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Date:   Thu, 15 Oct 2020 01:37:37 +0200
From:   Lukasz Stelmach <l.stelmach@...sung.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Bartłomiej Żolnierkiewicz 
        <b.zolnierkie@...sung.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH] net: phy: Prevent reporting advertised modes when
 autoneg is off

It was <2020-10-14 śro 23:04>, when Russell King - ARM Linux admin wrote:
> On Wed, Oct 14, 2020 at 04:39:47PM +0200, Lukasz Stelmach wrote:
>> It was <2020-10-14 śro 14:32>, when Russell King - ARM Linux admin wrote:
>> > In any case, the mii.c code does fill in the advertising mask even
>> > when autoneg is disabled, because, rightly or wrongly, the advertising
>> > mask contains more than just the link modes, it includes the
>> > interface(s) as well. Your change means phylib no longer reports the
>> > interface modes which is at odds with the mii.c code.
>> 
>> I am afraid you are wrong. There is a rather big if()[1], which
>> depending on AN beeing enabled fills more or less information. Yes this
>> if() looks like it has been yanked from mii_ethtool_gset(). When
>> advertising is converted and copied to cmd->link_modes.advertising at
>> the end of mii_ethtool_get_link_ksettings() it is 0[2] if autonegotiation
>> is disabled.
>> 
>> [1]
>> https://protect2.fireeye.com/v1/url?k=d1d33944-8c07852c-d1d2b20b-0cc47a3356b2-cede44db8a43e7c3&q=1&e=d1dea895-d1df-46b9-8413-ac329b0bbfa7&u=https%3A%2F%2Felixir.bootlin.com%2Flinux%2Fv5.9%2Fsource%2Fdrivers%2Fnet%2Fmii.c%23L174
>> [2]
>> https://protect2.fireeye.com/v1/url?k=c840e942-9594552a-c841620d-0cc47a3356b2-510c0749446ea7df&q=1&e=d1dea895-d1df-46b9-8413-ac329b0bbfa7&u=https%3A%2F%2Felixir.bootlin.com%2Flinux%2Fv5.9%2Fsource%2Fdrivers%2Fnet%2Fmii.c%23L215
>
> I'm very sorry, but I have to disagree.  I'll quote the code here:
>
>         advertising = ADVERTISED_TP | ADVERTISED_MII;
>
> 	// This is your big if()
>         if (bmcr & BMCR_ANENABLE) {
> 		advertising |= ADVERTISED_Autoneg;
> 		...
> 	} else {
> 		...
> 	}
>
> 	ethtool_convert_legacy_u32_to_link_mode(cmd->link_modes.advertising,
> 	                                        advertising);
>
> So, when AN is disabled, we still end up with TP and MII in the
> advertised link modes. I call TP and MII "interface modes" above
> to separate them from the "link modes" that describe the speed and
> duplex etc.

Yes.

> Note that only lp_advertising is zeroed in the "else" clause of
> the above "if" statement - advertising remains as-is with TP and MII
> set.

Yes.

> Your patch, on the other hand, merely avoids setting anything in
> cmd->link_modes.advertising, which means we do not end up with the
> "interface modes".
>
> I hope that this helps you see my point.

Yes.

Thanks.

tl;dr: v2 is coming.

Let's take a look at what ethtool prints. With AN enabled

--8<---------------cut here---------------start------------->8---
Settings for enx00249b14f06e:
        Supported ports: [ TP MII ]
        Supported link modes:   10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Half 1000baseT/Full 
        Supported pause frame use: No
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Full 
        Advertised pause frame use: Symmetric
        Advertised auto-negotiation: Yes

[...]

        Link detected: yes
--8<---------------cut here---------------end--------------->8---

with AN disabled

--8<---------------cut here---------------start------------->8---
Settings for enx00249b14f06e:
        Supported ports: [ TP MII ]
        Supported link modes:   10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Half 1000baseT/Full 
        Supported pause frame use: No
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  Not reported
        Advertised pause frame use: No
        Advertised auto-negotiation: No

[...]

        Link detected: yes
--8<---------------cut here---------------end--------------->8---

The driver is ax88179_178a which calls mii_ethtool_get_link_ksettings()

The first line that differs is "Advertised link modes". Apparently[1]
ethtool does not consider ADVERTISED_TP and ADVERTISED_MII as
interesting modes. Indeed I should add an else clause to clear
cmd->link_modes.advertising (and lp_advertising) and set only
ETHTOOL_LINK_MODE_TP_BIT and ETHTOOL_LINK_MODE_MII_BIT.


[1] https://git.kernel.org/pub/scm/network/ethtool/ethtool.git/tree/ethtool.c?h=v5.8#n656
-- 
Łukasz Stelmach
Samsung R&D Institute Poland
Samsung Electronics

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