[PATCH v2] powerpc: Remove eieio() in PowerPC IO functions

[Segher Boessenkool]

On Wed, Jan 29, 2025 at 10:45:10AM +0100, Julian Vetter wrote:
> Remove the eieio() calls in IO functions for PowerPC. While other
> architectures permit prefetching, combining, and reordering, the eieio()
> calls on PowerPC prevent such optimizations.

Yes, and it is crucial to prevent combining, it is part of the semantics
of these functions.  This is a much bigger problem on PowerPC than on
architectures which optimise memory accesses much less.  So most other
archs can get away with it much easier (but it is still completely wrong
there).

You are keeping the trap;isync things, which a) have a way bigger
performance impact, and b) are merely a debugging aid (if some i/o
access kills the system, it will be clear where that came from).  And
that isn't even the biggest thing of course, there is a heavyweight
sync in there as well.  Is there any benefit to this patch, or is it
only sabotage?


Segher