[PATCH v7 0/2] Support zero-sized HDM decoders

[PATCH v7 0/2] Support zero-sized HDM decoders

[Richard Cheng]

CXL r4.0 §8.2.4.20.12 ("Committing Decoder Programming") and
§14.13.10 ("CXL HDM Decoder Zero Size Commit") permit committing an
HDM decoder with size 0. Firmware may commit and lock such decoders so
the OS cannot program regions through them; this is a platform design
choice rather than a specification requirement.

The kernel currently rejects these decoders with -ENXIO during port
enumeration and aborts enumeration of the entire port. On affected
systems, this prevents the CXL topology from appearing in userspace.

This series enumerates empty committed decoders into the topology while
keeping them out of region assembly.

Patch 1 makes zero-size DPA reservations first-class. Since the resource
tree cannot represent an empty range, an empty decoder is backed by a
separately allocated resource. This keeps port->hdm_end, skip handling,
and in-order DPA accounting consistent. Operations requiring non-empty
capacity, including region attachment and poison queries, are guarded
appropriately.

Patch 2 adds the mock_zero_size_decoders cxl_test module parameter,
disabled by default. When enabled, it installs committed, locked,
zero-sized endpoint, switch, and host-bridge decoders under
host-bridge0.

Testing on arm64 covered enumeration of eight locked zero-sized
decoders, DPA reservation and release after those decoders, ACPI
unbind/rebind, module reload, poison handling, and rejection of
incompatible cxl_test module-parameter combinations. The core patch was
also previously validated on a Montage CXL Type 3 device.

Changelog:

v6->v7:
- Rebase onto cxl/for-7.3/cxl-type2-test.
- Patch 1 is unchanged.
- Adapt patch 2 to the refactored Type-2/Type-3 decoder initialization
  paths.
- Integrate mock_zero_size_decoders with special-topology parameter
  validation and reject its incompatible use with fail_autoassemble.

Richard Cheng (2):
  cxl/hdm: Allow zero sized HDM decoders
  tools/testing/cxl: Enable zero sized decoders under hb0

 drivers/cxl/core/hdm.c       |  52 +++++++++++------
 drivers/cxl/core/mbox.c      |   3 +
 drivers/cxl/core/region.c    |  49 ++++++++++------
 drivers/cxl/cxl.h            |  10 ++++
 drivers/cxl/port.c           |   3 +
 tools/testing/cxl/test/cxl.c | 109 ++++++++++++++++++++++++++++++-----
 6 files changed, 177 insertions(+), 49 deletions(-)


base-commit: 1c6b4ceafc3b994871c29340e0c1ddb0af5800e7
-- 
2.43.0

[PATCH v7 1/2] cxl/hdm: Allow zero sized HDM decoders

[Richard Cheng]

CXL r4.0 §8.2.4.20.12 ("Committing Decoder Programming") and §14.13.10
("CXL HDM Decoder Zero Size Commit") permit committing an HDM decoder
with size 0. BIOS may commit and lock such decoders so the OS cannot
program regions through them, this is a design choice rather than a spec
requirement.
The kernel rejected these with -ENXIO during port enumeration and aborted
the whole port, so affected systems showed nothing under 'cxl list'.

Treat empty decoders as first class instead of special casing them, back
them with a kmalloc'd resource, since the resource tree can't represent
an empty range, and keep the skip and hdm_end accounting intact. Guard
the paths an empty decoder can't serve, e.g. region attach, DPA free, and
poison queries.

Suggested-by: Dan Williams <djbw@kernel.org>
Signed-off-by: Vishal Aslot <vaslot@nvidia.com>
Signed-off-by: Richard Cheng <icheng@nvidia.com>
Reviewed-by: Dan Williams <djbw@kernel.org>
---
Changelog:
v6->v7:
	- Rebase onto cxl/for-7.3/cxl-type2-test.

v5->v6:
     - Comment wording fixups, no functional change: reword cxled_empty()
       per Dan Williams' review and tidy the poison-walk comment.
v4->v5:
     - Rework per Dan Williams' suggestion: make zero-size DPA
       reservations first class instead of bypassing reservation.
       cxl_dpa_request_region() returns a kmalloc'd resource for size 0;
       cxl_dpa_release_region() kfrees it on teardown. hdm_end now
       advances and releases for zero-size decoders under cxl_rwsem.dpa,
       and their skip registers are honored.
     - Corrections to the posted sketch: add the missing return after
       kfree() (use-after-free), use IORESOURCE_MEM flags so
       resource_contains() can resolve cxled->part.
     - cxl_mem_get_poison() returns success for zero-length queries
       before issuing a mailbox command.
     - Fully-burned device (all decoders zero-size at DPA 0): cxled->part
       cannot resolve; gate poison_by_decoder() queries on part >= 0
       (the part[-1] OOB read was already reachable for sized decoders
       that fail partition containment) and scan all partitions when the
       walk reaches commit_end with nothing mapped (sashiko report).
     - Drop v4's poison_by_decoder() restructure and the hdm_end
       non-advance approach; superseded by the above.

v3->v4:
     - Drop the port->hdm_end advance for zero-size decoders (sashiko AI
       review). The write was outside cxl_rwsem.dpa, and advancing the
       watermark without a balanced release strands hdm_end and breaks
       LIFO teardown. It is also unnecessary -- zero-size committed slots
       are trailing and locked, so the in-order check in
       __cxl_dpa_reserve() is never reached past them.

v2->v3:
     - Advance port->hdm_end for the committed zero-size decoder so a
       following committed decoder still passes the in-order check in
       __cxl_dpa_reserve() (it was left un-incremented in v2).
     - Fold the poison fix into this patch: commit_end may now reference a
       zero-size decoder with no DPA resource, so poison_by_decoder() falls
       through to run cxl_get_poison_unmapped() and scan the unmapped DPA
       tail.

