Re: [PATCH RFC 3/4] hwmon: Add driver for Raspberry Pi RP1 fan tachometer

[Andrea della Porta]

Hi Uwe,

On 17:32 Tue 27 Oct     , Uwe Kleine-König wrote:
> Hello,
> 
> On Fri, Oct 23, 2026 at 12:15:56PM +0200, Andrea della Porta wrote:
> > Add a hardware monitoring driver to support the fan tachometer on the
> > Raspberry Pi RP1 southbridge, found on the Raspberry Pi 5.
> > 
> > The RP1 PWM controller block contains specific MMIO registers that hold
> > the current fan speed in RPM. These registers are periodically updated by
> > the RP1 internal firmware based on the tachometer pulses.
> > 
> > Since the Linux PWM subsystem does not currently offer a framework for
> > exposing integrated hardware tachometer readings, this driver is
> > implemented as a standalone hwmon device. It parses the
> > 'raspberrypi,pwm-tachometer' DT property to find the supplier PWM device,
> > establishes a device link to ensure proper lifecycle management, and uses
> > RP1-specific PWM functions to retrieve the RPM values.
> 
> I wonder if the tachometer readout should be a counter driver? That
> would allow this driver to not be specific to the RP1 hardware.

I suspect the counter driver won't work with userspace daemons (e.g.
fancontrol) which use hwmon as an interface.
OTOH this driver is indeed specific to the RP1 driver: the fan rpm are
read from the same registers range and hw of teh PWM controller.

Regards,
Andrea 

> 
> Best regards
> Uwe