Re: [PATCH v1 net-next] net: stmmac: Add support for MDIO interrupts

[Florian Fainelli <f.fainelli@gmail.com>]

On 8/28/19 10:07 AM, Voon, Weifeng wrote:
>>>> DW EQoS v5.xx controllers added capability for interrupt generation
>>>> when MDIO interface is done (GMII Busy bit is cleared).
>>>> This patch adds support for this interrupt on supported HW to avoid
>>>> polling on GMII Busy bit.
>>>>
>>>> stmmac_mdio_read() & stmmac_mdio_write() will sleep until wake_up()
>>>> is called by the interrupt handler.
>>>
>>> Hi Voon
>>>
>>> I _think_ there are some order of operation issues here. The mdiobus
>>> is registered in the probe function. As soon as of_mdiobus_register()
>>> is called, the MDIO bus must work. At that point MDIO read/writes can
>>> start to happen.
>>>
>>> As far as i can see, the interrupt handler is only requested in
>>> stmmac_open(). So it seems like any MDIO operations after probe, but
>>> before open are going to fail?
>>
>> AFAIR, wait_event_timeout() will continue to busy loop and wait until
>> the timeout, but not return an error because the polled condition was
>> true, at least that is my recollection from having the same issue with
>> the bcmgenet driver before it was moved to connecting to the PHY in the
>> ndo_open() function.
>> --
>> Florian
> 
> Florian is right as the poll condition is still true after the timeout. 
> Hence, any mdio operation after probe and before ndo_open will still work.
> The only cons here is that attaching the PHY will takes a full length of 
> timeout time for each mdio_read and mdio_write. 
> So we should attach the phy only after the interrupt handler is requested?

From a power management/resource utilization perspective, it is better
to initialize as close as possible from the time where you are actually
going to use the hardware, therefore ndo_open().

This may not be convenient or possible given how widely use stmmac is,
and I do not know if parts of the Ethernet MAC require the PHY to supply
the clock, in which case, you may have some chicke and egg conditions if
the design does not allow for MDIO to work independently from the data
plane. Also, I would be worried about introducing bugs.

You could do a couple of things:

- continue to probe the device with interrupts disabled and add a
condition around the call to wait_event_timeout() to do a busy-loop
without going to the maximum defined timeout, if the interrupt line is
requested, use wait_event_timeout()

- request the interrupt during the probe function, but only
unmask/enable the MDIO interrupts for the probe to succeed and leave the
data path interrupts for a later enabling during ndo_open()
-- 
Florian