v1->v2:
    - Add zero-size committed decoders to the topology instead of
      skipping them. Drop v1's -ENOSPC sentinel and the matching
"continue" in devm_cxl_enumerate_decoders(); fall through so
add_hdm_decoder() registers the decoder.
    - Set port->commit_end unconditionally for any committed decoder,
      not only non-zero-size ones, so subsequent decoders satisfy the
out-of-order check.
    - Add an explicit early-return before devm_cxl_dpa_reserve() in the
      endpoint-decoder path. __cxl_dpa reserve() rejects zero-size
decoders
    - Spell out TSP and TCB and cite spec sections in commit message
    - Reorder series, implementation first
---
 drivers/cxl/core/hdm.c    | 52 ++++++++++++++++++++++++++-------------
 drivers/cxl/core/mbox.c   |  3 +++
 drivers/cxl/core/region.c | 49 +++++++++++++++++++++++-------------
 drivers/cxl/cxl.h         | 10 ++++++++
 drivers/cxl/port.c        |  3 +++
 5 files changed, 83 insertions(+), 34 deletions(-)

diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
index 0c80b76a5f9b..ccbab2e21f06 100644
--- a/drivers/cxl/core/hdm.c
+++ b/drivers/cxl/core/hdm.c
@@ -240,6 +240,18 @@ static resource_size_t __adjust_skip(struct cxl_dev_state *cxlds,
 }
 #define release_skip(c, b, l) __adjust_skip((c), (b), (l), NULL)
 
+static void cxl_dpa_release_region(struct resource *parent,
+				   struct resource *res)
+{
+	/* zero sized decoders are not tracked in the resource tree */
+	if (resource_size(res) == 0) {
+		kfree(res);
+		return;
+	}
+
+	__release_region(parent, res->start, resource_size(res));
+}
+
 /*
  * Must be called in a context that synchronizes against this decoder's
  * port ->remove() callback (like an endpoint decoder sysfs attribute)
@@ -256,7 +268,7 @@ static void __cxl_dpa_release(struct cxl_endpoint_decoder *cxled)
 
 	/* save @skip_start, before @res is released */
 	skip_start = res->start - cxled->skip;
-	__release_region(&cxlds->dpa_res, res->start, resource_size(res));
+	cxl_dpa_release_region(&cxlds->dpa_res, res);
 	if (cxled->skip)
 		release_skip(cxlds, skip_start, cxled->skip);
 	cxled->skip = 0;
@@ -336,6 +348,23 @@ static int request_skip(struct cxl_dev_state *cxlds,
 	return -EBUSY;
 }
 
+static struct resource *cxl_dpa_request_region(struct resource *parent,
+					       resource_size_t start,
+					       resource_size_t n,
+					       const char *name)
+{
+	if (!n) {
+		struct resource *res = kmalloc_obj(*res);
+
+		if (!res)
+			return NULL;
+		*res = DEFINE_RES_NAMED(start, 0, name, IORESOURCE_MEM);
+		return res;
+	}
+
+	return __request_region(parent, start, n, name, 0);
+}
+
 static int __cxl_dpa_reserve(struct cxl_endpoint_decoder *cxled,
 			     resource_size_t base, resource_size_t len,
 			     resource_size_t skipped)
@@ -349,12 +378,6 @@ static int __cxl_dpa_reserve(struct cxl_endpoint_decoder *cxled,
 
 	lockdep_assert_held_write(&cxl_rwsem.dpa);
 
-	if (!len) {
-		dev_warn(dev, "decoder%d.%d: empty reservation attempted\n",
-			 port->id, cxled->cxld.id);
-		return -EINVAL;
-	}
-
 	if (cxled->dpa_res) {
 		dev_dbg(dev, "decoder%d.%d: existing allocation %pr assigned\n",
 			port->id, cxled->cxld.id, cxled->dpa_res);
@@ -378,8 +401,8 @@ static int __cxl_dpa_reserve(struct cxl_endpoint_decoder *cxled,
 		if (rc)
 			return rc;
 	}
-	res = __request_region(&cxlds->dpa_res, base, len,
-			       dev_name(&cxled->cxld.dev), 0);
+	res = cxl_dpa_request_region(&cxlds->dpa_res, base, len,
+				     dev_name(&cxled->cxld.dev));
 	if (!res) {
 		dev_dbg(dev, "decoder%d.%d: failed to reserve allocation\n",
 			port->id, cxled->cxld.id);
@@ -402,7 +425,8 @@ static int __cxl_dpa_reserve(struct cxl_endpoint_decoder *cxled,
 				break;
 			}
 
-	if (cxled->part < 0)
+	/* Empty decoders may not be contained by a partition boundary */
+	if (cxled->part < 0 && resource_size(res))
 		dev_warn(dev, "decoder%d.%d: %pr does not map any partition\n",
 			 port->id, cxled->cxld.id, res);
 
@@ -545,7 +569,7 @@ int cxl_dpa_free(struct cxl_endpoint_decoder *cxled)
 	struct device *dev = &cxled->cxld.dev;
 
 	guard(rwsem_write)(&cxl_rwsem.dpa);
-	if (!cxled->dpa_res)
+	if (cxled_empty(cxled))
 		return 0;
 	if (cxled->cxld.region) {
 		dev_dbg(dev, "decoder assigned to: %s\n",
@@ -1031,12 +1055,6 @@ static int init_hdm_decoder(struct cxl_port *port, struct cxl_decoder *cxld,
 			return -ENXIO;
 		}
 
-		if (size == 0) {
-			dev_warn(&port->dev,
-				 "decoder%d.%d: Committed with zero size\n",
-				 port->id, cxld->id);
-			return -ENXIO;
-		}
 		port->commit_end = cxld->id;
 	} else {
 		if (cxled) {
diff --git a/drivers/cxl/core/mbox.c b/drivers/cxl/core/mbox.c
index 97b1e61ad018..ffd24992273d 100644
--- a/drivers/cxl/core/mbox.c
+++ b/drivers/cxl/core/mbox.c
@@ -1384,6 +1384,9 @@ int cxl_mem_get_poison(struct cxl_memdev *cxlmd, u64 offset, u64 len,
 	int nr_records = 0;
 	int rc;
 
+	if (!len)
+		return 0;
+
 	ACQUIRE(mutex_intr, lock)(&mds->poison.mutex);
 	if ((rc = ACQUIRE_ERR(mutex_intr, &lock)))
 		return rc;
diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index 1e211542b6b6..6c7d9a52707c 100644
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -2115,7 +2115,7 @@ static int cxl_region_attach(struct cxl_region *cxlr,
 		return -ENXIO;
 	}
 
-	if (!cxled->dpa_res) {
+	if (cxled_empty(cxled)) {
 		dev_dbg(&cxlr->dev, "%s:%s: missing DPA allocation.\n",
 			dev_name(&cxlmd->dev), dev_name(&cxled->cxld.dev));
 		return -ENXIO;
@@ -2959,28 +2959,35 @@ static int poison_by_decoder(struct device *dev, void *arg)
 	if (!cxled->dpa_res)
 		return rc;
 
