Re: [PATCH v4 1/2] cxl/region: Find free cxl decoder by device_for_each_child()

[Dan Williams]

Dan Williams wrote:
[..]
> So, while regionB would be the next decoder to allocate after regionC is
> torn down, it is not a free decoder while decoderC and regionC have not been
> reclaimed.

The "simple" conversion is bug compatible with the current
implementation, but here's a path to both constify the
device_find_child() argument, *and* prevent unwanted allocations by
allocating decoders precisely by id.  Something like this, it passes a
quick unit test run:

diff --git a/drivers/cxl/core/port.c b/drivers/cxl/core/port.c
index 1d5007e3795a..749a281819b4 100644
--- a/drivers/cxl/core/port.c
+++ b/drivers/cxl/core/port.c
@@ -1750,7 +1750,8 @@ static int cxl_decoder_init(struct cxl_port *port, struct cxl_decoder *cxld)
 	struct device *dev;
 	int rc;
 
-	rc = ida_alloc(&port->decoder_ida, GFP_KERNEL);
+	rc = ida_alloc_max(&port->decoder_ida, CXL_DECODER_NR_MAX - 1,
+			   GFP_KERNEL);
 	if (rc < 0)
 		return rc;
 
diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index 21ad5f242875..1f7b3a9ebfa3 100644
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -794,26 +794,16 @@ static size_t show_targetN(struct cxl_region *cxlr, char *buf, int pos)
 	return rc;
 }
 
-static int match_free_decoder(struct device *dev, void *data)
+static int match_decoder_id(struct device *dev, void *data)
 {
 	struct cxl_decoder *cxld;
-	int *id = data;
+	int id = *(int *) data;
 
 	if (!is_switch_decoder(dev))
 		return 0;
 
 	cxld = to_cxl_decoder(dev);
-
-	/* enforce ordered allocation */
-	if (cxld->id != *id)
-		return 0;
-
-	if (!cxld->region)
-		return 1;
-
-	(*id)++;
-
-	return 0;
+	return cxld->id == id;
 }
 
 static int match_auto_decoder(struct device *dev, void *data)
@@ -840,16 +830,29 @@ cxl_region_find_decoder(struct cxl_port *port,
 			struct cxl_region *cxlr)
 {
 	struct device *dev;
-	int id = 0;
-
 	if (port == cxled_to_port(cxled))
 		return &cxled->cxld;
 
 	if (test_bit(CXL_REGION_F_AUTO, &cxlr->flags))
 		dev = device_find_child(&port->dev, &cxlr->params,
 					match_auto_decoder);
-	else
-		dev = device_find_child(&port->dev, &id, match_free_decoder);
+	else {
+		int id, last;
+
+		/*
+		 * Find next available decoder, but fail new decoder
+		 * allocations if out-of-order region destruction has
+		 * occurred
+		 */
+		id = find_first_zero_bit(port->decoder_alloc,
+					 CXL_DECODER_NR_MAX);
+		last = find_last_bit(port->decoder_alloc, CXL_DECODER_NR_MAX);
+
+		if (id >= CXL_DECODER_NR_MAX ||
+		    (last < CXL_DECODER_NR_MAX && id != last + 1))
+			return NULL;
+		dev = device_find_child(&port->dev, &id, match_decoder_id);
+	}
 	if (!dev)
 		return NULL;
 	/*
@@ -943,6 +946,9 @@ static void cxl_rr_free_decoder(struct cxl_region_ref *cxl_rr)
 
 	dev_WARN_ONCE(&cxlr->dev, cxld->region != cxlr, "region mismatch\n");
 	if (cxld->region == cxlr) {
+		struct cxl_port *port = to_cxl_port(cxld->dev.parent);
+
+		clear_bit(cxld->id, port->decoder_alloc);
 		cxld->region = NULL;
 		put_device(&cxlr->dev);
 	}
@@ -977,6 +983,7 @@ static int cxl_rr_ep_add(struct cxl_region_ref *cxl_rr,
 	cxl_rr->nr_eps++;
 
 	if (!cxld->region) {
+		set_bit(cxld->id, port->decoder_alloc);
 		cxld->region = cxlr;
 		get_device(&cxlr->dev);
 	}
diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
index 9afb407d438f..750cd027d0b0 100644
--- a/drivers/cxl/cxl.h
+++ b/drivers/cxl/cxl.h
@@ -578,6 +578,9 @@ struct cxl_dax_region {
 	struct range hpa_range;
 };
 
+/* Max as of CXL 3.1 (8.2.4.20.1 CXL HDM Decoder Capability Register) */
+#define CXL_DECODER_NR_MAX 32
+
 /**
  * struct cxl_port - logical collection of upstream port devices and
  *		     downstream port devices to construct a CXL memory
@@ -591,6 +594,7 @@ struct cxl_dax_region {
  * @regions: cxl_region_ref instances, regions mapped by this port
  * @parent_dport: dport that points to this port in the parent
  * @decoder_ida: allocator for decoder ids
+ * @decoder_alloc: decoder busy/free (@cxld->region set) bitmap
  * @reg_map: component and ras register mapping parameters
  * @nr_dports: number of entries in @dports
  * @hdm_end: track last allocated HDM decoder instance for allocation ordering
@@ -611,6 +615,7 @@ struct cxl_port {
 	struct xarray regions;
 	struct cxl_dport *parent_dport;
 	struct ida decoder_ida;
+	DECLARE_BITMAP(decoder_alloc, CXL_DECODER_NR_MAX);
 	struct cxl_register_map reg_map;
 	int nr_dports;
 	int hdm_end;