netdev - Re: [PATCH net-next 3/9] net: phy: c45: don't accept disabled EEE modes in genphy_c45_ethtool_set

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Message-ID: <b1d56e22-5bbb-4881-abc1-6f8832bb575d@gmail.com>
Date: Sat, 11 Jan 2025 16:33:35 +0100
From: Heiner Kallweit <hkallweit1@...il.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Paolo Abeni <pabeni@...hat.com>, Jakub Kicinski <kuba@...nel.org>,
 David Miller <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Simon Horman <horms@...nel.org>, Andrew Lunn <andrew@...n.ch>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 3/9] net: phy: c45: don't accept disabled EEE
 modes in genphy_c45_ethtool_set_eee

On 11.01.2025 16:13, Russell King (Oracle) wrote:
> On Sat, Jan 11, 2025 at 02:19:04PM +0100, Heiner Kallweit wrote:
>> On 11.01.2025 11:01, Russell King (Oracle) wrote:
>>> On Sat, Jan 11, 2025 at 10:44:25AM +0100, Heiner Kallweit wrote:
>>>> On 11.01.2025 10:21, Russell King (Oracle) wrote:
>>>>> On Sat, Jan 11, 2025 at 10:06:02AM +0100, Heiner Kallweit wrote:
>>>>>> Link modes in phydev->eee_disabled_modes are filtered out by
>>>>>> genphy_c45_write_eee_adv() and won't be advertised. Therefore
>>>>>> don't accept such modes from userspace.
>>>>>
>>>>> Why do we need this? Surely if the MAC doesn't support modes, then they
>>>>> should be filtered out of phydev->supported_eee so that userspace knows
>>>>> that the mode is not supported by the network interface as a whole, just
>>>>> like we do for phydev->supported.
>>>>>
>>>>> That would give us the checking here.
>>>>>
>>>> Removing EEE modes to be disabled from supported_eee is problematic
>>>> because of how genphy_c45_write_eee_adv() works.
>>>>
>>>> Let's say we have a 2.5Gbps PHY and want to disable EEE at 2.5Gbps. If we
>>>> remove 2.5Gbps from supported_eee, then the following check is false:
>>>> if (linkmode_intersects(phydev->supported_eee, PHY_EEE_CAP2_FEATURES))
>>>> What would result in the 2.5Gbps mode not getting disabled.
>>>
>>> Ok. Do we at least remove the broken modes from the supported mask
>>> reported to userspace?
>>>
>> I think that's something we could do in addition, to provide a hint to the
>> user about unavailable modes. It wouldn't remove the need for the check here.
>> ethtool doesn't check the advertisement against the supported modes.
>> And even if it would, we must not rely on input from user space being sane.
> 
> I disagree with some of this. Userspace should expect:
> 
> - read current settings
> - copy supported modes to advertised modes
> - write current settings
> 
> to work. If it fails, then how does ethtool, or even the user, work out
> which link modes are actually supported or not.
> 
> If we're introducing a failure on the "disabled" modes, then that is
> a user-breaking change, and we need to avoid that. The current code
> silently ignored the broken modes, your new code would error out on
> the above action - and that's a bug.
> 
OK, then I think what we can/should do:
- filter out disabled EEE modes when populating data->supported in
  genphy_c45_ethtool_get_eee
- silently filter out disabled EEE modes from user space provided
  EEE advertisement in genphy_c45_ethtool_set_eee