Re: [PATCH] net: phy: fix auto-negotiation in case of 'down-shift'

[Antonio Borneo <antonio.borneo@st.com>]

On Tue, 2020-11-24 at 15:17 +0000, Russell King - ARM Linux admin wrote:
> On Tue, Nov 24, 2020 at 04:03:40PM +0100, Heiner Kallweit wrote:
> > Am 24.11.2020 um 15:38 schrieb Antonio Borneo:
> > > If the auto-negotiation fails to establish a gigabit link, the phy
> > > can try to 'down-shift': it resets the bits in MII_CTRL1000 to
> > > stop advertising 1Gbps and retries the negotiation at 100Mbps.
> > > 
> > I see that Russell answered already. My 2cts:
> > 
> > Are you sure all PHY's supporting downshift adjust the
> > advertisement bits? IIRC an Aquantia PHY I dealt with does not.
> > And if a PHY does so I'd consider this problematic:
> > Let's say you have a broken cable and the PHY downshifts to
> > 100Mbps. If you change the cable then the PHY would still negotiate
> > 100Mbps only.
> 
> From what I've seen, that is not how downshift works, at least on
> the PHYs I've seen.
> 
> When the PHY downshifts, it modifies the advertisement registers,
> but it also remembers the original value. When the cable is
> unplugged, it restores the setting to what was previously set.

In fact, at least rtl8211f is able to recover the original settings and
returns to 1Gbps once a decent cable gets plugged-in.

> 
> It is _far_ from nice, but the fact is that your patch that Antonio
> identified has broken previously working support, something that I
> brought up when I patched one of the PHY drivers that was broken by
> this very same problem by your patch.

The idea to fix it for a general case was indeed triggered by the fact that
before commit 5502b218e001 this was the norm. I considered it as a
regression.

> 
> That said, _if_ the PHY has a way to read the resolved state rather
> than reading the advertisement registers, that is what should be
> used (as I said previously) rather than trying to decode the
> advertisement registers ourselves. That is normally more reliable
> for speed and duplex.
> 

Wrt rtl8211f I don't have info other then the public datasheet, and there I
didn't found any way other than reading the advertisement register.

I have read the latest comment from Heiner. I will check aqr107!

Thanks
Antonio