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

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Date:   Thu, 7 Apr 2022 21:54:43 +0800
From:   "huangguangbin (A)" <huangguangbin2@...wei.com>
To:     Andrew Lunn <andrew@...n.ch>,
        Oleksij Rempel <o.rempel@...gutronix.de>
CC:     <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 2022/4/1 20:14, Andrew Lunn wrote:
>>> 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
> .
> 
Hi Andrew,
The PHY we test is RTL8211F, it supports 10 half. This problem actually is,
our board has MAC connected with PHY, when loopback test, packet flow is
MAC->PHY->MAC, it needs speed of MAC and PHY should be same when they work.

If PHY speed is unknown when PHY goes down, we will not set MAC speed in
adjust_link interface. In this case, we hope that PHY speed should not be
changed, as the old code of function genphy_loopback() before patch
"net: phy: genphy_loopback: add link speed configuration".

If PHY has never link, MAC speed has never be set in adjust_link interface,
yeah, in this case, MAC and PHY may has different speed, and they can not work.
I think we can accept this situation.

I think it is general problem if there is MAC connected with PHY.