Re: [PATCH net-next 1/7] dma: compile-out DMA sync op calls when not used

[Robin Murphy]

On 2024-01-29 11:07 am, Alexander Lobakin wrote:
> From: Christoph Hellwig <hch@xxxxxx>
> Date: Mon, 29 Jan 2024 07:11:36 +0100
>
> > On Fri, Jan 26, 2024 at 02:54:50PM +0100, Alexander Lobakin wrote:
> > > Some platforms do have DMA, but DMA there is always direct and coherent.
> > > Currently, even on such platforms DMA sync operations are compiled and
> > > called.
> > > Add a new hidden Kconfig symbol, DMA_NEED_SYNC, and set it only when
> > > either sync operations are needed or there is DMA ops or swiotlb
> > > enabled. Set dma_need_sync() and dma_skip_sync() (stub for now)
> > > depending on this symbol state and don't call sync ops when
> > > dma_skip_sync() is true.
> > > The change allows for future optimizations of DMA sync calls depending
> > > on compile-time or runtime conditions.
> >
> > So the idea of compiling out the calls sounds fine to me.  But what
> > is the point of the extra indirection through the __-prefixed calls?
>
> Because dma_sync_* ops are external functions, not inlines, and in the
> next patch I'm adding a check there.
>
> > And if we need that (please document it in the commit log), please
> > make the wrappers proper inline functions and not macros.

In fact those wrappers could perhaps subsume the existing stub 
definitions, by starting with a refactor along these lines:

static inline dma_sync_x(...)
{
	if (IS_ENABLED(CONFIG_NEED_DMA_SYNC))
		__dma_sync_x(...);
}

Cheers,
Robin.