netdev - Re: [PATCH net-next] net: phy: improve link partner capability detection

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Date:   Sat, 6 Apr 2019 13:21:21 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Heiner Kallweit <hkallweit1@...il.com>,
        Andrew Lunn <andrew@...n.ch>
Cc:     David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net: phy: improve link partner capability
 detection



On 4/6/2019 9:25 AM, Heiner Kallweit wrote:
> On 05.04.2019 23:54, Florian Fainelli wrote:
>> On 4/5/19 2:52 PM, Heiner Kallweit wrote:
>>> On 05.04.2019 23:38, Heiner Kallweit wrote:
>>>> On 05.04.2019 23:27, Andrew Lunn wrote:
>>>>>> +	if (linkmode_test_bit(ETHTOOL_LINK_MODE_1000baseT_Half_BIT,
>>>>>> +			      phydev->supported))
>>>>>> +		phydev->is_gigabit_capable = 1;
>>>>>> +	if (linkmode_test_bit(ETHTOOL_LINK_MODE_1000baseT_Full_BIT,
>>>>>> +			      phydev->supported))
>>>>>> +		phydev->is_gigabit_capable = 1;
>>>>>> +
>>>>>
>>>>> What i'm trying to get at is, why do we need this bit of the patch?
>>>>> Why do we need this flag? The hardware should tell us if it can do
>>>>> gigabit.
>>>>>
>>>> The code to query BMSR_ESTATEN and MII_ESTATUS is in genphy_read_status.
>>>> However we also have to cover the case that this function isn't used.
>>>> Therefore I query phydev->supported before the speed could be limited.
>>>> (relying on the PHY driver not lying about gigabit capability)
>>>> This part of the patch is directly before of_set_phy_supported().
>>>>
>>>> I just see that we can re-use is_gigabit_capable also in
>>>> genphy_config_advert.
>>>>
>>>> Of course I can read in every place the hardware for gigabit support.
>>>> But IMO this creates unnecessary code duplication.
>>>>
>>> I just see that we can reuse is_gigabit_capable also in
>>> genphy_config_advert().
>>>
>>> And when checking occurrences of BMSR_ESTATEN there seems to be more
>>> work waiting: In swphy BMSR_ESTATEN is set, but MII_ESTATUS isn't
>>> configured. It just works by chance becausing reading this register
>>> returns the default 0xffff.
>>
>> It is not clear to me what we are trying to optimize for here, given
>> everything is pretty much a slow path anyway. I am concerned though
>> about mis-behaving PHYs where caching of BMSR_ESTATEN could result in
>> incorrect behaviors, I don't have any data to back that claim, but
>> reading the registers should always be the safest way to determine what
>> HW is capable of (famous last words). Not feeling strongly about either
>> direction though...
>>
> 
> At this point I have to quote Einstein:
> "If you can't explain it simply, you don't understand it well enough."
> Means I'll spend few more thoughts on it ..

I have re-read the patch more carefully now and understand what it is
fixing, sorry for the pointless discussion.
-- 
Florian