[PATCH V4] iommu/io-pgtable-arm: Optimise non-coherent unmap

[Will Deacon]

On Tue, Aug 06, 2024 at 10:51:35AM +0000, Ashish Mhetre wrote:
> The current __arm_lpae_unmap() function calls dma_sync() on individual
> PTEs after clearing them. Overall unmap performance can be improved by
> around 25% for large buffer sizes by combining the syncs for adjacent
> leaf entries.
> Optimize the unmap time by clearing all the leaf entries and issuing a
> single dma_sync() for them.
> Below is detailed analysis of average unmap latency(in us) with and
> without this optimization obtained by running dma_map_benchmark for
> different buffer sizes.
> 
> 		UnMap Latency(us)
> Size	Without		With		% gain with
> 	optimiztion	optimization	optimization
> 
> 4KB	3		3		0
> 8KB	4		3.8		5
> 16KB	6.1		5.4		11.48
> 32KB	10.2		8.5		16.67
> 64KB	18.5		14.9		19.46
> 128KB	35		27.5		21.43
> 256KB	67.5		52.2		22.67
> 512KB	127.9		97.2		24.00
> 1MB	248.6		187.4		24.62
> 2MB	65.5		65.5		0
> 4MB	119.2		119		0.17
> 
> Reviewed-by: Robin Murphy <robin.murphy at arm.com>
> Signed-off-by: Ashish Mhetre <amhetre at nvidia.com>
> ---
> Changes in V2:
> - Updated the commit message to be imperative.
> - Fixed ptep at incorrect index getting cleared for non-leaf entries.
> 
> Changes in V3:
> - Used loop-local variables and removed redundant function variables.
> - Added check for zero-sized dma_sync in __arm_lpae_clear_pte().
> - Merged both patches into this single patch by adding check for a
>   NULL gather in __arm_lpae_unmap() itself.
> 
> Changes in V4:
> - Updated the subject in commit message to correctly reflect the changes
>   made in this patch.
> ---
>  drivers/iommu/io-pgtable-arm.c | 31 +++++++++++++++++--------------
>  1 file changed, 17 insertions(+), 14 deletions(-)

Acked-by: Will Deacon <will at kernel.org>

Joerg, please can you pick this one up for -next?

Cheers,

Will