[PATCH net-next v4 2/6] eth: fbnic: cache hwmon sensor readings

[Zinc Lim <limzhineng2@gmail.com>]

Each hwmon attribute access triggers its own TSENE firmware mailbox
round-trip, so reading the full set of attributes or polling them at a
high rate floods the firmware mailbox with quick, successive IPC messages
for data that barely changes between ticks.

Cache the last temperature and voltage reading and serve reads from it
for the remainder of the current jiffy. A single TSENE response carries
both readings, so one transaction on a miss refreshes both and satisfies
a whole batch of reads. The cache is seeded with the FBNIC_SENSOR_NO_DATA
sentinel in fbnic_mbx_init() so the first read always refreshes.

Signed-off-by: Zinc Lim <limzhineng2@gmail.com>
---
 drivers/net/ethernet/meta/fbnic/fbnic.h       |  7 +++++
 drivers/net/ethernet/meta/fbnic/fbnic_fw.c    |  7 +++++
 drivers/net/ethernet/meta/fbnic/fbnic_fw.h    |  7 +++++
 drivers/net/ethernet/meta/fbnic/fbnic_hwmon.c | 31 +++++++++++++------
 4 files changed, 42 insertions(+), 10 deletions(-)

diff --git a/drivers/net/ethernet/meta/fbnic/fbnic.h b/drivers/net/ethernet/meta/fbnic/fbnic.h
index 1f9fbd71447f..857a64b518f3 100644
--- a/drivers/net/ethernet/meta/fbnic/fbnic.h
+++ b/drivers/net/ethernet/meta/fbnic/fbnic.h
@@ -22,11 +22,18 @@ struct fbnic_napi_vector;
 #define FBNIC_MAX_NAPI_VECTORS		128u
 #define FBNIC_MBX_CMPL_SLOTS		4
 
+struct fbnic_hwmon_cache {
+	unsigned long last_read;
+	s32 temp_mdeg;
+	s32 volt_mv;
+};
+
 struct fbnic_dev {
 	struct device *dev;
 	struct net_device *netdev;
 	struct dentry *dbg_fbd;
 	struct device *hwmon;
+	struct fbnic_hwmon_cache hwmon_cache;
 	struct devlink_health_reporter *fw_reporter;
 	struct devlink_health_reporter *otp_reporter;
 
diff --git a/drivers/net/ethernet/meta/fbnic/fbnic_fw.c b/drivers/net/ethernet/meta/fbnic/fbnic_fw.c
index 49b8813a3564..82307f212f58 100644
--- a/drivers/net/ethernet/meta/fbnic/fbnic_fw.c
+++ b/drivers/net/ethernet/meta/fbnic/fbnic_fw.c
@@ -105,6 +105,13 @@ void fbnic_mbx_init(struct fbnic_dev *fbd)
 	/* Reset FW Capabilities */
 	memset(&fbd->fw_cap, 0, sizeof(fbd->fw_cap));
 
+	/* Seed cache with sentinel so the first read always refreshes. The
+	 * cache mirrors what firmware reported, so drop it when the mailbox
+	 * restarts.
+	 */
+	fbd->hwmon_cache.temp_mdeg = FBNIC_SENSOR_NO_DATA;
+	fbd->hwmon_cache.volt_mv = FBNIC_SENSOR_NO_DATA;
+
 	/* Reinitialize mailbox memory */
 	for (i = 0; i < FBNIC_IPC_MBX_INDICES; i++)
 		memset(&fbd->mbx[i], 0, sizeof(struct fbnic_fw_mbx));
diff --git a/drivers/net/ethernet/meta/fbnic/fbnic_fw.h b/drivers/net/ethernet/meta/fbnic/fbnic_fw.h
index f47669129c9d..ddedb7c0e0a7 100644
--- a/drivers/net/ethernet/meta/fbnic/fbnic_fw.h
+++ b/drivers/net/ethernet/meta/fbnic/fbnic_fw.h
@@ -6,6 +6,7 @@
 
