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