Re: [PATCH net-next 1/3] net: mdio: add support for passing a PTP system timestamp to the mii_bus driver

[Andrew Lunn <andrew@lunn.ch>]

> > > How expensive is ptp_read_system_prets()? My original suggestion was
> > > to unconditionally call it here, and then let the driver overwrite it
> > > if it supports finer grained time stamping. MDIO is slow, so as long
> > > as ptp_read_system_prets() is not too expensive, i prefer KISS.
> > >
> > >    Andrew
> >
> > While that works for the pre_ts, it doesn't work for the post_ts (the
> > MDIO bus core will unconditionally overwrite the system timestamp from
> > the driver).
> > Unless you're suggesting to keep the pre_ts unconditional and the
> > post_ts under the "if" condition, which is a bit odd.
> > According to my tests with a scope (measuring the width between SPI
> > transfers with and without the ptp_read_system_*ts calls), two calls
> > to ktime_get_real_ts64 amount to around 750 ns on a 1200 MHz Cortex A7
> > core, or around 90 clock cycles.
> 
> 900 clock cycles, my bad.

That is quite a lot. I was just expecting it to read a free running
clock and maybe do some unit conversions. 900 cycles suggests it is
doing a lot more.

So please keep with the idea of the bus driver indicating if it
supports the time stamping. But please make it a generic bus->flags,
and bit 0 indicating time stamping. At some point in the future, it
would be useful to indicate if the bus supports c45, which would be
another use of flags.

Thanks
	Andrew