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