-	cxlmd = cxled_to_memdev(cxled);
-	cxlds = cxlmd->cxlds;
-	mode = cxlds->part[cxled->part].mode;
+	/*
+	 * Handle the degenerate case of a device with only empty decoders. An
+	 * empty decoder can still map a non-zero skip range, so advance the
+	 * walk to commit_end either way.
+	 */
+	if (cxled->part >= 0) {
+		cxlmd = cxled_to_memdev(cxled);
+		cxlds = cxlmd->cxlds;
+		mode = cxlds->part[cxled->part].mode;
+
+		if (cxled->skip) {
+			offset = cxled->dpa_res->start - cxled->skip;
+			length = cxled->skip;
+			rc = cxl_mem_get_poison(cxlmd, offset, length, NULL);
+			if (rc == -EFAULT && mode == CXL_PARTMODE_RAM)
+				rc = 0;
+			if (rc)
+				return rc;
+		}
 
-	if (cxled->skip) {
-		offset = cxled->dpa_res->start - cxled->skip;
-		length = cxled->skip;
-		rc = cxl_mem_get_poison(cxlmd, offset, length, NULL);
+		offset = cxled->dpa_res->start;
+		length = cxled->dpa_res->end - offset + 1;
+		rc = cxl_mem_get_poison(cxlmd, offset, length, cxled->cxld.region);
 		if (rc == -EFAULT && mode == CXL_PARTMODE_RAM)
 			rc = 0;
 		if (rc)
 			return rc;
 	}
 
-	offset = cxled->dpa_res->start;
-	length = cxled->dpa_res->end - offset + 1;
-	rc = cxl_mem_get_poison(cxlmd, offset, length, cxled->cxld.region);
-	if (rc == -EFAULT && mode == CXL_PARTMODE_RAM)
-		rc = 0;
-	if (rc)
-		return rc;
-
 	/* Iterate until commit_end is reached */
 	if (cxled->cxld.id == ctx->port->commit_end) {
 		ctx->offset = cxled->dpa_res->end + 1;
@@ -3002,9 +3009,17 @@ int cxl_get_poison_by_endpoint(struct cxl_port *port)
 	};
 
 	rc = device_for_each_child(&port->dev, &ctx, poison_by_decoder);
-	if (rc == 1)
+	if (rc == 1) {
+		/*
+		 * No decoder with a sized DPA reservation was walked
+		 * (every committed decoder is zero-size): scan all
+		 * partitions in full.
+		 */
+		if (ctx.part < 0)
+			ctx.part = 0;
 		rc = cxl_get_poison_unmapped(to_cxl_memdev(port->uport_dev),
 					     &ctx);
+	}
 
 	return rc;
 }
diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
index c0e5308e4d1b..cef53d9bd5c0 100644
--- a/drivers/cxl/cxl.h
+++ b/drivers/cxl/cxl.h
@@ -336,6 +336,16 @@ struct cxl_endpoint_decoder {
 	int pos;
 };
 
+/*
+ * The common case is decoders with no reservation, but also handle
+ * decoders with a zero-sized reservation that firmware may install for
+ * security lockdown purposes.
+ */
+static inline bool cxled_empty(struct cxl_endpoint_decoder *cxled)
+{
+	return !cxled->dpa_res || !resource_size(cxled->dpa_res);
+}
+
 /**
  * struct cxl_switch_decoder - Switch specific CXL HDM Decoder
  * @cxld: base cxl_decoder object
diff --git a/drivers/cxl/port.c b/drivers/cxl/port.c
index 99cf77b6b699..c12fd0b89883 100644
--- a/drivers/cxl/port.c
+++ b/drivers/cxl/port.c
@@ -46,6 +46,9 @@ static int discover_region(struct device *dev, void *unused)
 	if (cxled->state != CXL_DECODER_STATE_AUTO)
 		return 0;
 
+	if (cxled_empty(cxled))
+		return 0;
+
 	/*
 	 * Region enumeration is opportunistic, if this add-event fails,
 	 * continue to the next endpoint decoder.
-- 
2.43.0

[PATCH v7 2/2] tools/testing/cxl: Enable zero sized decoders under hb0

[Richard Cheng]

The kernel now allows committed zero-size HDM decoders so BIOS can lock
empty decoders; cxl_test needs to exercise that path.

Add a mock_zero_size_decoders module parameter (default off). When set,
the special endpoints under host-bridge0 (cxl_mem.0 and cxl_mem.4)
commit decoders 1 and 2 as zero-size + locked above the decoder[0]
auto-region, mirrored on the parent switch and host bridge. The mocks
take a real zero-size DPA reservation, like enumeration of real
hardware, so commit_end lands on a zero-size decoder and the
reservation, poison-by-endpoint, and teardown paths all run.

Signed-off-by: Vishal Aslot <vaslot@nvidia.com>
Signed-off-by: Richard Cheng <icheng@nvidia.com>
---
Changelog:

v6->v7:
    - Rebase onto cxl/for-7.3/cxl-type2-test.
	- Integrate mock_zero_size_decoders with special-topology parameter
	  validation and reject its incompatible use with fail_autoassemble.

v5->v6:
     - Reword a mock comment, no functional change.
v4->v5:
      - Mirror the v5 core semantics: mock zero-size decoders take a real
        zero-size DPA reservation via devm_cxl_dpa_reserve() so
        dpa_res/hdm_end/cxled->part match real hardware enumeration and
        the poison-by-endpoint walk reaches commit_end.
      - Set cxled->state = CXL_DECODER_STATE_AUTO to match
        init_hdm_decoder()'s fall-through (undoes the v2 switch to
        MANUAL).
      - cxld_registry_restore(): reserve unconditionally for enabled
        endpoint decoders; enabled now implies reserved, sized or not.

v3->v4:
     - No change.

v2->v3:
      - Gate the zero-size + locked decoder injection behind a new
        mock_zero_size_decoders module parameter (default off). v2
        applied it unconditionally on the host-bridge0 auto-region
        endpoints, which the region test suite reuses, regressing 7 of
        17 cxl unit tests; defaulting off leaves the shared topology
        untouched.

