netdev - Re: Ethtool question

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Date:   Thu, 12 Oct 2017 15:00:38 -0700
From:   Roopa Prabhu <roopa@...ulusnetworks.com>
To:     Ben Greear <greearb@...delatech.com>
Cc:     David Miller <davem@...emloft.net>,
        "John W. Linville" <linville@...driver.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Ethtool question

On Thu, Oct 12, 2017 at 2:45 PM, Ben Greear <greearb@...delatech.com> wrote:
> On 10/11/2017 01:49 PM, David Miller wrote:
>>
>> From: "John W. Linville" <linville@...driver.com>
>> Date: Wed, 11 Oct 2017 16:44:07 -0400
>>
>>> On Wed, Oct 11, 2017 at 09:51:56AM -0700, Ben Greear wrote:
>>>>
>>>> I noticed today that setting some ethtool settings to the same value
>>>> returns an error code.  I would think this should silently return
>>>> success instead?  Makes it easier to call it from scripts this way:
>>>>
>>>> [root@...313-6477 lanforge]# ethtool -L eth3 combined 1
>>>> combined unmodified, ignoring
>>>> no channel parameters changed, aborting
>>>> current values: tx 0 rx 0 other 1 combined 1
>>>> [root@...313-6477 lanforge]# echo $?
>>>> 1
>>>
>>>
>>> I just had this discussion a couple of months ago with someone. My
>>> initial feeling was like you, a no-op is not a failure. But someone
>>> convinced me otherwise...I will now endeavour to remember who that
>>> was and how they convinced me...
>>>
>>> Anyone else have input here?
>>
>>
>> I guess this usually happens when drivers don't support changing the
>> settings at all.  So they just make their ethtool operation for the
>> 'set' always return an error.
>>
>> We could have a generic ethtool helper that does "get" and then if the
>> "set" request is identical just return zero.
>>
>> But from another perspective, the error returned from the "set" in this
>> situation also indicates to the user that the driver does not support
>> the "set" operation which has value and meaning in and of itself.  And
>> we'd lose that with the given suggestion.
>
>
> In my case, the driver (igb) does support the set, my program just made the
> same
> ethtool call several times and it fails after the initial change (that
> actually
> changes something), as best as I can figure.


This error is returned by ethtool user-space. It does a get, check and
then set if user has requested changes.