netdev - Re: [PATCH net-next 1/4] net: ethernet: validate pause autoneg setting

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Message-ID: <2afd5ca8-c913-6b58-7085-365036098468@gmail.com>
Date:   Tue, 12 May 2020 19:37:01 -0700
From:   Doug Berger <opendmb@...il.com>
To:     Michal Kubecek <mkubecek@...e.cz>, netdev@...r.kernel.org
Cc:     Andrew Lunn <andrew@...n.ch>,
        "David S. Miller" <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        bcm-kernel-feedback-list@...adcom.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 1/4] net: ethernet: validate pause autoneg
 setting

On 5/12/2020 12:08 PM, Michal Kubecek wrote:
> On Tue, May 12, 2020 at 11:31:39AM -0700, Doug Berger wrote:
>> On 5/11/2020 5:47 PM, Andrew Lunn wrote:
>>> On Mon, May 11, 2020 at 05:24:07PM -0700, Doug Berger wrote:
>>>> A comment in uapi/linux/ethtool.h states "Drivers should reject a
>>>> non-zero setting of @autoneg when autoneogotiation is disabled (or
>>>> not supported) for the link".
>>>>
>>>> That check should be added to phy_validate_pause() to consolidate
>>>> the code where possible.
>>>>
>>>> Fixes: 22b7d29926b5 ("net: ethernet: Add helper to determine if pause configuration is supported")
>>>
>>> Hi Doug
>>>
>>> If this is a real fix, please submit this to net, not net-next.
>>>
>>>    Andrew
>>>
>> This was intended as a fix, but I thought it would be better to keep it
>> as part of this set for context and since net-next is currently open.
>>
>> The context is trying to improve the phylib support for offloading
>> ethtool pause configuration and this is something that could be checked
>> in a single location rather than by individual drivers.
>>
>> I included it here to get feedback about its appropriateness as a common
>> behavior. I should have been more explicit about that.
>>
>> Personally, I'm actually not that fond of this change since it can
>> easily be a source of confusion with the ethtool interface because the
>> link autonegotiation and the pause autonegotiation are controlled by
>> different commands.
>>
>> Since the ethtool -A command performs a read/modify/write of pause
>> parameters, you can get strange results like these:
>> # ethtool -s eth0 speed 100 duplex full autoneg off
>> # ethtool -A eth0 tx off
>> Cannot set device pause parameters: Invalid argument
>> #
>> Because, the get read pause autoneg as enabled and only the tx_pause
>> member of the structure was updated.
> 
> This would be indeed unfortunate. We could use extack to make the error
> message easier to understand but the real problem IMHO is that
> ethtool_ops::get_pauseparam() returns value which is rejected by
> ethtool_ops::set_pauseparam(). This is something we should avoid.
> 
> If we really wanted to reject ethtool_pauseparam::autoneg on when
> general autonegotiation is off, we would have to disable pause
> autonegotiation whenever general autonegotiation is disabled. I don't
> like that idea, however, as that would mean that
> 
>   ethtool -s dev autoneg off ...
>   ethtool -s dev autoneg on ...
> 
> would reset the setting of pause autonegotiation.
> 
> Therefore I believe the comment should be rather replaced by a warning
> that even if ethtool_pauseparam::autoneg is enabled, pause
> autonegotiation is only active if general autonegotiation is also
> enabled.
> 
> Michal
> 
Thanks for your reply.

I agree with your concerns and will remove this commit from the set when
I resubmit. I also favor replacing the comment in ethtool.h.

-Doug