v1->v2:
      - Replace second_decoder(), third_decoder() with a single
        match_decoder_by_index() helper, so all lookups share one
        matcher.
      - Use DEFINE_RANGE() for the empty range instead of an open-coded
        struct.
      - Set cxled->state = CXL_DECODER_STATE_MANUAL rather than
        STATE_AUTO.
      - Set CXL_DECODER_F_LOCK on the mock zero-size decoders to model
        the BIOS-burns-slots case.
---
 tools/testing/cxl/test/cxl.c | 109 ++++++++++++++++++++++++++++++-----
 1 file changed, 94 insertions(+), 15 deletions(-)

diff --git a/tools/testing/cxl/test/cxl.c b/tools/testing/cxl/test/cxl.c
index 8ab2ce1262f3..fff2004a08c8 100644
--- a/tools/testing/cxl/test/cxl.c
+++ b/tools/testing/cxl/test/cxl.c
@@ -18,6 +18,7 @@ static int interleave_arithmetic;
 static bool extended_linear_cache;
 static bool fail_autoassemble;
 static bool type2_test;
+static bool mock_zero_size_decoders;
 
 #define FAKE_QTG_ID	42
 
@@ -872,14 +873,13 @@ static int cxld_registry_restore(struct cxl_decoder *cxld,
 		cxld_copy(cxld, &td->cxled.cxld);
 		cxled->state = td->cxled.state;
 		cxled->skip = td->cxled.skip;
-		if (range_len(&td->dpa_range)) {
-			rc = devm_cxl_dpa_reserve(cxled, td->dpa_range.start,
-						  range_len(&td->dpa_range),
-						  td->cxled.skip);
-			if (rc) {
-				init_disabled_mock_decoder(cxld);
-				return rc;
-			}
+		/* enabled endpoint decoders hold a reservation, sized or not */
+		rc = devm_cxl_dpa_reserve(cxled, td->dpa_range.start,
+					  range_len(&td->dpa_range),
+					  td->cxled.skip);
+		if (rc) {
+			init_disabled_mock_decoder(cxld);
+			return rc;
 		}
 		port->commit_end = cxld->id;
 	}
@@ -1072,16 +1072,49 @@ static void default_mock_decoder(struct cxl_decoder *cxld)
 	WARN_ON_ONCE(!cxld_registry_new(cxld));
 }
 
-static int first_decoder(struct device *dev, const void *data)
+static int match_decoder_by_index(struct device *dev, const void *data)
 {
+	int target_id = *(const int *)data;
 	struct cxl_decoder *cxld;
 
 	if (!is_switch_decoder(dev))
 		return 0;
 	cxld = to_cxl_decoder(dev);
-	if (cxld->id == 0)
-		return 1;
-	return 0;
+	return cxld->id == target_id;
+}
+
+/*
+ * Mock a committed, locked, empty decoder
+ * (CXL r4.0 8.2.4.20.12). Gated by the mock_zero_size_decoders module
+ * param so the default cxl_test topology, shared by the region test
+ * suite, is left undisturbed.
+ */
+static void size_zero_mock_decoder_ep(struct cxl_decoder *cxld, u64 base)
+{
+	struct cxl_endpoint_decoder *cxled = to_cxl_endpoint_decoder(&cxld->dev);
+
+	cxld->hpa_range = DEFINE_RANGE(base, base - 1);
+	cxld->interleave_ways = 2;
+	cxld->interleave_granularity = 4096;
+	cxld->target_type = CXL_DECODER_HOSTONLYMEM;
+	cxld->flags = CXL_DECODER_F_ENABLE | CXL_DECODER_F_LOCK;
+	cxled->state = CXL_DECODER_STATE_AUTO;
+	/* decoder[0] reserved [0, size/2), empty decoders sit at that watermark */
+	devm_cxl_dpa_reserve(cxled, mock_auto_region_size / 2, 0, 0);
+	cxld->commit = mock_decoder_commit;
+	cxld->reset = mock_decoder_reset;
+}
+
+static void size_zero_mock_decoder_sw(struct cxl_decoder *cxld, u64 base,
+				      int level)
+{
+	cxld->flags = CXL_DECODER_F_ENABLE | CXL_DECODER_F_LOCK;
+	cxld->target_type = CXL_DECODER_HOSTONLYMEM;
+	cxld->interleave_ways = level == 0 ? 2 : 1;
+	cxld->interleave_granularity = 4096;
+	cxld->hpa_range = DEFINE_RANGE(base, base - 1);
+	cxld->commit = mock_decoder_commit;
+	cxld->reset = mock_decoder_reset;
 }
 
 enum cxld_init_type {
@@ -1114,7 +1147,7 @@ static enum cxld_init_type get_decoder_init_type(struct cxl_decoder *cxld,
 	 * See 'cxl list -BMPu -m cxl_mem.0,cxl_mem.4'
 	 */
 	if (!is_endpoint_decoder(&cxld->dev) || !hb0 || pdev->id % 4 ||
-	    pdev->id > 4 || cxld->id > 0)
+	    pdev->id > 4 || cxld->id > (mock_zero_size_decoders ? 2 : 0))
 		return MOCK_DECODER_INIT_DEFAULT;
 
 	return type2_test ? MOCK_DECODER_INIT_TYPE2_AUTO :
@@ -1147,6 +1180,7 @@ static void mock_init_hdm_type2_cxled(struct cxl_endpoint_decoder *cxled,
 	struct cxl_port *root_port;
 	struct device *dev;
 	u64 base;
+	int id = 0;
 
 	base = window->base_hpa;
 	cxld->hpa_range = (struct range) {
@@ -1172,7 +1206,7 @@ static void mock_init_hdm_type2_cxled(struct cxl_endpoint_decoder *cxled,
 	 */
 	dport = port->parent_dport;
 	root_port = dport->port;
-	dev = device_find_child(&root_port->dev, NULL, first_decoder);
+	dev = device_find_child(&root_port->dev, &id, match_decoder_by_index);
 	/*
 	 * Ancestor ports are guaranteed to be enumerated before
 	 * @port, and all ports have at least one decoder.
@@ -1232,6 +1266,20 @@ static void mock_init_hdm_type3_cxled(struct cxl_endpoint_decoder *cxled,
 	base = window->base_hpa;
 	if (extended_linear_cache)
 		base += mock_auto_region_size;
+
+	/*
+	 * With mock_zero_size_decoders, decoders 1 and 2 of the special
+	 * endpoints mock committed, locked, empty decoders above the
+	 * decoder[0] auto-region (CXL r4.0 8.2.4.20.12). commit_end then
+	 * points at a zero-size decoder, exercising the zero-size
+	 * reservation and poison-by-endpoint code paths.
+	 */
+	if (cxld->id == 1 || cxld->id == 2) {
+		size_zero_mock_decoder_ep(cxld, base);
+		port->commit_end = cxld->id;
+		WARN_ON_ONCE(!cxld_registry_new(cxld));
+		return;
+	}
 	cxld->hpa_range = (struct range) {
 		.start = base,
 		.end = base + mock_auto_region_size - 1,
@@ -1255,9 +1303,11 @@ static void mock_init_hdm_type3_cxled(struct cxl_endpoint_decoder *cxled,
 	 */
 	iter = port;
 	for (i = 0; i < 2; i++) {
+		int id = 0;
+
 		dport = iter->parent_dport;
 		iter = dport->port;
-		dev = device_find_child(&iter->dev, NULL, first_decoder);
+		dev = device_find_child(&iter->dev, &id, match_decoder_by_index);
 		/*
 		 * Ancestor ports are guaranteed to be enumerated before
 		 * @port, and all ports have at least one decoder.
@@ -1307,6 +1357,26 @@ static void mock_init_hdm_type3_cxled(struct cxl_endpoint_decoder *cxled,
 
