Reading CPU temperature on Beaglebone Black

Question

I have a Beaglebone Black running Ubuntu 14.04. I flashed it using a guide to flashing a pre-built image to eMMC.

From the guides I've seen, I should be able to read /sys/class/hwmon/hwmon0/device/temp1_input to get the CPU temperature. But, I have no such path on my system; I'm guessing a kernel module for the Beaglebone Black's AM3358 ARM CPU provides this functionality and that my image lacks such a module, but I don't know where to start to get such a module. In fact, /sys/class/hwmon is an empty directory.

These are the modules that I do have:

$ lsmod | sort
6lowpan_iphc           10154  1 bluetooth
bluetooth             316797  10 bnep,rfcomm
bnep                   11946  2
can_dev                 7430  1 c_can
c_can                   9400  1 c_can_platform
c_can_platform          5927  0
g_multi                 3238  0
iptable_filter          1149  1
ip_tables              11857  1 iptable_filter
libcomposite           38715  5 usb_f_acm,usb_f_ecm,usb_f_rndis,g_multi,usb_f_mass_storage
musb_am335x             1075  0
musb_dsps               8369  0
musb_hdrc              76236  1 musb_dsps
rfcomm                 46571  0
rfkill                 14659  2 bluetooth
u_ether                 9524  3 usb_f_ecm,usb_f_rndis,g_multi
usb_f_acm               5687  1
usb_f_ecm               7901  1
usb_f_mass_storage     34664  2 g_multi
usb_f_rndis            17711  2 g_multi
u_serial                9631  1 usb_f_acm
x_tables               12575  3 ip_tables,xt_multiport,iptable_filter
xt_multiport            2051  1

...and overall status:

$ uname -a
Linux hostname 3.14.26-ti-r43 #1 SMP PREEMPT Wed Dec 24 05:27:12 UTC 2014 armv7l armv7l armv7l GNU/Linux
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 14.04.1 LTS
Release:        14.04
Codename:       trusty

So, how can I read the CPU temperature on a Beaglebone Black that is missing this file?

Answer 1

I've done my bit of searching the interwebs and have come up empty handed.

What I have found sofar is that the current Debian (I'm using Linux bbone 4.1.12-ti-r29 #1 SMP PREEMPT Mon Nov 9 22:46:19 UTC 2015 armv7l GNU/Linux) and probably all derivatives of it AND probably also recent predecessors of it (at least as far back as your kernel 3.14.26) no longer populate /sys/class/hwmon.

Also lm-sensors doesn't find any sensors on the board, whatsoever, so that's a dead-end aswell.

And what I gather from TI, is that the temperature sensor was never really meant to be used by the end-user. TI even prescribes a method of measuring the board temperature via an external sensor (see here).

So, that's a bummer, but there you have it.

EDIT: I've also found that Debian 8.2 shows the behaviour I described here, however, today I flashed Debian 7.9 (from here; Linux bbone 3.8.13-bone79 #1 SMP Tue Oct 13 20:44:55 UTC 2015 armv7l GNU/Linux) and that comes with a fully populated /sys including /sys/class/hwmon/hwmon0/device/temp1_input which is a temperature reading in millidegreesC. Mind you, this is probably a relative temperature that shouldn't be relied upon (acc. TI).