netdev - Re: [PATCH 2/5] net: phy: hook up clause 45 autonegotiation restart

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Message-ID: <23c238e0-2af8-3171-9986-af02e9c19129@gmail.com>
Date:   Thu, 1 Jun 2017 10:16:17 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Andrew Lunn <andrew@...n.ch>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH 2/5] net: phy: hook up clause 45 autonegotiation restart

On 06/01/2017 09:24 AM, Russell King - ARM Linux wrote:
> On Thu, Jun 01, 2017 at 04:47:35PM +0100, Russell King - ARM Linux wrote:
>> On Thu, Jun 01, 2017 at 03:19:55PM +0200, Andrew Lunn wrote:
>>> On Thu, Jun 01, 2017 at 02:09:00PM +0100, Russell King - ARM Linux wrote:
>>>> On Thu, Jun 01, 2017 at 03:05:27PM +0200, Andrew Lunn wrote:
>>>>> So you are saying a 10G PHY driver always needs to have a aneg_done
>>>>> callback, even if it just needs to call phygen_c45_aneg_done?
>>>>>
>>>>> This seems a bit error prone. I can see somebody writing a 10G driver,
>>>>> leaving out aneg_done() and having the c22 version called. Is the read
>>>>> of MII_BMSR likely to return 0xffff, since the register does not
>>>>> exist? If so, genphy_aneg_done() is likely to always return
>>>>> BMSR_ANEGCOMPLETE.
>>>>
>>>> Don't forget that the read will fail, so phy_read() will return a
>>>> negative number.
>>>
>>> By fail, you mean return something like -EIO or -ETIMEOUT? Is this
>>> guaranteed in the code somewhere? This particular Marvell PHY only
>>> does c45. But i could imagine some other PHYs answering a c22 request
>>> with 0xffff.
>>
>> Yes, C45 allows the PHYs to answer C22 as well, but then they have to
>> implement the C22 register set.  Such a PHY would be out of spec,
>> especially as what you're suggesting is that it answers C22 cycles
>> and fails to implement MII_BMSR.  I also think that there's a comment
>> in the 802.3 specs that says that unimplemented registers are to
>> return zero, not 0xffff.
> 
> Checking 802.3-2015, in "22.2.4 Management functions", the first
> sentence requires all PHYs that respond to Clause 22 cycles to
> implement BMCR and BMSR.  However, my statement about unimplemented
> registers returning zero seems to be a C45 thing, not a C22 thing,
> according to the C22 PICS and "45.2 MDIO Interface Registers"
> 
> However, digging a bit further, "22.2.4.2.10 Auto-Negotiation complete"
> states that bit 5 shall be zero if aneg has not completed or if aneg is
> unimplemented.
> 

So how about we are more defensive than that and if we are presented
with a C45 PHY driver that does not have an aneg_done() function pointer
set we return an error/warning during driver registration?
-- 
Florian