Beaglebone Black PRU with linux-4.19.81

Dear all,

To make a system with a minimal footprint, I am compiling my own kernel for the BeagleBone black.
With this kernel I am trying to use the PRU using the uio-pruss (which memory maps the PRU memory and offers interrupts to userspace).

However, I cannot access the PRU memory.
I tried to use devmem (part of busybox) to read the address to which the PRU is mapped and I get back:


devmem 0x4a300000

[ 568.572896] Unhandled fault: external abort on non-linefetch (0x1018) at 0xb6fd5000
[ 568.580634] pgd = 17e21f9e
[ 568.583361] [b6fd5000] *pgd=9d4e4831, *pte=4a300183, *ppte=4a300a32
Bus error

What I found for this type of error is that it could be caused by not having the right clocks enabled (

When I boot with the default ubuntu image for the BeagleBone Black (4.14.108-ti-r113), I am able to read the PRU memory.

Therefore, I compared all clocks on the default ubuntu and adapted uio_pruss to enable all these clocks.
This did not solve the issue.

Anybody has some suggestions on what might be different between a linux-4.14.108 kernel and dtb and the default ubuntu image?

Best wishes,

Hi all,

I have finally figured out why it was not working.
I compared the beagleboard linux kernel with the kernel mainline.

This led to the pruss_soc_bus module.
When this module is loaded I could read and otherwise it would fail.

By looking at the code I figured out that in my case the PRUSS reset was never deasserted.
So I had to set RM_PER_RSTCTRL defined in section of the AM335x technical reference manual.
(The correct clock also wasn’t enabled, but that could have been my undoing.)

The solution confirms that I am still a newbie at this.
I post the solution anyway as it might help someone else in the future.

Best wishes,

I did not deassert the reset line.