netdev - Re: [PATCH net v2 1/2] net: dsa: lantiq_gswip: Don't use PHY auto polling

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Message-ID: <CAFBinCAACrUO+89TnPs=5hhkV_73N5hxz=18u9o-NERUC_wpGg@mail.gmail.com>
Date:   Fri, 9 Apr 2021 21:43:34 +0200
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Hauke Mehrtens <hauke@...ke-m.de>, andrew@...n.ch,
        f.fainelli@...il.com, vivien.didelot@...il.com,
        netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        linux@...linux.org.uk, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH net v2 1/2] net: dsa: lantiq_gswip: Don't use PHY auto polling

Hello Vladimir,

On Fri, Apr 9, 2021 at 12:46 AM Vladimir Oltean <olteanv@...il.com> wrote:
>
> On Thu, Apr 08, 2021 at 08:38:27PM +0200, Martin Blumenstingl wrote:
> > PHY auto polling on the GSWIP hardware can be used so link changes
> > (speed, link up/down, etc.) can be detected automatically. Internally
> > GSWIP reads the PHY's registers for this functionality. Based on this
> > automatic detection GSWIP can also automatically re-configure it's port
> > settings. Unfortunately this auto polling (and configuration) mechanism
> > seems to cause various issues observed by different people on different
> > devices:
> > - FritzBox 7360v2: the two Gbit/s ports (connected to the two internal
> >   PHY11G instances) are working fine but the two Fast Ethernet ports
> >   (using an AR8030 RMII PHY) are completely dead (neither RX nor TX are
> >   received). It turns out that the AR8030 PHY sets the BMSR_ESTATEN bit
> >   as well as the ESTATUS_1000_TFULL and ESTATUS_1000_XFULL bits. This
> >   makes the PHY auto polling state machine (rightfully?) think that the
> >   established link speed (when the other side is Gbit/s capable) is
> >   1Gbit/s.
>
> Why do you say "rightfully"? The PHY is gigabit capable, and it reports
> that via the Extended Status register. This is one of the reasons why
> the "advertising" and "supported" link modes are separate concepts,
> because even though you support gigabit, you don't advertise it because
> you are in RMII mode.
according to the marketing materials of the AR8030 it is a "Ultra
low-power single RMII Fast Ethernet PHY"
based on that I am referring to this PHY as "not Gbit/s capable"
(other PHYs from the AR803x series are Gbit/s capable though)

> How does turning off the auto polling feature help circumvent the
> Atheros PHY reporting "issue"? Do we even know that is the problem, or
> is it simply a guess on your part based on something that looked strange?
I have a patch in my queue (which I'll send for the next -net-next
cycle) which adds "ethtool -d" (.get_regs) support to the GSWIP
driver.
There are multiple status registers, one of them indicates that the
link speed (as result of the auto polling mechanism) is 1Gbit/s

[...]
> > Switch to software based configuration instead of PHY auto polling (and
> > letting the GSWIP hardware configure the ports automatically) for the
> > following link parameters:
> > - link up/down
> > - link speed
> > - full/half duplex
> > - flow control (RX / TX pause)
>
> What does the auto polling feature consist of, exactly? Is there some
> sort of microcontroller accessing the MDIO bus simultaneously with
> Linux?
I believe the answer is yes, but there's no clear description in the
datasheet for a newer GSWIP revision [0]
"Figure 8" on page 41 (or page 39 if you go by the numbers at the
bottom of each page) has a description of the state machine logic.
If I understood Hauke correct the "not fiber" part is only checked for
newer GSWIP revisions

Please note that the datasheet from [0] refers to part number GSW140
which is a stand-alone IC.
The GSWIP driver (currently) supports an older revision (at least two
generations older) of GSWIP which is built into Lantiq xRX200 and
xRX300 SoCs.


Best regards,
Martin


[0] https://www.maxlinear.com/document/index?id=23266&languageid=1033&type=Datasheet&partnumber=GSW140&filename=617930_GSW140_DS_Rev1.7.pdf&part=GSW140