netdev - Re: [RFC] net: dsa: mv88e6xxx: ptp: improve phc2sys precision for mv88e6xxx switch in combination with imx6-fec

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Date:   Mon, 5 Aug 2019 16:50:12 +0200
From:   Hubert Feurstein <h.feurstein@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     mlichvar@...hat.com, Richard Cochran <richardcochran@...il.com>,
        Andrew Lunn <andrew@...n.ch>, netdev <netdev@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC] net: dsa: mv88e6xxx: ptp: improve phc2sys precision for
 mv88e6xxx switch in combination with imx6-fec

Hi Vladimir,

Am Mo., 5. Aug. 2019 um 11:55 Uhr schrieb Vladimir Oltean <olteanv@...il.com>:
[...]
> You guessed correctly (since you copied me) that I'm battling much of
> the same issues with the sja1105 and its spi-fsl-dspi controller
> driver.
I've copied you, because of this discussion on github:
https://github.com/openil/linuxptp/issues/5
There you said: "In fact any MDIO access will make the latency
unpredictable and hence
 throw off the time."
I thought you might be interested in how to make MDIO latency predictable.

[...]
> - You said you patched linuxptp master. Could you share the patch? Is
> there anything else that's needed except compiling against the board's
> real kernel headers (aka point KBUILD_OUTPUT to the extracted location
> of /sys/kernel/kheaders.tar.xz)? I've done that and I do see phc2sys
> probing and using the new ioctl, but I don't see a big improvement in
> my case (that's probably because my SPI interface has real jitter
> caused by peripheral clock instability, but I really need to put a
> scope on it to be sure, so that's a discussion for another time).

My compiler used kernel headers where the ioctl was not yet defined.
So I simply defined it in the linuxptp source directly:

diff --git a/sysoff.c b/sysoff.c
index b993ee9..b23ad2f 100644
--- a/sysoff.c
+++ b/sysoff.c
@@ -27,6 +27,20 @@

#define NS_PER_SEC 1000000000LL

+#ifndef PTP_SYS_OFFSET_EXTENDED
+struct ptp_sys_offset_extended {
+    unsigned int n_samples; /* Desired number of measurements. */
+    unsigned int rsv[3];    /* Reserved for future use. */
+    /*
+     * Array of [system, phc, system] time stamps. The kernel will provide
+     * 3*n_samples time stamps.
+     */
+    struct ptp_clock_time ts[PTP_MAX_SAMPLES][3];
+};
+#define PTP_SYS_OFFSET_EXTENDED \
+    _IOWR(PTP_CLK_MAGIC, 9, struct ptp_sys_offset_extended)
+#endif
+
#ifdef PTP_SYS_OFFSET

static int64_t pctns(struct ptp_clock_time *t)

> - I really wonder what your jitter is caused by. Maybe it is just
> contention on the mii_bus->mdio_lock? If that's the case, maybe you
> don't need to go all the way down to the driver level, and taking the
> ptp_sts at the subsystem (MDIO, SPI, I2C, etc) level is "good enough".

I would say there are many places, where we introduce unpredictable jitter:
- The busy-flag polling
- The locking of the chip- and mdio-bus-mutex
- The mdio_done completion in the imx_fec
- Scheduling latencies

> - Along the lines of the above, I wonder how badly would the
> maintainers shout at the proposal of adding a struct
> ptp_system_timestamp pointer and its associated lock in struct device.
> That way at least you don't have to change the API, like you did with
> mdiobus_write_nested_ptp. Relatively speaking, this is the least
> amount of intrusion (although, again, far from beautiful).

It is important to make sure to hook up the right mdio_write access, that is
why the ptp_sts pointer is passed under the mdio_lock. Of course It would
be nicer, to pass through the pointer as an argument, instead of bypassing it to
the mii_bus struct. In the case of SPI it could be added to the spi_transfer
struct.

> - The software timestamps help you in the particular case of phc2sys,
> but are they enough to cover all your needs?
For now it's all I need.

> I haven't spent even 1
> second looking at Marvell switch datasheets, but is its free-running
> timer only used for PTP timestamping? At least the sja1105 does also
> support time-based egress scheduling and ingress policing, so for that
> scenario, the timecounter/cyclecounter implementation will no longer
> cut it (you do need to synchronize the hardware clock). If your
> hardware supports these PTP-based features as well, I can only assume
> the reason why mv88e6xxx went for a timecounter is the same as the
> reason why I did: the MDIO/SPI jitter while accessing the frequency
> and offset correction registers is bad enough that the servo loop goes
> haywire. And implementing gettimex64 will not solve that.
>
[...]

Hubert