Re: [PATCH net-next v3 2/7] dma: avoid redundant calls for sync operations

[Robin Murphy]

On 15/02/2024 5:08 am, Christoph Hellwig wrote:
> On Wed, Feb 14, 2024 at 05:55:23PM +0000, Robin Murphy wrote:
> > >    #define DMA_F_PCI_P2PDMA_SUPPORTED     (1 << 0)
> > > +#define DMA_F_CAN_SKIP_SYNC		BIT(1)
> >
> > Yuck, please be consistent - either match the style of the existing code,
> > or change that to BIT(0) as well.
>
> Just don't use BIT() ever.  It doesn't save any typing and creates a
> totally pointless mental indirection.
>
> > I guess this was the existing condition from dma_need_sync(), but now it's
> > on a one-off slow path it might be nice to check the sync_sg_* ops as well
> > for completeness, or at least comment that nobody should be implementing
> > those without also implementing the sync_single_* ops.
>
> Implementing only one and not the other doesn't make any sense.  Maybe
> a debug check for that is ok, but thing will break badly if they aren't
> in sync anyway.

In principle we *could* have an implementation which used bouncing 
purely to merge coherent scatterlist segments, thus didn't need to do 
anything for single mappings. I agree that it wouldn't seem like a 
particularly realistic thing to do these days, but I don't believe the 
API rules it out, so it might be nice to enforce that assumption 
somewhere if we are actually relying on it (although I also concur that 
this may not necessarily be the ideal place to do that in general).

Thanks,
Robin.