[PATCH 2/3] hwmon: (coretemp) Read TjMax only when refreshing the temperature

[Ricardo Neri]

show_temp() reads TjMax before checking whether the cached temperature is
still fresh, but consumes it only when recomputing that temperature. On a
cache hit the value is discarded.

Discarding it is not free. On CPUs that report TjMax in
MSR_IA32_TEMPERATURE_TARGET, get_tjmax() returns the value it just read
without storing it in tdata->tjmax, so every call reads the MSR again.
That is a cross-CPU call whenever the CPU owning the sensor is not the
current one, and it is made while holding tdata->update_lock.

Read TjMax where it is used. The value cannot go stale in the meantime:
bits 23:16 of MSR_IA32_TEMPERATURE_TARGET are read-only, and the register
is package-scoped, so every CPU of the package reads the same value.

The temperature reported to userspace does not change.

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
 drivers/hwmon/coretemp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hwmon/coretemp.c b/drivers/hwmon/coretemp.c
index ace51e08e72d..0ab6bbff5637 100644
--- a/drivers/hwmon/coretemp.c
+++ b/drivers/hwmon/coretemp.c
@@ -402,9 +402,9 @@ static ssize_t show_temp(struct device *dev,
 
 	mutex_lock(&tdata->update_lock);
 
-	tjmax = get_tjmax(tdata, dev);
 	/* Check whether the time interval has elapsed */
 	if (time_after(jiffies, tdata->last_updated + HZ)) {
+		tjmax = get_tjmax(tdata, dev);
 		rdmsrq_on_cpu(tdata->cpu, tdata->status_reg, &val.q);
 		/*
 		 * Ignore the valid bit. In all observed cases the register

-- 
2.43.0