linux-kernel - Re: [PATCH] ARM: dts: imx6dl: SolidRun: add phy node with 100Mb/s max-speed

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Date:   Tue, 17 Sep 2019 20:39:34 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Mark Rutland <mark.rutland@....com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Baruch Siach <baruch@...s.co.il>,
        Fabio Estevam <festevam@...il.com>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        tinywrkb <tinywrkb@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        NXP Linux Team <linux-imx@....com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Shawn Guo <shawnguo@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] ARM: dts: imx6dl: SolidRun: add phy node with 100Mb/s
 max-speed

> > Well, the _correct_ driver needs to be used for the PHY specific
> > features to be properly controlled.  Using the generic driver
> > in this situation will not be guaranteed to work.

I fully agree about the PHY driver. I'm expect this device is
violating c22 when it modifies the advertisement register itself. So
all bets are off for the genphy.

> Well, this hasn't worked, but not for the obvious reason.  Register 0x14
> is documented as read/write.  Bits 15:6 are reserved, bit 5 is the
> smart speed enable, 4:2 configures the attempts, bit 1 sets the link
> stable condition, bit 0 is reserved.
> 
> Writing 0x80c results in the register reading back 0x82c.  Writing
> 0x800 results in the same.  Writing 0 reads back 0x2c.  Writing 0xffff
> seems to prevent packets being passed - and at that point I lost
> control so I couldn't see what the result was.
> 
> There is nothing in the data sheet which suggests that there is any
> gating of this register.  So it looks like we're stuck with smartspeed
> enabled.
> 
> So, I think there's only two remaining ways forward - to revert commit
> 5502b218e001 to restore the old behaviour, read back the advertisement
> from the PHY along with the rest of the status, as I've previously
> stated.  It means that phylib will modify phydev->advertising at
> random points, just as it modifies phydev->lp_advertising, so locking
> may become an issue.  The revert approach is probably best until we
> have something working along those lines.

We have a couple of other PHYs which support downshift. We should see
if we can follow what they do. What is i think important is that
read_status return the correct speed. So we probably cannot use
genphy_read_status() as is. Maybe we should split genphy_read_status()
into two, so the register reading bit can be done unconditionally by
phy drivers for hardware which don't report link down when they
should?

    Andrew