[PATCH v2] cxl/region: Fix bad logic for finding a free switch cxl decoder

[Zijun Hu]

From: Zijun Hu <quic_zijuhu@xxxxxxxxxxx>

It is bad for current match_free_decoder()'s logic to find a free switch
cxl decoder as explained below:

 - If all child decoders are sorted by ID in ascending order, then
   current logic can be simplified as below one:

   static int match_free_decoder(struct device *dev, void *data)
   {
	struct cxl_decoder *cxld;

	if (!is_switch_decoder(dev))
		return 0;

	cxld = to_cxl_decoder(dev);
	return cxld->region ? 0 : 1;
   }
   dev = device_find_child(&port->dev, NULL, match_free_decoder);

   which does not also need to modify device_find_child()'s match data.

 - If all child decoders are NOT sorted by ID in ascending order, then
   current logic are wrong as explained below:

   F: free, (cxld->region == NULL)    B: busy, (cxld->region != NULL)
   S(n)F : State of switch cxl_decoder with ID n is Free
   S(n)B : State of switch cxl_decoder with ID n is Busy

   Provided there are 2 child decoders: S(1)F -> S(0)B, then current logic
   will fail to find a free decoder even if there are a free one with ID 1

Anyway, current logic is not good, fixed by finding a free switch cxl
decoder with minimal ID.

Fixes: 384e624bb211 ("cxl/region: Attach endpoint decoders")
Closes: https://lore.kernel.org/all/cdfc6f98-1aa0-4cb5-bd7d-93256552c39b@xxxxxxxxxx/
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Zijun Hu <quic_zijuhu@xxxxxxxxxxx>
---
Changes in v2:
- Correct title and commit message
- Link to v1: https://lore.kernel.org/r/20240903-fix_cxld-v1-1-61acba7198ae@xxxxxxxxxxx
---
 drivers/cxl/core/region.c | 27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index 21ad5f242875..b9607b4fc40b 100644
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -797,21 +797,26 @@ static size_t show_targetN(struct cxl_region *cxlr, char *buf, int pos)
 static int match_free_decoder(struct device *dev, void *data)
 {
 	struct cxl_decoder *cxld;
-	int *id = data;
+	struct cxl_decoder *target_cxld;
+	struct device **target_device = data;
 
 	if (!is_switch_decoder(dev))
 		return 0;
 
 	cxld = to_cxl_decoder(dev);
-
-	/* enforce ordered allocation */
-	if (cxld->id != *id)
+	if (cxld->region)
 		return 0;
 
-	if (!cxld->region)
-		return 1;
-
-	(*id)++;
+	if (!*target_device) {
+		*target_device = get_device(dev);
+		return 0;
+	}
+	/* enforce ordered allocation */
+	target_cxld = to_cxl_decoder(*target_device);
+	if (cxld->id < target_cxld->id) {
+		put_device(*target_device);
+		*target_device = get_device(dev);
+	}
 
 	return 0;
 }
@@ -839,8 +844,7 @@ cxl_region_find_decoder(struct cxl_port *port,
 			struct cxl_endpoint_decoder *cxled,
 			struct cxl_region *cxlr)
 {
-	struct device *dev;
-	int id = 0;
+	struct device *dev = NULL;
 
 	if (port == cxled_to_port(cxled))
 		return &cxled->cxld;
@@ -849,7 +853,8 @@ cxl_region_find_decoder(struct cxl_port *port,
 		dev = device_find_child(&port->dev, &cxlr->params,
 					match_auto_decoder);
 	else
-		dev = device_find_child(&port->dev, &id, match_free_decoder);
+		/* Need to put_device(@dev) after use */
+		device_for_each_child(&port->dev, &dev, match_free_decoder);
 	if (!dev)
 		return NULL;
 	/*

---
base-commit: 67784a74e258a467225f0e68335df77acd67b7ab
change-id: 20240903-fix_cxld-4f6575a90619

Best regards,
-- 
Zijun Hu <quic_zijuhu@xxxxxxxxxxx>