[RFC PATCH 1/1] cxl/edac: Fix the min_scrub_cycle of a region miscalculation

[Li Ming <ming.li@zohomail.com>]

When trying to update the scrub_cycle value of a cxl region, which means
updating the scrub_cycle value of each memdev under a cxl region. cxl
driver needs to guarantee the new scrub_cycle value is greater than the
min_scrub_cycle value of a memdev, otherwise the updating operation will
fail(Per Table 8-223 in CXL r3.2 section 8.2.10.9.11.1).

Current implementation logic of getting the min_scrub_cycle value of a
cxl region is that getting the min_scrub_cycle value of each memdevs
under the cxl region, then using the minimum min_scrub_cycle value as
the region's min_scrub_cycle. Checking if the new scrub_cycle value is
greater than this value. If yes, updating the new scrub_cycle value to
each memdevs. The issue is that the new scrub_cycle value is possibly
greater than the minimum min_scrub_cycle value of all memdevs but less
than the maximum min_scrub_cycle value of all memdevs if memdevs have
a different min_scrub_cycle value. The updating operation will always
fail on these memdevs which have a greater min_scrub_cycle than the new
scrub_cycle.

The correct implementation logic is to get the maximum value of these
memdevs' min_scrub_cycle, check if the new scrub_cycle value is greater
than the value. If yes, the new scrub_cycle value is fit for the region.

The change also impacts the result of
cxl_patrol_scrub_get_min_scrub_cycle(), the interface returned the
minimum min_scrub_cycle value among all memdevs under the region before
the change. The interface will return the maximum min_scrub_cycle value
among all memdevs under the region with the change.

Signed-off-by: Li Ming <ming.li@zohomail.com>
---
I made this change based on my understanding on the SPEC and current CXL
EDAC code, but I am not sure if it is a bug or it is designed this way.

base-commit: 9f153b7fb5ae45c7d426851f896487927f40e501 cxl/next
---
 drivers/cxl/core/edac.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/cxl/core/edac.c b/drivers/cxl/core/edac.c
index 2cbc664e5d62..ad243cfe00e7 100644
--- a/drivers/cxl/core/edac.c
+++ b/drivers/cxl/core/edac.c
@@ -103,10 +103,10 @@ static int cxl_scrub_get_attrbs(struct cxl_patrol_scrub_context *cxl_ps_ctx,
 				u8 *cap, u16 *cycle, u8 *flags, u8 *min_cycle)
 {
 	struct cxl_mailbox *cxl_mbox;
-	u8 min_scrub_cycle = U8_MAX;
 	struct cxl_region_params *p;
 	struct cxl_memdev *cxlmd;
 	struct cxl_region *cxlr;
+	u8 min_scrub_cycle = 0;
 	int i, ret;
 
 	if (!cxl_ps_ctx->cxlr) {
@@ -133,8 +133,12 @@ static int cxl_scrub_get_attrbs(struct cxl_patrol_scrub_context *cxl_ps_ctx,
 		if (ret)
 			return ret;
 
+		/*
+		 * The min_scrub_cycle of a region is the maximum value among
+		 * the min_scrub_cycle of all the memdevs under the region.
+		 */
 		if (min_cycle)
-			min_scrub_cycle = min(*min_cycle, min_scrub_cycle);
+			min_scrub_cycle = max(*min_cycle, min_scrub_cycle);
 	}
 
 	if (min_cycle)
-- 
2.34.1