[PATCH RFC 0/4] hwmon: Add fan tachometer support for Raspberry Pi RP1

[Andrea della Porta]

Hi,

This patch series adds fan tachometer / RPM monitoring support for the
Raspberry Pi RP1 southbridge found on the Raspberry Pi 5.
Please note that the patchset in [1] is a prerequisite for this one.

On the Raspberry Pi RP1, the active cooling fan speed is controlled via
the RP1 PWM controller. In addition to generating PWM signals, the RP1
monitors fan tachometer pulses. The pulse measurement and RPM calculation
are performed autonomously by the RP1's onboard Cortex-M microcontroller
core, which periodically writes the computed RPM value directly into
dedicated MMIO registers located within the RP1 PWM hardware block.

During earlier discussions, Uwe Kleine-König explicitly indicated a
preference to keep the core PWM controller driver (drivers/pwm/pwm-rp1.c)
focused strictly on PWM duties, rather than bloating it by registering an
hwmon device directly from within the PWM driver (see [2]).

Because the Linux PWM subsystem currently lacks a generic framework or
consumer API for reporting hardware-calculated tachometer readings, this
series implements a standalone hwmon driver (rp1-fan) using the following
structure:

-  The PWM driver exposes and exports rp1_pwm_read_tachometer() within a
   dedicated symbol namespace ('RP1_PWM') to read the RPM value from
   the MMIO block.
-  A companion DT binding (raspberrypi,rp1-fan.yaml) introduces a vendor
   property, 'raspberrypi,pwm-tachometer', to link the fan node to the
   PWM controller node without overloading or abusing the standard 'pwms'
   property.
-  The rp1-fan hwmon driver parses this phandle, establishes a device link
   for proper runtime and unbind dependency management, and registers the
   standard hwmon 'fan1_input' sysfs attribute.

An alternative architectural approach to the DT phandle + exported helper
method would be to model the tachometer as a child device using the
auxiliary bus framework (drivers/base/auxiliary.c). In that model, the RP1
PWM driver would instantiate an auxiliary device for the tachometer, and
the hwmon driver would bind to that auxiliary bus device without requiring
a separate DT node.

However, adopting the auxiliary bus would require a more significant
rework and refactoring of the existing RP1 PWM driver. 

I'd appreciate any feedback on whether the current companion driver with
a dedicated DT property is acceptable, or if an auxiliary bus-based
refactoring of the PWM driver is the preferred upstream path.

Patch Breakdown
---------------
Patch 1: pwm: rp1: Export helper to read fan tachometer RPM
Patch 2: dt-bindings: hwmon: Add Raspberry Pi RP1 fan binding
Patch 3: hwmon: Add driver for Raspberry Pi RP1 fan tachometer
Patch 4: arm64: dts: broadcom: bcm2712-rpi-5-b: Add RP1 fan tachometer node

Thanks,
Andrea


LINKS:
[1] - https://lore.kernel.org/all/cover.1790686331.git.andrea.porta@xxxxxxxx/
[2] - https://lore.kernel.org/all/adLTwOTbkJ0VQXy6@monoceros/

Andrea della Porta (4):
  pwm: rp1: Export helper to read fan tachometer RPM
  dt-bindings: hwmon: Add Raspberry Pi RP1 fan binding
  hwmon: Add driver for Raspberry Pi RP1 fan tachometer
  arm64: dts: broadcom: bcm2712-rpi-5-b: Add RP1 fan tachometer node

 .../bindings/hwmon/raspberrypi,rp1-fan.yaml   |  39 +++++
 Documentation/hwmon/index.rst                 |   1 +
 Documentation/hwmon/rp1-fan.rst               |  41 ++++++
 MAINTAINERS                                   |   1 +
 .../boot/dts/broadcom/bcm2712-rpi-5-b.dts     |   5 +
 drivers/hwmon/Kconfig                         |  11 ++
 drivers/hwmon/Makefile                        |   1 +
 drivers/hwmon/rp1-hwmon.c                     | 136 ++++++++++++++++++
 drivers/pwm/pwm-rp1.c                         |  27 ++++
 include/soc/bcm2835/raspberrypi-pwm-rp1.h     |  10 ++
 10 files changed, 272 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/hwmon/raspberrypi,rp1-fan.yaml
 create mode 100644 Documentation/hwmon/rp1-fan.rst
 create mode 100644 drivers/hwmon/rp1-hwmon.c
 create mode 100644 include/soc/bcm2835/raspberrypi-pwm-rp1.h

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2.35.3