netdev - Re: [PATCH net-next 1/2] ethtool: add PHY Fast Link Down support

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Message-ID: <20190326091721.GH26076@unicorn.suse.cz>
Date:   Tue, 26 Mar 2019 10:17:21 +0100
From:   Michal Kubecek <mkubecek@...e.cz>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Heiner Kallweit <hkallweit1@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        David Miller <davem@...emloft.net>,
        "John W. Linville" <linville@...driver.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 1/2] ethtool: add PHY Fast Link Down support

On Tue, Mar 26, 2019 at 09:24:38AM +0100, Andrew Lunn wrote:
> > > +#define ETHTOOL_PHY_FAST_LINK_DOWN_ON	0
> > > +#define ETHTOOL_PHY_FAST_LINK_DOWN_OFF	0xff
> > > +
> > >  enum phy_tunable_id {
> > >  	ETHTOOL_PHY_ID_UNSPEC,
> > >  	ETHTOOL_PHY_DOWNSHIFT,
> > > +	ETHTOOL_PHY_FAST_LINK_DOWN,
> > >  	/*
> > >  	 * Add your fresh new phy tunable attribute above and remember to update
> > >  	 * phy_tunable_strings[] in net/core/ethtool.c
> > 
> > It would be nice to have a short summary around here explaining how is
> > the value interpreted. While it's obvious from the second patch, one
> > shouldn't have to go into driver specific implementation to find out.
> > 
> > I also wonder if the range 0-254 ms is sufficient. Would it be possible
> > that there is some other hardware which would support e.g. 300 ms?
> 
> The default, as defined by the 802.3 standard, is i think 750ms.
> 
> The Marvel PHY also supports 50ms, 20ms and 0ms, if i remember
> correctly.

The reason why I asked about this is that PHY tunables are supposed to
be universal, not specific to a particular driver, and there might be
other hardware supporting the feature with different set of supported
values.

> One problem we have here is discovery. How does the user find out the
> values the driver supports. For a netlink socket API, extended errors
> could be used to pass back a string indicating the supported
> values. For the old ethtool, i think all we have is -EINVAL, which is
> not very helpful.

As supported values are determined by the driver, we would need to pass
extack to ethtool_ops handler - but that is something we will want to do
eventually (ideally, for all ethtool_ops handlers).

AFAICS the implementation in patch 2/2 rounds user supplied value to
closest value supported by hardware so that user doesn't have to guess
which values are supported. But it would still deserve a warning via
netlink extack, IMHO.

Michal