linux-kernel - Re: [PATCH net v1] net: phy: smsc: fix printing too many logs

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Message-ID: <20200618150832.GA25889@nuc8i5>
Date:   Thu, 18 Jun 2020 23:08:32 +0800
From:   Dejin Zheng <zhengdejin5@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        f.fainelli@...il.com, hkallweit1@...il.com, davem@...emloft.net,
        kuba@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Kevin Groeneveld <kgroeneveld@...il.com>
Subject: Re: [PATCH net v1] net: phy: smsc: fix printing too many logs

On Wed, Jun 17, 2020 at 11:36:42PM +0200, Andrew Lunn wrote:
> On Wed, Jun 17, 2020 at 09:24:50PM +0100, Russell King - ARM Linux admin wrote:
> > On Wed, Jun 17, 2020 at 08:43:34PM +0200, Andrew Lunn wrote:
> > > You have explained what the change does. But not why it is
> > > needed. What exactly is happening. To me, the key thing is
> > > understanding why we get -110, and why it is not an actual error we
> > > should be reporting as an error. That is what needs explaining.
> > 
> > The patch author really ought to be explaining this... but let me
> > have a go.  It's worth pointing out that the comments in the file
> > aren't good English either, so don't really describe what is going
> > on.
> > 
> > When this PHY is in EDPD mode, it doesn't always detect a connected
> > cable.  The workaround for it involves, when the link is down, and
> > at each read_status() call:
> > 
> > - disable EDPD mode, forcing the PHY out of low-power mode
> > - waiting 640ms to see if we have any energy detected from the media
> > - re-enable entry to EDPD mode
> > 
> > This is presumably enough to allow the PHY to notice that a cable is
> > connected, and resume normal operations to negotiate with the partner.
> > 
> > The problem is that when no media is detected, the 640ms wait times
> > out (as it should, we don't want to wait forever) and the kernel
> > prints a warning.
> 
> Hi Russell
> 
> Yes, that is what i was thinking. 
> 
> There probably should be a comment added just to prevent somebody
> swapping it back to phy_read_poll_timeout(). It is not clear that
> -ETIMEOUT is expected under some conditions.
> 
> 	  Andrew

Hi Andrew and Russell,

First of all, thanks very much for your comment!

I did not understand Andrew's comments correctly in the previous email,
sorry for my bad English. I found something in the commit 776829de90c597
("net: phy: workaround for buggy cable detection by LAN8700 after cable
plugging") about why it is not an actual error when get a timeout error
in this driver. the commit's link is here:

https://lore.kernel.org/patchwork/patch/590285/

As Russell said, it just for fix a hardware bug on LAN8700 device by wait
640ms, so it should not as an actual error when got timeout.

The following is my understanding:

It leads to unstable detection of plugging in Ethernet cable when LAN87xx
is in Energy Detect Power-Down mode. Because the ENERGYON bit is not set.

For fix this issue, it will disables Energy Detect Power-Down mode and
check ENERGYON bit to waiting for response on link pulses to detect
presence of plugged Ethernet cable in the 640ms. if got the ENERGYON bit
is set, I guess the cable plug-in event was detected and will report an
interrupt after exit lan87xx_read_status() funtion, if the ENERGYON bit
is not set, the plug-in event was not detected. after that, entry Energy
Detect Power-Down mode again.

it will re-call lan87xx_read_status() funtion by interrupt when got the
ENERGYON bit is set. and check link status again. then, It will get a
stable link status for LAN87xx device.

So the timeout for wait 640ms should not as an actual error.

Thanks!

Dejin