[PATCH v4] drm/xe/hwmon: document DG2 fan speed reporting quirk

[Zhan Wei <zhanwei919@gmail.com>]

On DG2 the driver always shows two fan channels, because the
FSC_READ_NUM_FANS command does not work on some cards. OEMs decide how
the fans map to tach channels, so two fans can share one tach line.
When that happens, the second channel reads 0 RPM even though the fan
is spinning.

Note this on the fan2_input ABI entry so the steady 0 RPM is not
mistaken for a driver bug.

Fixes: 28f79ac609de ("drm/xe/hwmon: expose fan speed")
Signed-off-by: Zhan Wei <zhanwei919@gmail.com>
Reviewed-by: Raag Jadav <raag.jadav@intel.com>
---
v4:
- Add Fixes: tag and collect Reviewed-by from Raag.
v3: https://lore.kernel.org/intel-xe/20260529172449.41504-1-zhanwei919@gmail.com/
- Drop the dedicated Documentation/gpu/xe/xe_hwmon.rst doc and the
  index.rst hunk; add a short note under the fan2_input entry in the
  existing ABI doc instead, per Raag's feedback.
v2: https://lore.kernel.org/intel-xe/20260529135028.20763-1-zhanwei919@gmail.com/
- Drop the code change that reported a single fan on DG2; document the
  shared-tach behaviour instead, per review feedback on v1.
v1: https://lore.kernel.org/intel-xe/20260527115311.13398-1-zhanwei919@gmail.com/
 Documentation/ABI/testing/sysfs-driver-intel-xe-hwmon | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-driver-intel-xe-hwmon b/Documentation/ABI/testing/sysfs-driver-intel-xe-hwmon
index 55ab45f669ac..0da739d9a816 100644
--- a/Documentation/ABI/testing/sysfs-driver-intel-xe-hwmon
+++ b/Documentation/ABI/testing/sysfs-driver-intel-xe-hwmon
@@ -251,6 +251,13 @@ Description:	RO. Fan 2 speed in RPM.
 
 		Only supported for particular Intel Xe graphics platforms.
 
+		On DG2 the driver always shows two fan channels, because the
+		FSC_READ_NUM_FANS command does not work on some cards. OEMs
+		decide how the fans map to tach channels, so two fans can share
+		one tach line. When that happens, the second channel
+		reads 0 RPM even though the fan is spinning. This is normal, not
+		a bug.
+
 What:		/sys/bus/pci/drivers/xe/.../hwmon/hwmon<i>/fan3_input
 Date:		March 2025
 KernelVersion:	6.16
-- 
2.43.0