Re: [PATCH 09/11] swiotlb-xen: simplify cache maintainance

[Christoph Hellwig]

On Fri, Sep 06, 2019 at 09:52:12AM -0400, Boris Ostrovsky wrote:
> We need nop definitions of these two for x86.
> 
> Everything builds now but that's probably because the calls are under
> 'if (!dev_is_dma_coherent(dev))' which is always false so compiler
> optimized is out. I don't think we should rely on that.

That is how a lot of the kernel works.  Provide protypes only for code
that is semantically compiled, but can't ever be called due to
IS_ENABLED() checks.  It took me a while to get used to it, but it
actually is pretty nice as the linker does the work for you to check
that it really is never called.  Much better than say a BUILD_BUG_ON().