-----Original Message-----
From: Andrew Lunn <andrew@lunn.ch>
Sent: Tuesday, 20 September, 2022 7:46 AM
To: Jamaluddin, Aminuddin <redacted>
Cc: Heiner Kallweit <hkallweit1@gmail.com>; Russell King
[off-list ref]; David S . Miller [off-list ref]; Eric
Dumazet [off-list ref]; Jakub Kicinski [off-list ref];
Paolo Abeni [off-list ref]; Ismail, Mohammad Athari
[off-list ref]; netdev@vger.kernel.org; linux-
kernel@vger.kernel.org; stable@vger.kernel.org; Tan, Tee Min
[off-list ref]; Zulkifli, Muhammad Husaini
[off-list ref]
Subject: Re: [PATCH net 1/1] net: phy: marvell: add link status check before
enabling phy loopback

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Its required cabled plug in, back to back connection.

Loopback should not require that. The whole point of loopback in the
PHY is you can do it without needing a cable.

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Have you tested this with the cable unplugged?

Yes we have and its expected to have the timeout. But the
self-test required the link to be up first before it can be run.

So you get an ETIMEDOUT, and then skip the code which actually sets
the LOOPBACK bit?

If cable unplugged, test result will be displayed as 1. See comments below.

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Please look at this again, and make it work without a cable.

Related to this the flow without cable, what we see in the codes during

debugging.

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After the phy loopback bit was set.
The test will be run through this __stmmac_test_loopback()
https://elixir.bootlin.com/linux/v5.19.8/source/drivers/net/ethernet/s
tmicro/stmmac/stmmac_selftests.c#L320
Here, it will have another set of checking in dev_direct_xmit(),

__dev_direct_xmit().

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returning value 1(NET_XMIT_DROP)
https://elixir.bootlin.com/linux/v5.19.8/source/net/core/dev.c#L4288
Which means the interface is not available or the interface link status is not

up.

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For this case the interface link status is not up.
Thus failing the phy loopback test.
https://elixir.bootlin.com/linux/v5.19.8/source/net/core/dev.c#L4296
Since we don't own this __stmmac_test_loopback(), we conclude the

behaviour was as expected.

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Maybe you are addressing the wrong issue? Is the PHY actually
performing loopback, but reporting the link is down? Maybe you need to

fake a link up?

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Maybe you need the self test to not care about the link state, all
it really needs is that packets get looped?

When bit 14 was set, the link will be broken.
But before the self-test was triggered it requires link to be up as stated

above comments.

You have not said anything about my comment:

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Maybe you need to fake a link up?

My guess is, some PHYs are going to report link up when put into loopback.
Others might not. For the Marvell PHY, it looks like you need to make
marvell_read_status() return that the link is up if loopback is enabled.

We able to do the PHY loopback test, after fake link up without 
cable plugged on as suggested above. 
We will provide version 2 patch with minimum code changes 
without having the status link check.
Only need to increase the msleep(200) to msleep(1000) inside
m88e1510_loopback() function.

Amin