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

[Raag Jadav]

On Fri, May 29, 2026 at 10:05:58PM +0800, 占wei wrote:
> +Cc Raag, who authored the fan support and reviewed v1.
> 
> Thanks for your help, this v2 drops the code change and documents the
> DG2 shared-tach behaviour instead, per your feedback on v1.

IMO it's a bit verbose to have a dedicated doc for this. Just add a small
comment in the existing ABI doc[1] under fan channel description.

[1] Documentation/ABI/testing/sysfs-driver-intel-xe-hwmon

Raag

> Zhan Wei <zhanwei919@xxxxxxxxx> 于2026年5月29日周五 21:50写道：
> >
> > 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@xxxxxxxxx>
> > ---
> > 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@xxxxxxxxx/
> >
> >  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
> >