Re: [PATCH 2/4] iommu/mediatek: Add iotlb_sync_range() support

[Robin Murphy <robin.murphy@arm.com>]

On 2020-10-19 12:30, Chao Hao wrote:
> MTK_IOMMU driver writes one page entry and does tlb flush at a time
> currently. More optimal would be to aggregate the writes and flush
> BUS buffer in the end.

That's exactly what iommu_iotlb_gather_add_page() is meant to achieve. 
Rather than jumping straight into hacking up a new API to go round the 
back of the existing API design, it would be far better to ask the 
question of why that's not behaving as expected.

> For 50MB buffer mapping, if mtk_iommu driver use iotlb_sync_range()
> instead of tlb_add_range() and tlb_flush_walk/leaf(), it can increase
> 50% performance or more(depending on size of every page size) in
> comparison to flushing after each page entry update. So we prefer to
> use iotlb_sync_range() to replace iotlb_sync(), tlb_add_range() and
> tlb_flush_walk/leaf() for MTK platforms.

In the case of mapping, it sounds like what you actually want to do is 
hook up .iotlb_sync_map and generally make IO_PGTABLE_QUIRK_TLBI_ON_MAP 
cleverer, because the current implementation is as dumb as it could 
possibly be. In fact if we simply passed an address range to 
.iotlb_sync_map, io-pgtable probably wouldn't need to be involved at all 
any more.

Robin.

> Signed-off-by: Chao Hao <chao.hao@mediatek.com>
> ---
>   drivers/iommu/mtk_iommu.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/iommu/mtk_iommu.c b/drivers/iommu/mtk_iommu.c
> index 785b228d39a6..d3400c15ff7b 100644
> --- a/drivers/iommu/mtk_iommu.c
> +++ b/drivers/iommu/mtk_iommu.c
> @@ -224,6 +224,11 @@ static void mtk_iommu_tlb_flush_range_sync(unsigned long iova, size_t size,
>   	}
>   }
>   
> +static void __mtk_iommu_tlb_flush_range_sync(unsigned long iova, size_t size)
> +{
> +	mtk_iommu_tlb_flush_range_sync(iova, size, 0, NULL)
> +}
> +
>   static void mtk_iommu_tlb_flush_page_nosync(struct iommu_iotlb_gather *gather,
>   					    unsigned long iova, size_t granule,
>   					    void *cookie)
> @@ -536,6 +541,7 @@ static const struct iommu_ops mtk_iommu_ops = {
>   	.map		= mtk_iommu_map,
>   	.unmap		= mtk_iommu_unmap,
>   	.flush_iotlb_all = mtk_iommu_flush_iotlb_all,
> +	.iotlb_sync_range = __mtk_iommu_tlb_flush_range_sync,
>   	.iotlb_sync	= mtk_iommu_iotlb_sync,
>   	.iova_to_phys	= mtk_iommu_iova_to_phys,
>   	.probe_device	= mtk_iommu_probe_device,
>