Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH net V2 1/1] net: phy: fix invalid phy id when probe using
 C22
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Heiner Kallweit <hkallweit1@gmail.com>,
        Wong Vee Khee <vee.khee.wong@intel.com>,
        Andrew Lunn <andrew@lunn.ch>,
        Russell King <linux@armlinux.org.uk>,
        "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Voon Weifeng <weifeng.voon@intel.com>,
        Ong Boon Leong <boon.leong.ong@intel.com>
References: <20210318090937.26465-1-vee.khee.wong@intel.com>
 <b63c5068-1203-fcb6-560d-1d2419bb39b0@gmail.com>
 <c921bf7f-e4d1-eefa-c5ae-024d5e8a4845@gmail.com> <YFOYclhA75hjmQHY@kroah.com>
From:   Florian Fainelli <f.fainelli@gmail.com>
Message-ID: <665e347d-b4fc-b893-0317-3d04624fc1ba@gmail.com>
Date:   Thu, 18 Mar 2021 11:58:07 -0700
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101
 Firefox/78.0 Thunderbird/78.8.1
MIME-Version: 1.0
In-Reply-To: <YFOYclhA75hjmQHY@kroah.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk


On 3/18/2021 11:14 AM, Greg KH wrote:
> On Thu, Mar 18, 2021 at 09:02:22AM -0700, Florian Fainelli wrote:
>>
>>
>> On 3/18/2021 6:25 AM, Heiner Kallweit wrote:
>>> On 18.03.2021 10:09, Wong Vee Khee wrote:
>>>> When using Clause-22 to probe for PHY devices such as the Marvell
>>>> 88E2110, PHY ID with value 0 is read from the MII PHYID registers
>>>> which caused the PHY framework failed to attach the Marvell PHY
>>>> driver.
>>>>
>>>> Fixed this by adding a check of PHY ID equals to all zeroes.
>>>>
>>>
>>> I was wondering whether we have, and may break, use cases where a PHY,
>>> for whatever reason, reports PHY ID 0, but works with the genphy
>>> driver. And indeed in swphy_read_reg() we return PHY ID 0, therefore
>>> the patch may break the fixed phy.
>>> Having said that I think your patch is ok, but we need a change of
>>> the PHY ID reported by swphy_read_reg() first.
>>> At a first glance changing the PHY ID to 0x00000001 in swphy_read_reg()
>>> should be sufficient. This value shouldn't collide with any real world
>>> PHY ID.
>>
>> It most likely would not, but it could be considered an ABI breakage,
>> unless we filter out what we report to user-space via SIOGCMIIREG and
>> /sys/class/mdio_bus/*/*/phy_id
>>
>> Ideally we would have assigned an unique PHY OUI to the fixed PHY but
>> that would have required registering Linux as a vendor, and the process
>> is not entirely clear to me about how to go about doing that.
> 
> If you need me to do that under the umbrella of the Linux Foundation,
> I'll be glad to do so if you point me at the proper group to do that
> with.
> 
> We did this for a few years with the USB-IF and have a vendor id
> assigned to us for Linux through them, until they kicked us out because.
> But as the number is in a global namespace, it can't be reused so we
> keep it :)

We would still be creating what is technically an user-space interface
breakage since prior to a given kernel version we would return 0 for
that PHY OUI, and after another point it would be something different. I
don't know what software out there may be expecting to find 0 and not
determine that the PHY was fixed already because it is under
/sys/class/mdio_bus/fixed-0
-- 
Florian