[PATCH v2] drm/xe/hwmon: document DG2 fan speed reporting quirk

[Zhan Wei <zhanwei919@gmail.com>]

The number of fanN_input attributes on DG2 is hardcoded to two because
FSC_READ_NUM_FANS returns an incorrect value on some boards. How the
physical fans map onto the tach channels is left to the board vendor:
some OEMs route multiple physical fans through a single shared tach
line, in which case the unwired channel's pulse counter never
accumulates and fanN_input reads a constant 0 RPM.

This is expected behaviour for such boards rather than a driver fault,
and the driver has no reliable way to distinguish a shared-tach layout
from a genuinely silent fan. Document this so the flat DG2 fan count is
not mistaken for a bug and "fixed" by lowering it, which would hide a
working fan2 on boards that do wire two tach lines.

Signed-off-by: Zhan Wei <zhanwei919@gmail.com>
---
v1 -> v2: Drop the code change. As pointed out in review, the same PCI
  device ID ships with both shared-tach (multiple physical fans on one
  channel) and 1:1 fan wiring, and FSC_READ_NUM_FANS is unreliable on
  some boards, so the DG2 fan count cannot be lowered without hiding a
  working fan2 on boards that do wire two tach lines. Document the
  behaviour instead of changing the reported fan count.

v1: https://lore.kernel.org/intel-xe/20260527115311.13398-1-zhanwei919@gmail.com/

 Documentation/gpu/xe/index.rst    |  1 +
 Documentation/gpu/xe/xe_hwmon.rst | 48 +++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+)
 create mode 100644 Documentation/gpu/xe/xe_hwmon.rst

diff --git a/Documentation/gpu/xe/index.rst b/Documentation/gpu/xe/index.rst
index 874ffcb6da3a..3c14cdcaa8a6 100644
--- a/Documentation/gpu/xe/index.rst
+++ b/Documentation/gpu/xe/index.rst
@@ -30,3 +30,4 @@ DG2, etc is provided to prototype the driver.
    xe-drm-usage-stats.rst
    xe_configfs
    xe_gt_stats
+   xe_hwmon
diff --git a/Documentation/gpu/xe/xe_hwmon.rst b/Documentation/gpu/xe/xe_hwmon.rst
new file mode 100644
index 000000000000..8cd48df59386
--- /dev/null
+++ b/Documentation/gpu/xe/xe_hwmon.rst
@@ -0,0 +1,48 @@
+.. SPDX-License-Identifier: (GPL-2.0+ OR MIT)
+
+=================
+Xe HWMON support
+=================
+
+The xe driver exposes hardware monitoring sensors (power, energy,
+temperature, voltage and fan speed) through the kernel hwmon subsystem,
+typically consumed via ``/sys/class/hwmon/hwmonX/`` or tools such as
+``sensors``.
+
+Fan speed reporting
+===================
+
+Fan speed (``fanN_input``) is reported in RPM and computed from a tach
+pulse counter: the driver reads an accumulating pulse register, divides
+the delta between two subsequent readings by two pulses per rotation,
+and time-averages the result.
+
+Number of fan channels
+-----------------------
+
+The number of ``fanN_input`` attributes exposed in sysfs is the fan
+count returned by the ``FSC_READ_NUM_FANS`` pcode command. On DG2 this
+command has been found to return an incorrect value on some boards, so
+the driver hardcodes a fan count of two there. As a result up to
+``fan1_input`` and ``fan2_input`` are always exposed on DG2 regardless
+of how many tach lines are actually wired.
+
+Zero RPM on DG2 is not necessarily a bug
+----------------------------------------
+
+How physical fans map onto the tach channels is left to the board
+vendor. Some OEMs route several physical fans through a single shared
+tach line, while others wire each fan to its own channel 1:1. The
+driver has no reliable way to tell these layouts apart, and the same PCI
+device ID can ship in either configuration.
+
+When a channel has no tach line driving it, its pulse counter never
+accumulates, so the corresponding ``fanN_input`` reads a constant 0 RPM.
+On DG2 this is most often seen on ``fan2_input`` for boards that drive
+both physical fans from a single tach line. This is expected behaviour
+for such boards, not a driver fault, and reflects the board wiring
+rather than a missing or stalled fan.
+
+For this reason the fan count on DG2 is intentionally left at a flat
+value rather than tracked per board: there is no driver-visible signal
+that distinguishes a shared-tach layout from a genuinely silent fan.
-- 
2.43.0