 		cxld_registry_update(cxld);
 		put_device(dev);
+
+		if (!mock_zero_size_decoders)
+			continue;
+
+		/*
+		 * Mirror the endpoint: commit the next two switch decoders
+		 * as zero-size + locked so the empty-decoder layout extends
+		 * end-to-end through the switch and host bridge.
+		 */
+		for (id = 1; id <= 2; id++) {
+			dev = device_find_child(&iter->dev, &id,
+						match_decoder_by_index);
+			if (WARN_ON(!dev))
+				continue;
+			cxld = to_cxl_decoder(dev);
+			size_zero_mock_decoder_sw(cxld, base, i);
+			iter->commit_end = id;
+			cxld_registry_update(cxld);
+			put_device(dev);
+		}
 	}
 }
 
@@ -2085,6 +2155,12 @@ static bool __init have_multiple_modparms(void)
 		count++;
 	if (type2_test)
 		count++;
+	if (mock_zero_size_decoders)
+		count++;
+
+	/* Zero-size decoders require decoder[0] to preserve commit order. */
+	if (fail_autoassemble && mock_zero_size_decoders)
+		return true;
 
 	return count > 1;
 }
@@ -2493,6 +2569,9 @@ module_param(fail_autoassemble, bool, 0444);
 MODULE_PARM_DESC(fail_autoassemble, "Simulate missing member of an auto-region");
 module_param(type2_test, bool, 0444);
 MODULE_PARM_DESC(type2_test, "Enable type 2 support testing");
+module_param(mock_zero_size_decoders, bool, 0444);
+MODULE_PARM_DESC(mock_zero_size_decoders,
+		 "Mock committed, locked, empty decoders under host-bridge0");
 module_init(cxl_test_init);
 module_exit(cxl_test_exit);
 MODULE_LICENSE("GPL v2");
-- 
2.43.0

Re: [PATCH v7 2/2] tools/testing/cxl: Enable zero sized decoders under hb0

[Alison Schofield]

On Wed, Jul 08, 2026 at 06:40:23PM +0800, Richard Cheng wrote:
> The kernel now allows committed zero-size HDM decoders so BIOS can lock
> empty decoders; cxl_test needs to exercise that path.
> 
> Add a mock_zero_size_decoders module parameter (default off). When set,
> the special endpoints under host-bridge0 (cxl_mem.0 and cxl_mem.4)
> commit decoders 1 and 2 as zero-size + locked above the decoder[0]
> auto-region, mirrored on the parent switch and host bridge. The mocks
> take a real zero-size DPA reservation, like enumeration of real
> hardware, so commit_end lands on a zero-size decoder and the
> reservation, poison-by-endpoint, and teardown paths all run.

At v5 review we chatted about a unit test for this.
Not sure what has happened since wrt the poison tail mappings, but
the check in cxl-topology.sh 'wants' to be in place to merge this.

