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

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Date:   Fri, 1 Apr 2022 14:14:37 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
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

> > 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?

You should not need a link partner for loopback to work. This is local
loopback. The PHY is also saying it has link, if the LSTATUS bit is
set. So i don't see why previous speed is relevant hear. This seems to
me to be an issue for this particular PHY.

What i don't like about this patch is that it is not deterministic
what mode the PHY will end up in if speed is unknown. Without the
patch, it is 10Mbps, which is historically a sensible default.

If this PHY has never had link, what speed does it use? Does it still
work in that case?

   Andrew