netdev - Re: Get MAC supported link modes for SFP port

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Message-ID: <20201126161348.GO1551@shell.armlinux.org.uk>
Date:   Thu, 26 Nov 2020 16:13:48 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Baruch Siach <baruch@...s.co.il>
Cc:     Andrew Lunn <andrew@...n.ch>, netdev@...r.kernel.org,
        Heiner Kallweit <hkallweit1@...il.com>
Subject: Re: Get MAC supported link modes for SFP port

On Thu, Nov 26, 2020 at 06:01:35PM +0200, Baruch Siach wrote:
> Hi Andrew,
> 
> On Thu, Nov 26 2020, Andrew Lunn wrote:
> > On Thu, Nov 26, 2020 at 05:37:22PM +0200, Baruch Siach wrote:
> >> I am trying to retrieve all MAC supported link modes
> >> (ETHTOOL_LINK_MODE_*) for network interfaces with SFP port. The
> >> 'supported' bit mask that ETHTOOL_GLINKSETTINGS provides in
> >> link_mode_masks[] changes to match the SFP module that happens to be
> >> plugged in. When no SFP module is plugged, the bit mask looks
> >> meaningless.
> >
> > That sounds like it is doing the correct thing.
> >
> >> I understand that ETHTOOL_LINK_MODE_* bits are meant to describe PHY
> >> level capabilities. So I would settle for a MAC level "supported rates"
> >> list.
> >
> > What is your use cases?
> 
> I would like to report the port supported data rates to the system
> user. I need to tell whether 10Gbps SFP module are supported in that
> port in a generic way. The driver has this information. It is necessary
> to implement the validate callback in phylink_mac_ops. But I see no way
> to read this information from userspace.

If we want to know whether 10Gbps SFPs are supported in a cage, and
that information is important, we really ought to use a different
DT compatible for it. I've debated about using "sff,sfp+" since that's
what they are - but I have no use practical for it, so I've not made
the change.

This is especially important as there are boards out there
(Macchiatobin) where the SFP+ sockets do not support SFP modules
from the electrical point of view, and if you happen to plug in a
SFP module, you may end up with its EEPROM being corrupted merely
as a result of plugging the module in and the ID trying to be read.

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