[PATCH v2 09/10] drm/xe/hw_engine: document top-down paging requirement

[Matthew Auld <matthew.auld@intel.com>]

We were doing this anyway, but going forward for paging engines,
agreement is to always reserve BCS instances in top down fashion. This
hopefully future proofs things for VFs, where in some low-level places
it might only have the physical BCS instance from hw pov. If we stick to
a consistent mapping scheme, it should make it possible to determine if
this a special paging engine, or not.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
---
 drivers/gpu/drm/xe/xe_hw_engine.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/xe/xe_hw_engine.c b/drivers/gpu/drm/xe/xe_hw_engine.c
index c279a7dc62e2..5583fd98b08d 100644
--- a/drivers/gpu/drm/xe/xe_hw_engine.c
+++ b/drivers/gpu/drm/xe/xe_hw_engine.c
@@ -665,7 +665,6 @@ static void hw_engine_setup_logical_and_paging_mapping(struct xe_gt *gt)
 		if (hwe->class == XE_ENGINE_CLASS_COPY)
 			num_copy_engines++;
 
-	/* We just reserve the highest BCS instance for USM */
 	if (num_copy_engines && xe->info.has_usm)
 		num_paging_engines = 1;
 
@@ -690,6 +689,16 @@ static void hw_engine_setup_logical_and_paging_mapping(struct xe_gt *gt)
 	xe_gt_assert(gt, !(num_copy_engines || num_paging_engines) ||
 		     (num_paging_engines < num_copy_engines));
 
+	/*
+	 * On PF, we just reserve the highest BCS instance for USM.
+	 *
+	 * Note: This is now a requirement going forward. The PF must ALWAYS
+	 * reserve BCS instances in top down order, that way the VF has a chance
+	 * of discovering the physical BCS instance mappings for paging engines,
+	 * in conjunction with vf_num_paging_engines. In some places we might
+	 * only have the physical instance, and from hw pov there is no such
+	 * thing as a paging engine.
+	 */
 	reserved_logical_bcs_start = num_copy_engines - num_paging_engines;
 
 	/* FIXME: Doing a simple logical mapping that works for most hardware */
-- 
2.54.0