Re: [PATCH v2 7/9] powerpc/powernv: Add platform support for stop instruction

Paul Mackerras
2016-05-20T05:25:14Z

On Tue, May 03, 2016 at 01:54:36PM +0530, Shreyas B. Prabhu wrote:
> POWER ISA v3 defines a new idle processor core mechanism. In summary,
> a) new instruction named stop is added. This instruction replaces
> instructions like nap, sleep, rvwinkle.
> b) new per thread SPR named PSSCR is added which controls the behavior
> of stop instruction.
>
> PSSCR has following key fields
> Bits 0:3 - Power-Saving Level Status. This field indicates the lowest
> power-saving state the thread entered since stop instruction was last
> executed.
>
> Bit 42 - Enable State Loss
> 0 - No state is lost irrespective of other fields
> 1 - Allows state loss
>
> Bits 44:47 - Power-Saving Level Limit
> This limits the power-saving level that can be entered into.
>
> Bits 60:63 - Requested Level
> Used to specify which power-saving level must be entered on executing
> stop instruction
>
> This patch adds support for stop instruction and PSSCR handling.

I notice that you have duplicated a whole lot of assembly code
relating to synchronizing between threads going into and out of
power-saving modes, saving/restoring SPRs, resyncing the timebase, and
so on.

Two questions arise:

- Are we really going to have to do all of that in the same way for
POWER9 as we did for POWER8? You even copied over a comment about
the fastsleep workaround, which I really hope we won't have to do on
POWER9. Also, on POWER9, the threads are much more independent, so
I was not expecting that there would still be shared registers.

- If we do have to do all that, could we use the same code as on
POWER8 rather than having another copy of all that code?

Paul.