Re: [PATCH v2 3/8] iommu/arm-smmu-v3: Optimize range invalidation for latency

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Jul 07, 2026 at 12:27:00AM -0700, Nicolin Chen wrote:
> On Mon, Jul 06, 2026 at 01:26:40PM -0300, Jason Gunthorpe wrote:
> > +	/*
> > +	 * There are at most 5 possible values for NUM based on SCALE. The
> 
> Mind elaborating the "at most 5 possible values for NUM"?

Oh, I guess it is at most 6 since it is +1

How about:

	/*
	 * The spec defines the invalidated range as:
	 *   Range = ((NUM+1) * 2^SCALE) * Translation_Granule_Size
	 * NUM is 5 bits, so (NUM+1) covers 1..32 granules and SCALE scales
	 * that by a power of two. We must pick NUM and SCALE so the range
	 * covers num_tg granules, i.e. (NUM+1) * 2^SCALE >= num_tg with
	 * NUM+1 <= 32. That constrains SCALE to:
	 *    ceil(num_tg / 2^SCALE) <= 32
	 *    SCALE >= ceil(log2(num_tg / 32))
	 * Choosing the smallest such SCALE gives the finest granularity and
	 * thus the tightest (least over-covering) range, and equivalently the
	 * largest NUM.
	 *
	 * Unlike other IOMMUs the spec has no alignment requirement on the
	 * address beyond alignment to tg (so long as TTL=0).
	 */

Jason