Beaglebone Black PRU with linux-4.19.81

[Hugo van den Brand]

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 (https://forums.xilinx.com/t5/Embedded-Linux/Unhandled-fault-external-abort-on-non-linefetch-0x1018-at/td-p/904286)

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,

Hugo

[Hugo van den Brand]

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 8.1.13.2.1 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,

Hugo

I did not deassert the reset line.

RM_PER_RSTCTRL