Re: [PATCH v3 03/13] iommu/dma: Force bouncing of the size is not cacheline-aligned

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Nov 07, 2022 at 01:26:21PM +0000, Robin Murphy wrote:
> On 2022-11-07 10:54, Catalin Marinas wrote:
> > On Mon, Nov 07, 2022 at 10:46:03AM +0100, Christoph Hellwig wrote:
> > > > +static inline bool dma_sg_kmalloc_needs_bounce(struct device *dev,
> > > > +					       struct scatterlist *sg, int nents,
> > > > +					       enum dma_data_direction dir)
> > > > +{
> > > > +	struct scatterlist *s;
> > > > +	int i;
> > > > +
> > > > +	if (!IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) ||
> > > > +	    dir == DMA_TO_DEVICE || dev_is_dma_coherent(dev))
> > > > +		return false;
> > > 
> > > This part should be shared with dma-direct in a well documented helper.
> > > 
> > > > +	for_each_sg(sg, s, nents, i) {
> > > > +		if (dma_kmalloc_needs_bounce(dev, s->length, dir))
> > > > +			return true;
> > > > +	}
> > > 
> > > And for this loop iteration I'd much prefer it to be out of line, and
> > > also not available in a global helper.
> > > 
> > > But maybe someone can come up with a nice tweak to the dma-iommu
> > > code to not require the extra sglist walk anyway.
> > 
> > An idea: we could add another member to struct scatterlist to track the
> > bounced address. We can then do the bouncing in a similar way to
> > iommu_dma_map_sg_swiotlb() but without the iova allocation. The latter
> > would be a common path for both the bounced and non-bounced cases.
> 
> FWIW I spent a little time looking at this as well; I'm pretty confident
> it can be done without the extra walk if the iommu-dma bouncing is
> completely refactored (and it might want a SWIOTLB helper to retrieve
> the original page from a bounced address).

Doesn't sg_page() provide the original page already? Either way, the
swiotlb knows it as it needs to do the copying between buffers.

> That's going to be a bigger
> job than I'll be able to finish this cycle, and I concluded that this
> in-between approach wouldn't be worth posting for its own sake, but as
> part of this series I think it's a reasonable compromise.

I'll drop my hack once you have something. Happy to carry it as part of
this series.

> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index 375a5e90d86a..87aaf8b5cdb4 100644
> --- a/include/linux/scatterlist.h
> +++ b/include/linux/scatterlist.h
> @@ -16,7 +16,7 @@ struct scatterlist {
>  #ifdef CONFIG_NEED_SG_DMA_LENGTH
>  	unsigned int	dma_length;
>  #endif
> -#ifdef CONFIG_PCI_P2PDMA
> +#ifdef CONFIG_NEED_SG_DMA_FLAGS
>  	unsigned int    dma_flags;
>  #endif

I initially had something similar but I decided it's overkill for a
patch that I expected to be NAK'ed.

I'll include your patch in my series in the meantime.

-- 
Catalin