Re: [PATCH] net-core: remove unnecessary ETHTOOL_GCHANNELS initialization

[Michal Kubecek <mkubecek@suse.cz>]

On Wed, Jan 22, 2020 at 04:18:56PM -0800, Luigi Rizzo wrote:
> On Wed, Jan 22, 2020 at 3:47 PM Andrew Lunn <andrew@lunn.ch> wrote:
> >
> > On Wed, Jan 22, 2020 at 02:33:26PM -0800, Luigi Rizzo wrote:
> > > struct ethtool_channels does not need .cmd to be set when calling the
> > > driver's ethtool methods. Just zero-initialize it.
> > >
> > > Tested: run ethtool -l and ethtool -L
> >
> > Hi Luigi
> >
> > This seems pretty risky. You are assuming ethtool is the only user of
> > this API. What is actually wrong with putting a sane cmd value, rather
> > than the undefined value 0.
> 
> Hi Andrew, if I understand correctly your suggestion is that even if
> the values are unused, it is better to stay compliant with the header
> file include/uapi/linux/ethtool.h, which does suggest a value for .cmd
> for the various structs

The point is that unless you check ethtool_ops::get_channels() of all in
tree drivers, you cannot be sure there isn't one which depends on .cmd
being set to expected value. And even then there could be some out of
tree drivers; we usually don't care too much about those but it's always
question of what you gain by the cleanup. AFAICS, in this case it might
be few CPU cycles and even that isn't really sure.

> and only replace the value in ethtool_set_channels() with the correct
> one ETHTOOL_SCHANNELS ?

That would be incorrect. In ethtool_set_channels(), the structure
initialized is curr which is used with ->get_channels() to get current
values (for comparison of new values against driver maximum) so that
ETHTOOL_GCHANNELS is the right command.

Michal