https://lore.kernel.org/linux-cxl/ajozhDmPogKi9hb5@MWDK4CY14F/

-- Alison


> 
> Signed-off-by: Vishal Aslot <vaslot@nvidia.com>
> Signed-off-by: Richard Cheng <icheng@nvidia.com>
> ---
> Changelog:
> 
> v6->v7:
>     - Rebase onto cxl/for-7.3/cxl-type2-test.
> 	- Integrate mock_zero_size_decoders with special-topology parameter
> 	  validation and reject its incompatible use with fail_autoassemble.
> 
> v5->v6:
>      - Reword a mock comment, no functional change.
> v4->v5:
>       - Mirror the v5 core semantics: mock zero-size decoders take a real
>         zero-size DPA reservation via devm_cxl_dpa_reserve() so
>         dpa_res/hdm_end/cxled->part match real hardware enumeration and
>         the poison-by-endpoint walk reaches commit_end.
>       - Set cxled->state = CXL_DECODER_STATE_AUTO to match
>         init_hdm_decoder()'s fall-through (undoes the v2 switch to
>         MANUAL).
>       - cxld_registry_restore(): reserve unconditionally for enabled
>         endpoint decoders; enabled now implies reserved, sized or not.
> 
> v3->v4:
>      - No change.
> 
> v2->v3:
>       - Gate the zero-size + locked decoder injection behind a new
>         mock_zero_size_decoders module parameter (default off). v2
>         applied it unconditionally on the host-bridge0 auto-region
>         endpoints, which the region test suite reuses, regressing 7 of
>         17 cxl unit tests; defaulting off leaves the shared topology
>         untouched.
> 
> v1->v2:
>       - Replace second_decoder(), third_decoder() with a single
>         match_decoder_by_index() helper, so all lookups share one
>         matcher.
>       - Use DEFINE_RANGE() for the empty range instead of an open-coded
>         struct.
>       - Set cxled->state = CXL_DECODER_STATE_MANUAL rather than
>         STATE_AUTO.
>       - Set CXL_DECODER_F_LOCK on the mock zero-size decoders to model
>         the BIOS-burns-slots case.
> ---
>  tools/testing/cxl/test/cxl.c | 109 ++++++++++++++++++++++++++++++-----
>  1 file changed, 94 insertions(+), 15 deletions(-)
> 
> diff --git a/tools/testing/cxl/test/cxl.c b/tools/testing/cxl/test/cxl.c
> index 8ab2ce1262f3..fff2004a08c8 100644
> --- a/tools/testing/cxl/test/cxl.c
> +++ b/tools/testing/cxl/test/cxl.c
> @@ -18,6 +18,7 @@ static int interleave_arithmetic;
>  static bool extended_linear_cache;
>  static bool fail_autoassemble;
>  static bool type2_test;
> +static bool mock_zero_size_decoders;
>  
>  #define FAKE_QTG_ID	42
>  
> @@ -872,14 +873,13 @@ static int cxld_registry_restore(struct cxl_decoder *cxld,
>  		cxld_copy(cxld, &td->cxled.cxld);
>  		cxled->state = td->cxled.state;
>  		cxled->skip = td->cxled.skip;
> -		if (range_len(&td->dpa_range)) {
> -			rc = devm_cxl_dpa_reserve(cxled, td->dpa_range.start,
> -						  range_len(&td->dpa_range),
> -						  td->cxled.skip);
> -			if (rc) {
> -				init_disabled_mock_decoder(cxld);
> -				return rc;
> -			}
> +		/* enabled endpoint decoders hold a reservation, sized or not */
> +		rc = devm_cxl_dpa_reserve(cxled, td->dpa_range.start,
> +					  range_len(&td->dpa_range),
> +					  td->cxled.skip);
> +		if (rc) {
> +			init_disabled_mock_decoder(cxld);
> +			return rc;
>  		}
>  		port->commit_end = cxld->id;
>  	}
> @@ -1072,16 +1072,49 @@ static void default_mock_decoder(struct cxl_decoder *cxld)
>  	WARN_ON_ONCE(!cxld_registry_new(cxld));
>  }
>  
> -static int first_decoder(struct device *dev, const void *data)
> +static int match_decoder_by_index(struct device *dev, const void *data)
>  {
> +	int target_id = *(const int *)data;
>  	struct cxl_decoder *cxld;
>  
>  	if (!is_switch_decoder(dev))
>  		return 0;
>  	cxld = to_cxl_decoder(dev);
> -	if (cxld->id == 0)
> -		return 1;
> -	return 0;
> +	return cxld->id == target_id;
> +}
> +
> +/*
> + * Mock a committed, locked, empty decoder
> + * (CXL r4.0 8.2.4.20.12). Gated by the mock_zero_size_decoders module
> + * param so the default cxl_test topology, shared by the region test
> + * suite, is left undisturbed.
> + */
> +static void size_zero_mock_decoder_ep(struct cxl_decoder *cxld, u64 base)
> +{
> +	struct cxl_endpoint_decoder *cxled = to_cxl_endpoint_decoder(&cxld->dev);
> +
> +	cxld->hpa_range = DEFINE_RANGE(base, base - 1);
> +	cxld->interleave_ways = 2;
> +	cxld->interleave_granularity = 4096;
> +	cxld->target_type = CXL_DECODER_HOSTONLYMEM;
> +	cxld->flags = CXL_DECODER_F_ENABLE | CXL_DECODER_F_LOCK;
> +	cxled->state = CXL_DECODER_STATE_AUTO;
> +	/* decoder[0] reserved [0, size/2), empty decoders sit at that watermark */
> +	devm_cxl_dpa_reserve(cxled, mock_auto_region_size / 2, 0, 0);
> +	cxld->commit = mock_decoder_commit;
> +	cxld->reset = mock_decoder_reset;
> +}
> +
> +static void size_zero_mock_decoder_sw(struct cxl_decoder *cxld, u64 base,
> +				      int level)
> +{
> +	cxld->flags = CXL_DECODER_F_ENABLE | CXL_DECODER_F_LOCK;
> +	cxld->target_type = CXL_DECODER_HOSTONLYMEM;
> +	cxld->interleave_ways = level == 0 ? 2 : 1;
> +	cxld->interleave_granularity = 4096;
> +	cxld->hpa_range = DEFINE_RANGE(base, base - 1);
> +	cxld->commit = mock_decoder_commit;
> +	cxld->reset = mock_decoder_reset;
>  }
>  
>  enum cxld_init_type {
> @@ -1114,7 +1147,7 @@ static enum cxld_init_type get_decoder_init_type(struct cxl_decoder *cxld,
>  	 * See 'cxl list -BMPu -m cxl_mem.0,cxl_mem.4'
>  	 */
>  	if (!is_endpoint_decoder(&cxld->dev) || !hb0 || pdev->id % 4 ||
> -	    pdev->id > 4 || cxld->id > 0)
> +	    pdev->id > 4 || cxld->id > (mock_zero_size_decoders ? 2 : 0))
>  		return MOCK_DECODER_INIT_DEFAULT;
>  
>  	return type2_test ? MOCK_DECODER_INIT_TYPE2_AUTO :
> @@ -1147,6 +1180,7 @@ static void mock_init_hdm_type2_cxled(struct cxl_endpoint_decoder *cxled,
>  	struct cxl_port *root_port;
>  	struct device *dev;
>  	u64 base;
> +	int id = 0;
>  
>  	base = window->base_hpa;
>  	cxld->hpa_range = (struct range) {
> @@ -1172,7 +1206,7 @@ static void mock_init_hdm_type2_cxled(struct cxl_endpoint_decoder *cxled,
>  	 */
>  	dport = port->parent_dport;
>  	root_port = dport->port;
> -	dev = device_find_child(&root_port->dev, NULL, first_decoder);
> +	dev = device_find_child(&root_port->dev, &id, match_decoder_by_index);
>  	/*
>  	 * Ancestor ports are guaranteed to be enumerated before
>  	 * @port, and all ports have at least one decoder.
> @@ -1232,6 +1266,20 @@ static void mock_init_hdm_type3_cxled(struct cxl_endpoint_decoder *cxled,
>  	base = window->base_hpa;
>  	if (extended_linear_cache)
>  		base += mock_auto_region_size;
> +
> +	/*
> +	 * With mock_zero_size_decoders, decoders 1 and 2 of the special
> +	 * endpoints mock committed, locked, empty decoders above the
> +	 * decoder[0] auto-region (CXL r4.0 8.2.4.20.12). commit_end then
> +	 * points at a zero-size decoder, exercising the zero-size
> +	 * reservation and poison-by-endpoint code paths.
> +	 */
> +	if (cxld->id == 1 || cxld->id == 2) {
> +		size_zero_mock_decoder_ep(cxld, base);
> +		port->commit_end = cxld->id;
> +		WARN_ON_ONCE(!cxld_registry_new(cxld));
> +		return;
> +	}
>  	cxld->hpa_range = (struct range) {
>  		.start = base,
>  		.end = base + mock_auto_region_size - 1,
> @@ -1255,9 +1303,11 @@ static void mock_init_hdm_type3_cxled(struct cxl_endpoint_decoder *cxled,
>  	 */
>  	iter = port;
>  	for (i = 0; i < 2; i++) {
> +		int id = 0;
> +
>  		dport = iter->parent_dport;
>  		iter = dport->port;
> -		dev = device_find_child(&iter->dev, NULL, first_decoder);
> +		dev = device_find_child(&iter->dev, &id, match_decoder_by_index);
>  		/*
>  		 * Ancestor ports are guaranteed to be enumerated before
>  		 * @port, and all ports have at least one decoder.
> @@ -1307,6 +1357,26 @@ static void mock_init_hdm_type3_cxled(struct cxl_endpoint_decoder *cxled,
>  
>  		cxld_registry_update(cxld);
>  		put_device(dev);
> +
> +		if (!mock_zero_size_decoders)
> +			continue;
> +
> +		/*
> +		 * Mirror the endpoint: commit the next two switch decoders
> +		 * as zero-size + locked so the empty-decoder layout extends
> +		 * end-to-end through the switch and host bridge.
> +		 */
> +		for (id = 1; id <= 2; id++) {
> +			dev = device_find_child(&iter->dev, &id,
> +						match_decoder_by_index);
> +			if (WARN_ON(!dev))
> +				continue;
> +			cxld = to_cxl_decoder(dev);
> +			size_zero_mock_decoder_sw(cxld, base, i);
> +			iter->commit_end = id;
> +			cxld_registry_update(cxld);
> +			put_device(dev);
> +		}
>  	}
>  }
>  
> @@ -2085,6 +2155,12 @@ static bool __init have_multiple_modparms(void)
>  		count++;
>  	if (type2_test)
>  		count++;
> +	if (mock_zero_size_decoders)
> +		count++;
> +
> +	/* Zero-size decoders require decoder[0] to preserve commit order. */
> +	if (fail_autoassemble && mock_zero_size_decoders)
> +		return true;
>  
>  	return count > 1;
>  }
> @@ -2493,6 +2569,9 @@ module_param(fail_autoassemble, bool, 0444);
>  MODULE_PARM_DESC(fail_autoassemble, "Simulate missing member of an auto-region");
>  module_param(type2_test, bool, 0444);
>  MODULE_PARM_DESC(type2_test, "Enable type 2 support testing");
> +module_param(mock_zero_size_decoders, bool, 0444);
> +MODULE_PARM_DESC(mock_zero_size_decoders,
> +		 "Mock committed, locked, empty decoders under host-bridge0");
>  module_init(cxl_test_init);
>  module_exit(cxl_test_exit);
>  MODULE_LICENSE("GPL v2");
> -- 
> 2.43.0
> 

