Re: [RFC PATCH v6 1/6] riscv: mm: dma-noncoherent: Switch using function pointers for cache management

[Christoph Hellwig]

On Sat, Jan 21, 2023 at 08:30:23PM +0100, Arnd Bergmann wrote:
> > That feels really odd, and might be worth a bug report to the
> > PPC maintainers.
> 
> Right, my first step would be to change all of the current
> outliers to use the same set of operations where possible.

Sounds good.

> > I'd rather avoid multiple callbacks if we can.  But maybe solve
> > the simple problem first and just pass the paddr and then
> > iterate from there.
> 
> Ok, fair enough. This means we can't easily put the kmap_atomic()
> into common code for highmem, though the per-page loop would
> still work.

Yes.  Given how messy many of the ops are I think one step at a time
is always good.

> I was thinking of using STATIC_CALL() as an optimization here, which
> I find easier to read and understand than alternatives. One advantage
> here is that this allows the actual cache operations to be declared
> locally in the architecture without letting drivers call them,
> but still update the common code to work without indirect branches.
> 
> The main downside is that this is currently only optimized on
> powerpc and x86, both of which don't actually need CPU specific
> callbacks. ARC, ARM, and MIPS on the other hand already
> have indirect function pointers, RISC-V would likely benefit the
> most from either alternatives or static_call, as it already
> uses alternatives and has one implementation that is clearly
> preferred over the others.

For now I'd just keep doing direct calls into the arch code, just
for the lower level invalidate, writeback, invalidate+writeback
calls as that helps cementinc the logic of which of those to use
in well documented core code.

And I'm not really sure I'd like to go beyond that - making it too
easy pluggable will make people feel more comfortable doing stupid
things here.  And yes, maybe that's personal because I've warned
the RISC-V people years ago that they'll need architectural
cache management instructions yesterday and the answer was that
no one is going to use them on modern CPUs.  *sigh*