netdev - Re: [PATCH] net: stmmac: fix MAC not working when system resume back with WoL enabled

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Date:   Thu, 2 Sep 2021 09:32:24 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Joakim Zhang <qiangqing.zhang@....com>
Cc:     Vladimir Oltean <olteanv@...il.com>,
        "peppe.cavallaro@...com" <peppe.cavallaro@...com>,
        "alexandre.torgue@...s.st.com" <alexandre.torgue@...s.st.com>,
        "joabreu@...opsys.com" <joabreu@...opsys.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "mcoquelin.stm32@...il.com" <mcoquelin.stm32@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "hkallweit1@...il.com" <hkallweit1@...il.com>,
        dl-linux-imx <linux-imx@....com>
Subject: Re: [PATCH] net: stmmac: fix MAC not working when system resume back
 with WoL enabled

On Thu, Sep 02, 2021 at 07:28:44AM +0000, Joakim Zhang wrote:
> 
> Hi Russell,
> 
> > -----Original Message-----
> > From: Russell King <linux@...linux.org.uk>
> > Sent: 2021年9月1日 21:26
> > To: Joakim Zhang <qiangqing.zhang@....com>
> > Cc: Vladimir Oltean <olteanv@...il.com>; peppe.cavallaro@...com;
> > alexandre.torgue@...s.st.com; joabreu@...opsys.com;
> > davem@...emloft.net; kuba@...nel.org; mcoquelin.stm32@...il.com;
> > netdev@...r.kernel.org; andrew@...n.ch; f.fainelli@...il.com;
> > hkallweit1@...il.com; dl-linux-imx <linux-imx@....com>
> > Subject: Re: [PATCH] net: stmmac: fix MAC not working when system resume
> > back with WoL enabled
> > 
> > This means you need to have the phy <-> mac link up during
> > suspend, and in that case, yes, you do not want to call
> > phylink_stop() or phylink_start().
> 
> I have a question here, why need to have the phy<->mac link up during suspend?

You need the link up because I think from reading the code, it is _not_
the PHY that is triggering the wakeup in the configuration you are using,
but the MAC.

If the link is down, the PHY can't pass the received packet to the MAC,
and the MAC can't recognise the magic packet.

FEC doesn't have this. FEC relies purely on the PHY detecting the magic
packet, which is much more power efficient, because it means the MAC
doesn't need to be powered up and operational while the rest of the
system is suspended.

> As you described in past thread, phylink_stop() and phylink_start() also need to be called even with
> WoL active.

That was with the assumption that the PHY was detecting the magic
packet. It isn't for stmmac - stmmac can be configured to bypass
the configuration of the PHY for this and uses the MAC to detect
this instead. If the MAC is doing the detecting for WoL, then you
need network connectivity to be functional from the network cable
through the PHY and up to the MAC.

So, bringing the link down at suspend in this case _will_ break
WoL. The PHY isn't the device detecting the magic packet, it is the
MAC, and the MAC must be able to see the network traffic.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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