Re: [RFC PATCH v7 2/5] iommu/dma: Add a new dma_map_ops of get_merge_boundary()

[Christoph Hellwig <hch@lst.de>]

On Fri, Jun 21, 2019 at 09:59:21AM +0200, Marek Szyprowski wrote:
> Hi,
> 
> On 2019-06-20 10:50, Yoshihiro Shimoda wrote:
> > This patch adds a new dma_map_ops of get_merge_boundary() to
> > expose the DMA merge boundary if the domain type is IOMMU_DOMAIN_DMA.
> >
> > Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
> > ---
> >   drivers/iommu/dma-iommu.c | 11 +++++++++++
> >   1 file changed, 11 insertions(+)
> >
> > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> > index 205d694..9950cb5 100644
> > --- a/drivers/iommu/dma-iommu.c
> > +++ b/drivers/iommu/dma-iommu.c
> > @@ -1091,6 +1091,16 @@ static int iommu_dma_get_sgtable(struct device *dev, struct sg_table *sgt,
> >   	return ret;
> >   }
> >   
> > +static unsigned long iommu_dma_get_merge_boundary(struct device *dev)
> > +{
> > +	struct iommu_domain *domain = iommu_get_dma_domain(dev);
> > +
> > +	if (domain->type != IOMMU_DOMAIN_DMA)
> > +		return 0;	/* can't merge */
> > +
> > +	return (1 << __ffs(domain->pgsize_bitmap)) - 1;
> > +}
> 
> I really wonder if there is any IOMMU, which doesn't support 4KiB pages. 
> Cannot you simply assume that the merge boundary is 4KiB and avoid 
> adding this new API?

No idea if we have one, but I would not be surprised if one shows
up on a system only built to run with 64k pages for example.

Either way the abstraction seems light and self-explanatory, so I see
now reason not to have it even if we assume it would always return
4k, especially as we'd also still need a flag at the dma_map_ops level
to indicate if segement merging is supported at all.