Re: [PATCH v7 2/2] tools/testing/cxl: Enable zero sized decoders under hb0

[Richard Cheng]

On Wed, Jul 08, 2026 at 11:35:55AM +0800, Alison Schofield wrote:
> On Wed, Jul 08, 2026 at 06:40:23PM +0800, Richard Cheng wrote:
> > The kernel now allows committed zero-size HDM decoders so BIOS can lock
> > empty decoders; cxl_test needs to exercise that path.
> > 
> > Add a mock_zero_size_decoders module parameter (default off). When set,
> > the special endpoints under host-bridge0 (cxl_mem.0 and cxl_mem.4)
> > commit decoders 1 and 2 as zero-size + locked above the decoder[0]
> > auto-region, mirrored on the parent switch and host bridge. The mocks
> > take a real zero-size DPA reservation, like enumeration of real
> > hardware, so commit_end lands on a zero-size decoder and the
> > reservation, poison-by-endpoint, and teardown paths all run.
> 
> At v5 review we chatted about a unit test for this.
> Not sure what has happened since wrt the poison tail mappings, but
> the check in cxl-topology.sh 'wants' to be in place to merge this.
> 
> https://lore.kernel.org/linux-cxl/ajozhDmPogKi9hb5@MWDK4CY14F/
> 
> -- Alison
>

Hi Alison,

Sorry I forgot about that,

Just have the work sent out here.

https://lore.kernel.org/linux-cxl/20260709022857.18732-1-icheng@nvidia.com/

Does this match what you expect ?

