Re: [PATCH v2 4/6] iommu: Add granule_ignore when tlb gather

Robin Murphy
2020-11-25T16:38:56Z

On 2020-11-19 06:18, Yong Wu wrote:
> Add a granule_ignore option when tlb gather for some HW which don't care
> about granule when it flush tlb.
>
> Signed-off-by: Yong Wu <yong.wu@mediatek.com>
> ---
> include/linux/iommu.h | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 794d4085edd3..1aad32238510 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -171,6 +171,7 @@ enum iommu_dev_features {
> * @start: IOVA representing the start of the range to be flushed
> * @end: IOVA representing the end of the range to be flushed (exclusive)
> * @pgsize: The interval at which to perform the flush
> + * @granule_ignore: For tlb flushing that could be regardless of granule.
> *
> * This structure is intended to be updated by multiple calls to the
> * ->unmap() function in struct iommu_ops before eventually being passed
> @@ -180,6 +181,7 @@ struct iommu_iotlb_gather {
> unsigned long start;
> unsigned long end;
> size_t pgsize;
> + bool granule_ignore;

I can't see that this would ever need to vary on a per-unmap-operation
basis, so this doesn't seem like the right level of abstraction. AFAICS
it should simply be hard-coded in the driver logic.

> };
>
> /**
> @@ -544,7 +546,7 @@ static inline void iommu_iotlb_gather_add_page(struct iommu_domain *domain,
> * a different granularity, then sync the TLB so that the gather
> * structure can be rewritten.
> */
> - if (gather->pgsize != size ||
> + if ((!gather->granule_ignore && gather->pgsize != size) ||

I also think this is a slippery slope in the wrong direction anyway -
there is likely to be a fair bit of hardware-dependent variation around
how low-level TLB maintenance works (also consider drivers that may want
to convert a sufficiently large range to an "invalidate all" operation),
so if a generic helper function doesn't do the right thing for a given
driver, that driver should simply not use the helper, and directly
implement the logic it does need.

Robin.

> end < gather->start || start > gather->end) {
> if (gather->pgsize)
> iommu_iotlb_sync(domain, gather);
>