Subject: [PATCH net-next 2/3] ice: avoid the PTP hardware semaphore in gettimex64 path

From: Michal Schmidt <[email-masked]>
Date: Mon, 26 Feb 2024 16:11:24 +0100

The PTP hardware semaphore (PFTSYN_SEM) is used to synchronize
operations that program the PTP timers. The operations involve issuing
commands to the sideband queue. The E810 does not have a hardware
sideband queue, so the admin queue is used. The admin queue is slow.
I have observed delays in hundreds of milliseconds waiting for
ice_sq_done.

When phc2sys reads the time from the ice PTP clock and PFTSYN_SEM is
held by a task performing one of the slow operations, ice_ptp_lock can
easily time out. phc2sys gets -EBUSY and the kernel prints:
  ice 0000:XX:YY.0: PTP failed to get time
These messages appear once every few seconds, causing log spam.

The E810 datasheet recommends an algorithm for reading the upper 64 bits
of the GLTSYN_TIME register. It matches what's implemented in
ice_ptp_read_src_clk_reg. It is robust against wrap-around, but not
necessarily against the concurrent setting of the register (with
GLTSYN_CMD_{INIT,ADJ}_TIME commands). Perhaps that's why
ice_ptp_gettimex64 also takes PFTSYN_SEM.

The race with time setters can be prevented without relying on the PTP
hardware semaphore. Using the "ice_adapter" from the previous patch,
we can have a common spinlock for the PFs that share the clock hardware.
It will protect the reading and writing to the GLTSYN_TIME register.
The writing is performed indirectly, by the hardware, as a result of
the driver writing GLTSYN_CMD_SYNC in ice_ptp_exec_tmr_cmd. I wasn't
sure if the ice_flush there is enough to make sure GLTSYN_TIME has been
updated, but it works well in my testing.

My test code can be seen here:
https://gitlab.com/mschmidt2/linux/-/commits/ice-ptp-host-side-lock
It consists of:
 - kernel threads reading the time in a busy loop and looking at the
   deltas between consecutive values, reporting new maxima.
   in the consecutive values;
 - a shell script that sets the time repeatedly;
 - a bpftrace probe to produce a histogram of the measured deltas.
Without the spinlock ptp_gltsyn_time_lock, it is easy to see tearing.
Deltas in the [2G, 4G) range appear in the histograms.
With the spinlock added, there is no tearing and the biggest delta I saw
was in the range [1M, 2M), that is under 2 ms.

Signed-off-by: Michal Schmidt <[email-masked]>
---
 drivers/net/ethernet/intel/ice/ice_adapter.c | 2 ++
 drivers/net/ethernet/intel/ice/ice_adapter.h | 6 ++++++
 drivers/net/ethernet/intel/ice/ice_ptp.c     | 8 +-------
 drivers/net/ethernet/intel/ice/ice_ptp_hw.c  | 3 +++
 4 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/drivers/net/ethernet/intel/ice/ice_adapter.c b/drivers/net/ethernet/intel/ice/ice_adapter.c
index deb063401238..4b9f5d29811c 100644
--- a/drivers/net/ethernet/intel/ice/ice_adapter.c
+++ b/drivers/net/ethernet/intel/ice/ice_adapter.c
@@ -5,6 +5,7 @@
 #include <linux/mutex.h>
 #include <linux/pci.h>
 #include <linux/slab.h>
+#include <linux/spinlock.h>
 #include <linux/xarray.h>
 #include "ice_adapter.h"
 
@@ -38,6 +39,7 @@ struct ice_adapter *ice_adapter_get(const struct pci_dev *pdev)
 	if (!a)
 		return NULL;
 
+	spin_lock_init(&a->ptp_gltsyn_time_lock);
 	refcount_set(&a->refcount, 1);
 
 	if (xa_is_err(xa_store(&ice_adapters, index, a, GFP_KERNEL))) {
diff --git a/drivers/net/ethernet/intel/ice/ice_adapter.h b/drivers/net/ethernet/intel/ice/ice_adapter.h
index cb5a02eb24c1..9d11014ec02f 100644
--- a/drivers/net/ethernet/intel/ice/ice_adapter.h
+++ b/drivers/net/ethernet/intel/ice/ice_adapter.h
@@ -4,15 +4,21 @@
 #ifndef _ICE_ADAPTER_H_
 #define _ICE_ADAPTER_H_
 
+#include <linux/spinlock_types.h>
 #include <linux/refcount_types.h>
 
 struct pci_dev;
 
 /**
  * struct ice_adapter - PCI adapter resources shared across PFs
+ * @ptp_gltsyn_time_lock: Spinlock protecting access to the GLTSYN_TIME
+ *                        register of the PTP clock.
  * @refcount: Reference count. struct ice_pf objects hold the references.
  */
 struct ice_adapter {
+	/* For access to the GLTSYN_TIME register */
+	spinlock_t ptp_gltsyn_time_lock;
+
 	refcount_t refcount;
 };
 
diff --git a/drivers/net/ethernet/intel/ice/ice_ptp.c b/drivers/net/ethernet/intel/ice/ice_ptp.c
index c11eba07283c..b6c7246245c6 100644
--- a/drivers/net/ethernet/intel/ice/ice_ptp.c
+++ b/drivers/net/ethernet/intel/ice/ice_ptp.c
@@ -374,6 +374,7 @@ ice_ptp_read_src_clk_reg(struct ice_pf *pf, struct ptp_system_timestamp *sts)
 	u8 tmr_idx;
 
 	tmr_idx = ice_get_ptp_src_clock_index(hw);
+	guard(spinlock_irqsave)(&pf->adapter->ptp_gltsyn_time_lock);
 	/* Read the system timestamp pre PHC read */
 	ptp_read_system_prets(sts);
 
@@ -1925,15 +1926,8 @@ ice_ptp_gettimex64(struct ptp_clock_info *info, struct timespec64 *ts,
 		   struct ptp_system_timestamp *sts)
 {
 	struct ice_pf *pf = ptp_info_to_pf(info);
-	struct ice_hw *hw = &pf->hw;
-
-	if (!ice_ptp_lock(hw)) {
-		dev_err(ice_pf_to_dev(pf), "PTP failed to get time\n");
-		return -EBUSY;
-	}
 
 	ice_ptp_read_time(pf, ts, sts);
-	ice_ptp_unlock(hw);
 
 	return 0;
 }
diff --git a/drivers/net/ethernet/intel/ice/ice_ptp_hw.c b/drivers/net/ethernet/intel/ice/ice_ptp_hw.c
index 187ce9b54e1a..a47dbbfadb74 100644
--- a/drivers/net/ethernet/intel/ice/ice_ptp_hw.c
+++ b/drivers/net/ethernet/intel/ice/ice_ptp_hw.c
@@ -274,6 +274,9 @@ void ice_ptp_src_cmd(struct ice_hw *hw, enum ice_ptp_tmr_cmd cmd)
  */
 static void ice_ptp_exec_tmr_cmd(struct ice_hw *hw)
 {
+	struct ice_pf *pf = container_of(hw, struct ice_pf, hw);
+
+	guard(spinlock_irqsave)(&pf->adapter->ptp_gltsyn_time_lock);
 	wr32(hw, GLTSYN_CMD_SYNC, SYNC_EXEC_CMD);
 	ice_flush(hw);
 }
-- 
2.43.2