Dell Inspiron 3593, Linux Ubuntu - how to change backlight PWM frequency? intel_reg read 0xC8254 doesn't work

Question

I would like to increase screen backlight PWM frequency in Dell Inspiron 3593 with factory installed Ubuntu (this laptop, like many other Dell models, probably has only 200 Hz). I read that it can be done with software by installing Intel GPU tools and editing a proper register: link I installed these tools as described here: by running sudo apt-get update and sudo apt-get install intel-gpu-tools. However, when I run intel_reg read 0xC8254, I get:

Warning: register spec not found in '/usr/share/intel-gpu-tools/registers'. Using builtin register spec.
(intel_reg:9325) intel-mmio-CRITICAL: Test assertion failure function intel_mmio_use_pci_bar, file ../../lib/intel_mmio.c:145:
(intel_reg:9325) intel-mmio-CRITICAL: Failed assertion: !(error != 0)
(intel_reg:9325) intel-mmio-CRITICAL: Last errno: 13, Permission denied
(intel_reg:9325) intel-mmio-CRITICAL: Couldn't map MMIO region
Stack trace:
  #0 [_init+0x137c7]
  #1 [_init+0xe620]
  #2 [_init+0xe6d1]
  #3 [_init+0x25cf]
  #4 [_init+0xf7d]
  #5 [__libc_start_main+0xe7]
  #6 [_init+0x1002]
Test (null) failed.
**** DEBUG ****
(intel_reg:9325) intel-chipset-DEBUG: Test requirement passed: pci_dev
(intel_reg:9325) intel-mmio-CRITICAL: Test assertion failure function intel_mmio_use_pci_bar, file ../../lib/intel_mmio.c:145:
(intel_reg:9325) intel-mmio-CRITICAL: Failed assertion: !(error != 0)
(intel_reg:9325) intel-mmio-CRITICAL: Last errno: 13, Permission denied
(intel_reg:9325) intel-mmio-CRITICAL: Couldn't map MMIO region
(intel_reg:9325) igt-core-INFO: Stack trace:
(intel_reg:9325) igt-core-INFO:   #0 [_init+0x137c7]
(intel_reg:9325) igt-core-INFO:   #1 [_init+0xe620]
(intel_reg:9325) igt-core-INFO:   #2 [_init+0xe6d1]
(intel_reg:9325) igt-core-INFO:   #3 [_init+0x25cf]
(intel_reg:9325) igt-core-INFO:   #4 [_init+0xf7d]
(intel_reg:9325) igt-core-INFO:   #5 [__libc_start_main+0xe7]
(intel_reg:9325) igt-core-INFO:   #6 [_init+0x1002]
****  END  ****
FAIL (-1.000s)

Could you please tell me if it's possible in this laptop and what's the correct register?

Answer 1

Looks like there is lack of proper permissions to map the memory. Is this command running by privileged user?

intel_reg read 0xC8254

This error suggests that:

(intel_reg:9325) intel-mmio-CRITICAL: Last errno: 13, Permission denied

Also here is the output from my machine (different vendor and model, but still Intel GPU):

$ intel_reg_read 0xC8254
Test assertion failure function intel_mmio_use_pci_bar, file intel_mmio.c:145:
Failed assertion: !(error != 0)
Last errno: 13, Permission denied
Couldn't map MMIO region

And with root privileges:

$ sudo intel_reg_read 0xC8254
0xC8254 : 0x5EB2

Notice, the syntax of the command may be different as I am using different distribution, but I think this should help you with the investigation.

EDIT: To be more explicit I use the 2.99.917 version of the intel-gpu-tools, which is significantly newer then your 1.14. I think that the debuging tool is not aware of your newer (I assume) GPU.

I'll try to update my answer if this doesn't solve your problem.