git: 45f70f98fc55 - main - iommu_gas_map_region(): add comment explaining the ma array shift

The branch main has been updated by kib:

URL: https://cgit.FreeBSD.org/src/commit/?id=45f70f98fc55b753b1e7a7374b4a061aa1f27392

commit 45f70f98fc55b753b1e7a7374b4a061aa1f27392
Author:     Konstantin Belousov <kib@FreeBSD.org>
AuthorDate: 2025-01-25 09:37:50 +0000
Commit:     Konstantin Belousov <kib@FreeBSD.org>
CommitDate: 2025-04-01 22:23:17 +0000

    iommu_gas_map_region(): add comment explaining the ma array shift
    
    and the difference between start and entry->start values at this point.
    
    Sponsored by:   Advanced Micro Devices (AMD)
    Sponsored by:   The FreeBSD Foundation
    MFC after:      3 days
---
 sys/dev/iommu/iommu_gas.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/sys/dev/iommu/iommu_gas.c b/sys/dev/iommu/iommu_gas.c
index d97bdee47b28..ffa8dc096adc 100644
--- a/sys/dev/iommu/iommu_gas.c
+++ b/sys/dev/iommu/iommu_gas.c
@@ -875,6 +875,11 @@ iommu_gas_map_region(struct iommu_domain *domain, struct iommu_map_entry *entry,
 	if (entry->end == entry->start)
 		return (0);
 
+	/*
+	 * iommu_gas_alloc_region() might clipped the entry start and
+	 * end positions.  Adjust the beginning of the ma array to map
+	 * the pages at the requested relative positions.
+	 */
 	error = domain->ops->map(domain, entry,
 	    ma + OFF_TO_IDX(start - entry->start), eflags,
 	    ((flags & IOMMU_MF_CANWAIT) != 0 ? IOMMU_PGF_WAITOK : 0));