Re: MIPS 20Kc and WAIT instruction

[Ralf Baechle <ralf@linux-mips.org>]

On Mon, Jun 11, 2007 at 11:54:07AM +0200, Aurelien Jarno wrote:

> The latest kernels from kernel.org or linux-mips.org do not enter low
> power mode when the CPU is idle on a MIPS 20Kc CPU.
> 
> Looking at the code in arch/mips/kernel/cpu-probe.c, the "case"
> corresponding to the 20Kc CPU is commented out:
> 
>         case CPU_5KC:
> /*      case CPU_20KC:*/
>         case CPU_24K:
> 
> According to the datasheet this CPU supports the WAIT instruction, so I
> don't really understand why it is disabled in the kernel. Does anybody
> know why?

CPU_20KC is MIPS64 so by definition supports a WAIT instruction - even
though an implementation would be legal as per architecture spec.

As for why it's commented out, I don't know.  The original patch to add
support was submitted by Carsten Langgaard (ex-carstenl@mips.com) in 2002
and already had the CPU_20KC commented out.  As for why one would do
something like that - my guess is as good as your's :-)  Trying to find
out more.

  Ralf