netdev - Re: [EXT] Re: Phylink flow control support on ports with MLO_AN

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Date:   Sun, 31 Jan 2021 11:12:14 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Stefan Chulski <stefanc@...vell.com>
Cc:     "andrew@...n.ch" <andrew@...n.ch>,
        Jakub Kicinski <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Marcin Wojtas (mw@...ihalf.com)" <mw@...ihalf.com>,
        Nadav Haklai <nadavh@...vell.com>,
        Yan Markman <ymarkman@...vell.com>
Subject: Re: [EXT] Re: Phylink flow control support on ports with
 MLO_AN_FIXED auto negotiation

On Sun, Jan 31, 2021 at 10:51:46AM +0000, Stefan Chulski wrote:
> 
> > > Hi,
> > >
> > > Armada has options for 1G/10G ports without PHY's(for example
> > community board Macchiato single shot).
> > > This port doesn't have PHY's and cannot negotiate Flow Control support,
> > but we can for example connect two ports without PHY's and manually(by
> > ethtool) configure FC.
> > 
> > On the Macchiatobin single shot, you use the existing SFP support rather
> > than forcing them to fixed link.
> > 
> > > Current phylink return error if I do this on ports with
> > MLO_AN_FIXED(callback phylink_ethtool_set_pauseparam):
> > > if (pl->cur_link_an_mode == MLO_AN_FIXED)
> > >                 return -EOPNOTSUPP;
> > >
> > > How can we enable FC configurations for these ports? Do you have any
> > suggestions or should I post my proposal as an RFC patch?
> > 
> > If you really must use fixed-link, you specify the pause modes via firmware,
> > just as you specify the speed and duplex - you specify the link partner's flow
> > control abilities.
> 
> In this case we cannot change this by ethtool during runtime?

I discussed it with Andrew earlier last year, and his response was:

 DT configuration of pause for fixed link probably is sufficient. I don't
 remember it ever been really discussed for DSA. It was a Melanox
 discussion about limiting pause for the CPU. So I think it is safe to
 not implement ethtool -A, at least until somebody has a real use case
 for it.

So I chose not to support it - no point supporting features that people
aren't using. If you have a "real use case" then it can be added.

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