hwmon: (coretemp) Add comments describing the handling of HT CPUs

The coretemp driver provides a single set of device attributes for each
physical core of a HT CPU to avoid duplicate sensors.  This
functionality was introduced with commit d883b9f09772 ("hwmon:
(coretemp) Skip duplicate CPU entries").

Commit e40cc4bdfd4b ("x86/hwmon: register alternate sibling upon CPU
removal") extends this functionality to register the HT sibling of a CPU
which is taken offline, to ensure that sensor attributes are provided if
at least one HT sibling of a core is online.

Add comments into the code describing the functionality in some more
detail.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 18 insertions, 2 deletions

diff --git a/drivers/hwmon/coretemp.c b/drivers/hwmon/coretemp.c
index a00245eb3fa0..9577c432e77f 100644
--- a/drivers/hwmon/coretemp.c
+++ b/drivers/hwmon/coretemp.c
@@ -506,7 +506,13 @@ static int create_core_data(struct platform_data *pdata,
         if (attr_no > MAX_CORE_DATA - 1)
                 return -ERANGE;
 
-        /* Skip if it is a HT core, Not an error */
+        /*
+         * Provide a single set of attributes for all HT siblings of a core
+         * to avoid duplicate sensors (the processor ID and core ID of all
+         * HT siblings of a core is the same).
+         * Skip if a HT sibling of this core is already online.
+         * This is not an error.
+         */
         if (pdata->core_data[attr_no] != NULL)
                 return 0;
 
@@ -763,10 +769,20 @@ static void __cpuinit put_core_offline(unsigned int cpu)
         if (pdata->core_data[indx] && pdata->core_data[indx]->cpu == cpu)
                 coretemp_remove_core(pdata, &pdev->dev, indx);
 
-        /* Online the HT version of this core, if any */
+        /*
+         * If a core is taken offline, but a HT sibling of the same core is
+         * still online, register the alternate sibling. This ensures that
+         * exactly one set of attributes is provided as long as at least one
+         * HT sibling of a core is online.
+         */
         for_each_sibling(i, cpu) {
                 if (i != cpu) {
                         get_core_online(i);
+                        /*
+                         * Display temperature sensor data for one HT sibling
+                         * per core only, so abort the loop after one such
+                         * sibling has been found.
+                         */
                         break;
                 }
         }