netdev - Re: [PATCH] net: phy: genphy_loopback: fix loopback failed when speed is unknown

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Date:   Fri, 1 Apr 2022 08:40:06 +0200
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     "huangguangbin (A)" <huangguangbin2@...wei.com>,
        davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, lipeng321@...wei.com,
        chenhao288@...ilicon.com
Subject: Re: [PATCH] net: phy: genphy_loopback: fix loopback failed when
 speed is unknown

On Thu, Mar 31, 2022 at 04:26:47PM +0200, Andrew Lunn wrote:
> > In this case, as speed and duplex both are unknown, ctl is just set to 0x4000.
> > However, the follow code sets mask to ~0 for function phy_modify():
> > int genphy_loopback(struct phy_device *phydev, bool enable)
> > {
> > 	if (enable) {
> > 		...
> > 		phy_modify(phydev, MII_BMCR, ~0, ctl);
> > 		...
> > }
> > so all other bits of BMCR will be cleared and just set bit 14, I use phy trace to
> > prove that:
> > 
> > $ cat /sys/kernel/debug/tracing/trace
> > # tracer: nop
> > #
> > # entries-in-buffer/entries-written: 923/923   #P:128
> > #
> > #                                _-----=> irqs-off/BH-disabled
> > #                               / _----=> need-resched
> > #                              | / _---=> hardirq/softirq
> > #                              || / _--=> preempt-depth
> > #                              ||| / _-=> migrate-disable
> > #                              |||| /     delay
> > #           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
> > #              | |         |   |||||     |         |
> >   kworker/u257:2-694     [015] .....   209.263912: mdio_access: mii-0000:bd:00.1 read  phy:0x03 reg:0x00 val:0x1040
> >   kworker/u257:2-694     [015] .....   209.263951: mdio_access: mii-0000:bd:00.1 read  phy:0x03 reg:0x01 val:0x7989
> >   kworker/u257:2-694     [015] .....   209.263990: mdio_access: mii-0000:bd:00.1 read  phy:0x03 reg:0x01 val:0x7989
> >   kworker/u257:2-694     [015] .....   209.264028: mdio_access: mii-0000:bd:00.1 read  phy:0x03 reg:0x09 val:0x0200
> >   kworker/u257:2-694     [015] .....   209.264067: mdio_access: mii-0000:bd:00.1 read  phy:0x03 reg:0x0a val:0x0000
> >          ethtool-1148    [007] .....   209.665693: mdio_access: mii-0000:bd:00.1 read  phy:0x03 reg:0x00 val:0x1040
> >          ethtool-1148    [007] .....   209.665706: mdio_access: mii-0000:bd:00.1 write phy:0x03 reg:0x00 val:0x1840
> >          ethtool-1148    [007] .....   210.588139: mdio_access: mii-0000:bd:00.1 read  phy:0x03 reg:0x00 val:0x1840
> >          ethtool-1148    [007] .....   210.588152: mdio_access: mii-0000:bd:00.1 write phy:0x03 reg:0x00 val:0x1040
> >          ethtool-1148    [007] .....   210.615900: mdio_access: mii-0000:bd:00.1 read  phy:0x03 reg:0x00 val:0x1040
> >          ethtool-1148    [007] .....   210.615912: mdio_access: mii-0000:bd:00.1 write phy:0x03 reg:0x00 val:0x4000 //here just set bit 14
> > 
> > So phy speed will be set to 10M in this case, if previous speed of
> > device before going down is 10M, loopback test is pass. Only
> > previous speed is 100M or 1000M, loopback test is failed.
> 
> O.K. So it should be set into 10M half duplex. But why does this cause
> it not to loopback packets? Does the PHY you are using not actually
> support 10 Half? Why does it need to be the same speed as when the
> link was up? And why does it actually set LSTATUS indicating there is
> link?
> 
> Is this a generic problem, all PHYs are like this, or is this specific
> to the PHY you are using? Maybe this PHY needs its own loopback
> function because it does something odd?

It looks for me like attempt to fix loopback test for setup without active
link partner. Correct?

Regards,
Oleksij
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