netdev - Re: [PATCH net-next v2 2/3] net: ethernet: fec: Allow configuration of MDIO bus speed

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Date:   Sat, 18 Apr 2020 14:07:22 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Heiner Kallweit <hkallweit1@...il.com>, fugang.duan@....com,
        Chris Healy <Chris.Healy@....aero>
Subject: Re: [PATCH net-next v2 2/3] net: ethernet: fec: Allow configuration
 of MDIO bus speed



On 4/18/2020 9:49 AM, Andrew Lunn wrote:
>>> I don't see how that would work. Each device on the bus needs to be
>>> able to receiver the transaction in order to decode the device
>>> address, and then either discard it, or act on it. So the same as I2C
>>> where the device address is part of the transaction. You need the bus
>>> to run as fast as the slowest device on the bus. So a bus property is
>>> the simplest. You could have per device properties, and during the bus
>>> scan, figure out what the slowest device is, but that seems to add
>>> complexity for no real gain. I2C does not have this either.
>>>
>>> If MDIO was more like SPI, with per device chip select lines, then a
>>> per device frequency would make sense.
>>
>> OK, that is a good point, but then again, just like patch #3 you need to
>> ensure that you are setting a MDIO bus controller frequency that is the
>> lowest common denominator of all MDIO slaves on the bus, which means that
>> you need to know about what devices do support.
> 
> Hi Florian
> 
> I've been following what I2C does, since MDIO and I2C is very similar.
> I2C has none of what you are asking for. If I2C does not need any of
> this, does MDIO? I2C assumes what whoever writes the DT knows what
> they are doing and will set a valid clock frequency which works for
> all devices on the bus. This seems to work for I2C, so why should it
> not work for MDIO?
> 
> My preference is KISS.

OK, you have convinced me.
-- 
Florian