[PATCH v1 net-next 2/3] net: ena: PHC silent reset

[David Arinzon <darinzon@amazon.com>]

Each PHC device kernel registration receives a unique kernel index,
which is associated with a new PHC device file located at
"/dev/ptp<index>".
This device file serves as an interface for obtaining PHC timestamps.
Examples of tools that use "/dev/ptp" include testptp [1]
and chrony [2].

A reset flow may occur in the ENA driver while PHC is active.
During a reset, the driver will unregister and then re-register the
PHC device with the kernel.
Under race conditions, particularly during heavy PHC loads,
the driver’s reset flow might complete faster than the kernel’s PHC
unregister/register process.
This can result in the PHC index being different from what it was prior
to the reset, as the PHC index is selected using kernel ID
allocation [3].

While driver rmmod/insmod are done by the user, a reset may occur
at anytime, without the user awareness, consequently, the driver
might receive a new PHC index after the reset, potentially compromising
the user experience.

To prevent this issue, the PHC flow will detect the reset during PHC
destruction and will skip the PHC unregister/register calls to preserve
the kernel PHC index.
During the reset flow, any attempt to get a PHC timestamp will fail as
expected, but the kernel PHC index will remain unchanged.

[1]: https://github.com/torvalds/linux/blob/v6.1/tools/testing/selftests/ptp/testptp.c
[2]: https://github.com/mlichvar/chrony
[3]: https://www.kernel.org/doc/html/latest/core-api/idr.html

Signed-off-by: Amit Bernstein <amitbern@amazon.com>
Signed-off-by: David Arinzon <darinzon@amazon.com>
---
 drivers/net/ethernet/amazon/ena/ena_phc.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/amazon/ena/ena_phc.c b/drivers/net/ethernet/amazon/ena/ena_phc.c
index 17c8bd0a..7e23c9c5 100644
--- a/drivers/net/ethernet/amazon/ena/ena_phc.c
+++ b/drivers/net/ethernet/amazon/ena/ena_phc.c
@@ -104,6 +104,10 @@ static int ena_phc_register(struct ena_adapter *adapter)
 	phc_info = adapter->phc_info;
 	clock_info = &phc_info->clock_info;
 
+	/* PHC may already be registered in case of a reset */
+	if (ena_phc_is_active(adapter))
+		return 0;
+
 	phc_info->adapter = adapter;
 
 	spin_lock_init(&phc_info->lock);
@@ -129,7 +133,8 @@ static void ena_phc_unregister(struct ena_adapter *adapter)
 {
 	struct ena_phc_info *phc_info = adapter->phc_info;
 
-	if (ena_phc_is_active(adapter)) {
+	/* During reset flow, PHC must stay registered to keep kernel's PHC index */
+	if (ena_phc_is_active(adapter) && !test_bit(ENA_FLAG_TRIGGER_RESET, &adapter->flags)) {
 		ptp_clock_unregister(phc_info->clock);
 		phc_info->clock = NULL;
 	}
-- 
2.40.1