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How to read back the fan speed?

Question asked by Allen Vandiver on Mar 7, 2019
Latest reply on Mar 8, 2019 by Allen Vandiver

I am on a custom board using an i.MX6. I am using Yocto (Pyro) to build my kernel (4.14.16).

I am using the generic imx6qdl.dtsi device tree entry for PWM2 to drive the fan and it appears to work fine. The fan has a Tachometer input, which is connected to GPIO2_7. How do I read the fan speed? I have seen device tree blobs for cooling devices, but none of the examples I found have a tachometer input to monitor the fan's speed.