[PATCH RFC v6 06/18] fs/resctrl: Let bandwidth resources default to min_bw at reset

Bandwidth resources reset to max_bw on group creation today, which is
the right default for MBA and SMBA. However, it is the wrong default for
hardware whose registers form a sum-constrained reservation: defaulting
every new group to max_bw would immediately violate the sum on the first
mkdir.

When default_to_min is set, resctrl_get_default_ctrl() returns min_bw
for the resource. The existing MBA and SMBA behavior is not changed.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Drew Fustini <[email protected]>
---
 include/linux/resctrl.h | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
index 9529ed0d1fdf..bcbc166412ef 100644
--- a/include/linux/resctrl.h
+++ b/include/linux/resctrl.h
@@ -247,7 +247,13 @@ enum membw_throttle_mode {
 /**
  * struct resctrl_membw - Memory bandwidth allocation related data
  * @min_bw:		Minimum memory bandwidth percentage user can request
- * @max_bw:		Maximum memory bandwidth value, used as the reset value
+ * @max_bw:		Maximum memory bandwidth value a group can be
+ *			configured with
+ * @default_to_min:	When true, the default control value for new
+ *			groups and reset is @min_bw instead of @max_bw.
+ *			Drivers whose hardware enforces a sum constraint
+ *			across groups (e.g. CBQRI MB_MIN) set this so
+ *			mkdir does not overflow the sum.
  * @bw_gran:		Granularity at which the memory bandwidth is allocated
  * @delay_linear:	True if memory B/W delay is in linear scale
  * @arch_needs_linear:	True if we can't configure non-linear resources
@@ -259,6 +265,7 @@ enum membw_throttle_mode {
 struct resctrl_membw {
 	u32				min_bw;
 	u32				max_bw;
+	bool				default_to_min;
 	u32				bw_gran;
 	u32				delay_linear;
 	bool				arch_needs_linear;
@@ -405,7 +412,7 @@ static inline u32 resctrl_get_default_ctrl(struct rdt_resource *r)
 	case RESCTRL_SCHEMA_BITMAP:
 		return BIT_MASK(r->cache.cbm_len) - 1;
 	case RESCTRL_SCHEMA_RANGE:
-		return r->membw.max_bw;
+		return r->membw.default_to_min ? r->membw.min_bw : r->membw.max_bw;
 	}
 
 	return WARN_ON_ONCE(1);

-- 
2.43.0