[RFC PATCH 4/4] ptp_vmclock: Feed reference to timekeeping for feed-forward discipline

[David Woodhouse]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

When a vmclock device provides valid time, call timekeeping_set_reference()
to enable feed-forward clock discipline. This eliminates drift between the
system clock and the vmclock reference.

The reference is set at probe time (after PTP registration) and updated
on each notification from the hypervisor (ACPI or DT interrupt).

If cycle_interval is not provided (set to 0), timekeeping_set_reference()
fills it from the current timekeeper.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 drivers/ptp/ptp_vmclock.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/drivers/ptp/ptp_vmclock.c b/drivers/ptp/ptp_vmclock.c
index 8b630eb916b5..225cdc4366dc 100644
--- a/drivers/ptp/ptp_vmclock.c
+++ b/drivers/ptp/ptp_vmclock.c
@@ -27,6 +27,7 @@
 #include <uapi/linux/vmclock-abi.h>
 
 #include <linux/ptp_clock_kernel.h>
+#include <linux/timekeeping_reference.h>
 
 #ifdef CONFIG_X86
 #include <asm/pvclock.h>
@@ -334,6 +335,27 @@ static const struct ptp_clock_info ptp_vmclock_info = {
 	.getcrosststamp = ptp_vmclock_getcrosststamp,
 };
 
+static void vmclock_set_tk_reference(struct vmclock_state *st)
+{
+	struct vmclock_abi *clk = st->clk;
+	struct tk_reference ref = {
+		.cs_id = st->cs_id,
+		.counter_value = le64_to_cpu(clk->counter_value),
+		.time_sec = le64_to_cpu(clk->time_sec),
+		.time_frac_sec = le64_to_cpu(clk->time_frac_sec),
+		.period_frac_sec = le64_to_cpu(clk->counter_period_frac_sec),
+		.period_shift = clk->counter_period_shift,
+	};
+
+	/* Convert TAI to UTC for comparison with xtime_sec */
+	if (clk->time_type == VMCLOCK_TIME_TAI &&
+	    (le64_to_cpu(clk->flags) & VMCLOCK_FLAG_TAI_OFFSET_VALID))
+		ref.time_sec += (int16_t)le16_to_cpu(clk->tai_offset_sec);
+
+	if (clk->clock_status != VMCLOCK_STATUS_UNRELIABLE)
+		timekeeping_set_reference(&ref);
+}
+
 static struct ptp_clock *vmclock_ptp_register(struct device *dev,
 					      struct vmclock_state *st)
 {
@@ -525,6 +547,7 @@ vmclock_acpi_notification_handler(acpi_handle __always_unused handle,
 	struct device *device = dev;
 	struct vmclock_state *st = device->driver_data;
 
+	vmclock_set_tk_reference(st);
 	wake_up_interruptible(&st->disrupt_wait);
 }
 
@@ -580,6 +603,7 @@ static irqreturn_t vmclock_of_irq_handler(int __always_unused irq, void *_st)
 {
 	struct vmclock_state *st = _st;
 
+	vmclock_set_tk_reference(st);
 	wake_up_interruptible(&st->disrupt_wait);
 	return IRQ_HANDLED;
 }
@@ -751,6 +775,8 @@ static int vmclock_probe(struct platform_device *pdev)
 			st->ptp_clock = NULL;
 			return ret;
 		}
+		if (st->ptp_clock)
+			vmclock_set_tk_reference(st);
 	}
 
 	if (!st->miscdev.minor && !st->ptp_clock) {
-- 
2.51.0