Messages in this thread | | | From | Vladimir Oltean <> | Date | Mon, 19 Aug 2019 16:34:46 +0300 | Subject | Re: [PATCH net-next 1/3] net: mdio: add support for passing a PTP system timestamp to the mii_bus driver |
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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
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