Get MAC supported link modes for SFP port

Get MAC supported link modes for SFP port

[Baruch Siach]

Hi netdev list,

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.

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.

Is there anything like that?

Thanks,
baruch

-- 
                                                     ~. .~   Tk Open Systems
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   - baruch@tkos.co.il - tel: +972.52.368.4656, http://www.tkos.co.il -

Re: Get MAC supported link modes for SFP port

[Russell King - ARM Linux admin]

On Thu, Nov 26, 2020 at 05:37:22PM +0200, Baruch Siach wrote:
> Hi netdev list,
> 
> 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.
> 
> 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.
> 
> Is there anything like that?

No, because there's a problem: the link modes that the MAC supports
is not a particularly certain thing. When there's no module inserted,
we don't know what interface mode may be in operation, and the
interface mode has a big handle in determining which link modes can
be supported.

For example, if it's operating as 1000BASE-X or 10GBASE-R, then we're
pretty limited to a single link mode if there's nothing else present.
If it's in SGMII, then 10/100/1000 speeds are possible but there's
no link mode to describe that - no PHY, therefore there's no copper
and therefore BASE-T is meaningless.

The ethtool link modes are, in reality, _media_ link modes. If there
is no media or socket that defines the media, there are no link modes.

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Re: Get MAC supported link modes for SFP port

[Andrew Lunn]

On Thu, Nov 26, 2020 at 05:37:22PM +0200, Baruch Siach wrote:
> Hi netdev list,
> 
> 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?

A MAC without some form a PHY, be it copper, fibre, or a faked
fixed-link, is useless. You need the combination of what the MAC can
do and what the PHY can do to have any meaning information.

       Andrew

Re: Get MAC supported link modes for SFP port

[Baruch Siach]

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.

> A MAC without some form a PHY, be it copper, fibre, or a faked
> fixed-link, is useless. You need the combination of what the MAC can
> do and what the PHY can do to have any meaning information.

I understand that. I probably need a higher level concept of data rate
supported.

baruch

-- 
                                                     ~. .~   Tk Open Systems
=}------------------------------------------------ooO--U--Ooo------------{=
   - baruch@tkos.co.il - tel: +972.52.368.4656, http://www.tkos.co.il -

Re: Get MAC supported link modes for SFP port

[Russell King - ARM Linux admin]

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/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!