netdev - Re: [PATCH] net-core: remove unnecessary ETHTOOL

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Message-ID: <20200123084000.GT22304@unicorn.suse.cz>
Date:   Thu, 23 Jan 2020 09:40:00 +0100
From:   Michal Kubecek <mkubecek@...e.cz>
To:     netdev@...r.kernel.org
Cc:     Luigi Rizzo <lrizzo@...gle.com>, Andrew Lunn <andrew@...n.ch>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] net-core: remove unnecessary ETHTOOL_GCHANNELS
 initialization

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@...n.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

Unless you check all in tree drivers, you cannot be sure there isn't one
which would in fact rely on .cmd being set to expected value. And even
then there could be some out of tree driver; we usually don't care too
much about them but it's always matter of what you gain by the cleanup.
AFAICS it might be just few CPU cycles in this case - if we are lucky.

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

That would be incorrect. The initialization in ethtool_set_channels() is
for curr variable which is passed to ->get_channels() (to get current
values for checking new values against maximum). Therefore the correct
command really is ETHTOOL_GCHANNELS.

Michal