 #include <linux/completion.h>
 #include <linux/if_ether.h>
+#include <linux/limits.h>
 #include <linux/types.h>
 
 struct fbnic_dev;
@@ -59,6 +60,12 @@ enum {
 	(BIT(FBNIC_FW_CAP_F_BMC_TCAM_REINIT) |	\
 	 BIT(FBNIC_FW_CAP_F_BMC_MACDA_SYNC))
 
+/* Sentinel for a sensor value the driver does not have: a threshold the
+ * firmware never populated (older firmware) or a cache entry not yet
+ * refreshed.
+ */
+#define FBNIC_SENSOR_NO_DATA			S32_MIN
+
 struct fbnic_fw_cap {
 	unsigned long state;
 	struct {
diff --git a/drivers/net/ethernet/meta/fbnic/fbnic_hwmon.c b/drivers/net/ethernet/meta/fbnic/fbnic_hwmon.c
index 6c8c66ab86c1..0c3fe2a37227 100644
--- a/drivers/net/ethernet/meta/fbnic/fbnic_hwmon.c
+++ b/drivers/net/ethernet/meta/fbnic/fbnic_hwmon.c
@@ -2,6 +2,7 @@
 /* Copyright (c) Meta Platforms, Inc. and affiliates. */
 
 #include <linux/hwmon.h>
+#include <linux/jiffies.h>
 
 #include "fbnic.h"
 #include "fbnic_mac.h"
@@ -30,26 +31,31 @@ static umode_t fbnic_hwmon_is_visible(const void *drvdata,
 
 static int fbnic_hwmon_sensor_read(struct fbnic_dev *fbd, int id, long *val)
 {
+	struct fbnic_hwmon_cache *cache = &fbd->hwmon_cache;
 	struct fbnic_fw_completion *fw_cmpl;
 	int err = 0;
-	s32 *sensor;
-
-	fw_cmpl = fbnic_fw_alloc_cmpl(FBNIC_TLV_MSG_ID_TSENE_READ_RESP);
-	if (!fw_cmpl)
-		return -ENOMEM;
+	s32 *cached;
 
 	switch (id) {
 	case FBNIC_SENSOR_TEMP:
-		sensor = &fw_cmpl->u.tsene.millidegrees;
+		cached = &cache->temp_mdeg;
 		break;
 	case FBNIC_SENSOR_VOLTAGE:
-		sensor = &fw_cmpl->u.tsene.millivolts;
+		cached = &cache->volt_mv;
 		break;
 	default:
-		err = -EINVAL;
-		goto exit_free;
+		return -EINVAL;
+	}
+
+	if (*cached != FBNIC_SENSOR_NO_DATA && cache->last_read == jiffies) {
+		*val = *cached;
+		return 0;
 	}
 
+	fw_cmpl = fbnic_fw_alloc_cmpl(FBNIC_TLV_MSG_ID_TSENE_READ_RESP);
+	if (!fw_cmpl)
+		return -ENOMEM;
+
 	err = fbnic_fw_xmit_tsene_read_msg(fbd, fw_cmpl);
 	if (err) {
 		dev_err(fbd->dev,
@@ -72,7 +78,12 @@ static int fbnic_hwmon_sensor_read(struct fbnic_dev *fbd, int id, long *val)
 		goto exit_cleanup;
 	}
 
-	*val = *sensor;
+	/* FW returns both readings in one response, cache both. */
+	cache->temp_mdeg = fw_cmpl->u.tsene.millidegrees;
+	cache->volt_mv = fw_cmpl->u.tsene.millivolts;
+	cache->last_read = jiffies;
+
+	*val = *cached;
 exit_cleanup:
 	fbnic_mbx_clear_cmpl(fbd, fw_cmpl);
 exit_free:
-- 
2.53.0-Meta