Re: [PATCH 0/6] dma mapping/iommu: Allow IOMMU IOVA rcache range to be configured

[Christoph Hellwig]

On Fri, Mar 19, 2021 at 09:25:42PM +0800, John Garry wrote:
> For streaming DMA mappings involving an IOMMU and whose IOVA len regularly
> exceeds the IOVA rcache upper limit (meaning that they are not cached),
> performance can be reduced. 
> 
> This is much more pronounced from commit 4e89dce72521 ("iommu/iova: Retry
> from last rb tree node if iova search fails"), as discussed at [0].
> 
> IOVAs which cannot be cached are highly involved in the IOVA aging issue,
> as discussed at [1].

I'm confused.  If this a limit in the IOVA allocator, dma-iommu should
be able to just not grow the allocation so larger without help from
the driver.

If contrary to the above description it is device-specific, the driver
could simply use dma_get_max_seg_size().