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R9 390 (Hawaii/CIK) - Memory clock stuck at highest clockspeed

Brief summary of the problem:

The R9 390's memory clock is always stuck at the highest speed of 1500 MHz. It doesn't drop down to 150 MHz like it used to. There are no GPU intensive tasks running.

Hardware description:

  • CPU: Intel
  • GPU: XFX R9 390
  • System Memory: 16 GB
  • Type of Display Connection: DisplayPort

System information:

  • Distro name and Version: Arch Linux
  • Kernel version: 5.10-rc1
  • AMD package version: No package

How to reproduce the issue:

  • Boot 5.10-rc1 kernel, and observe memory clock

In the Linux 5.9 kernel, this bug isn't present. I have bisected and discovered that the commit that introduces the problem is 8d89b96fe79713fc5fc2b8d9c9b9f93842b55695.

The R9 390 memory width is 512 bits, and the commit mentioned above doesn't take appropriate action for that particular case.