I didn't have
> access to the full documentation and used a vendor reference
> implementation as a basis... I managed to get my hands on the proper
> doc and the control word being used looks to be the usxgmii control
> word, which matches with the offset you are seeing.
> 
> Do you have a patch or should I send a followup ?

I don't have a patch.

> Out of curiosity, on which hardware did you find this ?

No, it's something I noticed when reviewing some documentation I have.

> I still have some patches of PCH extensions around, but didn't get any
> room in my schedule to move forward with it. Is it something that you
> plan on using ?

No plans, sorry! :D

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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