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

[Vladimir Oltean <olteanv@gmail.com>]

Hi Andrew,

On Mon, 19 Aug 2019 at 16:17, Andrew Lunn <andrew@lunn.ch> wrote:
>
> On Fri, Aug 16, 2019 at 06:31:55PM +0200, Hubert Feurstein wrote:
> > In order to improve the synchronisation precision of phc2sys (from
> > the linuxptp project) for devices like switches which are attached
> > to the MDIO bus, it is necessary the get the system timestamps as
> > close as possible to the access which causes the PTP timestamp
> > register to be snapshotted in the switch hardware. Usually this is
> > triggered by an MDIO write access, the snapshotted timestamp is then
> > transferred by several MDIO reads.
> >
> > This patch adds the required infrastructure to solve the problem described
> > above.
> >
> > Signed-off-by: Hubert Feurstein <h.feurstein@gmail.com>
> > ---
> >  drivers/net/phy/mdio_bus.c | 105 +++++++++++++++++++++++++++++++++++++
> >  include/linux/mdio.h       |   7 +++
> >  include/linux/phy.h        |  25 +++++++++
> >  3 files changed, 137 insertions(+)
> >
> > diff --git a/drivers/net/phy/mdio_bus.c b/drivers/net/phy/mdio_bus.c
> > index bd04fe762056..167a21f267fa 100644
> > --- a/drivers/net/phy/mdio_bus.c
> > +++ b/drivers/net/phy/mdio_bus.c
> > @@ -34,6 +34,7 @@
> >  #include <linux/phy.h>
> >  #include <linux/io.h>
> >  #include <linux/uaccess.h>
> > +#include <linux/ptp_clock_kernel.h>
> >
> >  #define CREATE_TRACE_POINTS
> >  #include <trace/events/mdio.h>
> > @@ -697,6 +698,110 @@ int mdiobus_write(struct mii_bus *bus, int addr, u32 regnum, u16 val)
> >  }
> >  EXPORT_SYMBOL(mdiobus_write);
> >
> > +/**
> > + * __mdiobus_write_sts - Unlocked version of the mdiobus_write_sts function
> > + * @bus: the mii_bus struct
> > + * @addr: the phy address
> > + * @regnum: register number to write
> > + * @val: value to write to @regnum
> > + * @sts: the ptp system timestamp
> > + *
> > + * Write a MDIO bus register and request the MDIO bus driver to take the
> > + * system timestamps when sts-pointer is valid. When the bus driver doesn't
> > + * support this, the timestamps are taken in this function instead.
> > + *
> > + * In order to improve the synchronisation precision of phc2sys (from
> > + * the linuxptp project) for devices like switches which are attached
> > + * to the MDIO bus, it is necessary the get the system timestamps as
> > + * close as possible to the access which causes the PTP timestamp
> > + * register to be snapshotted in the switch hardware. Usually this is
> > + * triggered by an MDIO write access, the snapshotted timestamp is then
> > + * transferred by several MDIO reads.
> > + *
> > + * Caller must hold the mdio bus lock.
> > + *
> > + * NOTE: MUST NOT be called from interrupt context.
> > + */
> > +int __mdiobus_write_sts(struct mii_bus *bus, int addr, u32 regnum, u16 val,
> > +                     struct ptp_system_timestamp *sts)
> > +{
> > +     int retval;
> > +
> > +     WARN_ON_ONCE(!mutex_is_locked(&bus->mdio_lock));
> > +
> > +     if (!bus->ptp_sts_supported)
> > +             ptp_read_system_prets(sts);
>
> 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.

Regards,
-Vladimir