--Richard
 
> 
> > 
> > Signed-off-by: Vishal Aslot <vaslot@nvidia.com>
> > Signed-off-by: Richard Cheng <icheng@nvidia.com>
> > ---
> > Changelog:
> > 
> > v6->v7:
> >     - Rebase onto cxl/for-7.3/cxl-type2-test.
> > 	- Integrate mock_zero_size_decoders with special-topology parameter
> > 	  validation and reject its incompatible use with fail_autoassemble.
> > 
> > v5->v6:
> >      - Reword a mock comment, no functional change.
> > v4->v5:
> >       - Mirror the v5 core semantics: mock zero-size decoders take a real
> >         zero-size DPA reservation via devm_cxl_dpa_reserve() so
> >         dpa_res/hdm_end/cxled->part match real hardware enumeration and
> >         the poison-by-endpoint walk reaches commit_end.
> >       - Set cxled->state = CXL_DECODER_STATE_AUTO to match
> >         init_hdm_decoder()'s fall-through (undoes the v2 switch to
> >         MANUAL).
> >       - cxld_registry_restore(): reserve unconditionally for enabled
> >         endpoint decoders; enabled now implies reserved, sized or not.
> > 
> > v3->v4:
> >      - No change.
> > 
> > v2->v3:
> >       - Gate the zero-size + locked decoder injection behind a new
> >         mock_zero_size_decoders module parameter (default off). v2
> >         applied it unconditionally on the host-bridge0 auto-region
> >         endpoints, which the region test suite reuses, regressing 7 of
> >         17 cxl unit tests; defaulting off leaves the shared topology
> >         untouched.
> > 
> > v1->v2:
> >       - Replace second_decoder(), third_decoder() with a single
> >         match_decoder_by_index() helper, so all lookups share one
> >         matcher.
> >       - Use DEFINE_RANGE() for the empty range instead of an open-coded
> >         struct.
> >       - Set cxled->state = CXL_DECODER_STATE_MANUAL rather than
> >         STATE_AUTO.
> >       - Set CXL_DECODER_F_LOCK on the mock zero-size decoders to model
> >         the BIOS-burns-slots case.
> > ---
> >  tools/testing/cxl/test/cxl.c | 109 ++++++++++++++++++++++++++++++-----
> >  1 file changed, 94 insertions(+), 15 deletions(-)
> > 
> > diff --git a/tools/testing/cxl/test/cxl.c b/tools/testing/cxl/test/cxl.c
> > index 8ab2ce1262f3..fff2004a08c8 100644
> > --- a/tools/testing/cxl/test/cxl.c
> > +++ b/tools/testing/cxl/test/cxl.c
> > @@ -18,6 +18,7 @@ static int interleave_arithmetic;
> >  static bool extended_linear_cache;
> >  static bool fail_autoassemble;
> >  static bool type2_test;
> > +static bool mock_zero_size_decoders;
> >  
> >  #define FAKE_QTG_ID	42
> >  
> > @@ -872,14 +873,13 @@ static int cxld_registry_restore(struct cxl_decoder *cxld,
> >  		cxld_copy(cxld, &td->cxled.cxld);
> >  		cxled->state = td->cxled.state;
> >  		cxled->skip = td->cxled.skip;
> > -		if (range_len(&td->dpa_range)) {
> > -			rc = devm_cxl_dpa_reserve(cxled, td->dpa_range.start,
> > -						  range_len(&td->dpa_range),
> > -						  td->cxled.skip);
> > -			if (rc) {
> > -				init_disabled_mock_decoder(cxld);
> > -				return rc;
> > -			}
> > +		/* enabled endpoint decoders hold a reservation, sized or not */
> > +		rc = devm_cxl_dpa_reserve(cxled, td->dpa_range.start,
> > +					  range_len(&td->dpa_range),
> > +					  td->cxled.skip);
> > +		if (rc) {
> > +			init_disabled_mock_decoder(cxld);
> > +			return rc;
> >  		}
> >  		port->commit_end = cxld->id;
> >  	}
> > @@ -1072,16 +1072,49 @@ static void default_mock_decoder(struct cxl_decoder *cxld)
> >  	WARN_ON_ONCE(!cxld_registry_new(cxld));
> >  }
> >  
> > -static int first_decoder(struct device *dev, const void *data)
> > +static int match_decoder_by_index(struct device *dev, const void *data)
> >  {
> > +	int target_id = *(const int *)data;
> >  	struct cxl_decoder *cxld;
> >  
> >  	if (!is_switch_decoder(dev))
> >  		return 0;
> >  	cxld = to_cxl_decoder(dev);
> > -	if (cxld->id == 0)
> > -		return 1;
> > -	return 0;
> > +	return cxld->id == target_id;
> > +}
> > +
> > +/*
> > + * Mock a committed, locked, empty decoder
> > + * (CXL r4.0 8.2.4.20.12). Gated by the mock_zero_size_decoders module
> > + * param so the default cxl_test topology, shared by the region test
> > + * suite, is left undisturbed.
> > + */
> > +static void size_zero_mock_decoder_ep(struct cxl_decoder *cxld, u64 base)
> > +{
> > +	struct cxl_endpoint_decoder *cxled = to_cxl_endpoint_decoder(&cxld->dev);
> > +
> > +	cxld->hpa_range = DEFINE_RANGE(base, base - 1);
> > +	cxld->interleave_ways = 2;
> > +	cxld->interleave_granularity = 4096;
> > +	cxld->target_type = CXL_DECODER_HOSTONLYMEM;
> > +	cxld->flags = CXL_DECODER_F_ENABLE | CXL_DECODER_F_LOCK;
> > +	cxled->state = CXL_DECODER_STATE_AUTO;
> > +	/* decoder[0] reserved [0, size/2), empty decoders sit at that watermark */
> > +	devm_cxl_dpa_reserve(cxled, mock_auto_region_size / 2, 0, 0);
> > +	cxld->commit = mock_decoder_commit;
> > +	cxld->reset = mock_decoder_reset;
> > +}
> > +
> > +static void size_zero_mock_decoder_sw(struct cxl_decoder *cxld, u64 base,
> > +				      int level)
> > +{
> > +	cxld->flags = CXL_DECODER_F_ENABLE | CXL_DECODER_F_LOCK;
> > +	cxld->target_type = CXL_DECODER_HOSTONLYMEM;
> > +	cxld->interleave_ways = level == 0 ? 2 : 1;
> > +	cxld->interleave_granularity = 4096;
> > +	cxld->hpa_range = DEFINE_RANGE(base, base - 1);
> > +	cxld->commit = mock_decoder_commit;
> > +	cxld->reset = mock_decoder_reset;
> >  }
> >  
> >  enum cxld_init_type {
> > @@ -1114,7 +1147,7 @@ static enum cxld_init_type get_decoder_init_type(struct cxl_decoder *cxld,
> >  	 * See 'cxl list -BMPu -m cxl_mem.0,cxl_mem.4'
> >  	 */
> >  	if (!is_endpoint_decoder(&cxld->dev) || !hb0 || pdev->id % 4 ||
> > -	    pdev->id > 4 || cxld->id > 0)
> > +	    pdev->id > 4 || cxld->id > (mock_zero_size_decoders ? 2 : 0))
> >  		return MOCK_DECODER_INIT_DEFAULT;
> >  
> >  	return type2_test ? MOCK_DECODER_INIT_TYPE2_AUTO :
> > @@ -1147,6 +1180,7 @@ static void mock_init_hdm_type2_cxled(struct cxl_endpoint_decoder *cxled,
> >  	struct cxl_port *root_port;
> >  	struct device *dev;
> >  	u64 base;
> > +	int id = 0;
> >  
> >  	base = window->base_hpa;
> >  	cxld->hpa_range = (struct range) {
> > @@ -1172,7 +1206,7 @@ static void mock_init_hdm_type2_cxled(struct cxl_endpoint_decoder *cxled,
> >  	 */
> >  	dport = port->parent_dport;
> >  	root_port = dport->port;
> > -	dev = device_find_child(&root_port->dev, NULL, first_decoder);
> > +	dev = device_find_child(&root_port->dev, &id, match_decoder_by_index);
> >  	/*
> >  	 * Ancestor ports are guaranteed to be enumerated before
> >  	 * @port, and all ports have at least one decoder.
> > @@ -1232,6 +1266,20 @@ static void mock_init_hdm_type3_cxled(struct cxl_endpoint_decoder *cxled,
> >  	base = window->base_hpa;
> >  	if (extended_linear_cache)
> >  		base += mock_auto_region_size;
> > +
> > +	/*
> > +	 * With mock_zero_size_decoders, decoders 1 and 2 of the special
> > +	 * endpoints mock committed, locked, empty decoders above the
> > +	 * decoder[0] auto-region (CXL r4.0 8.2.4.20.12). commit_end then
> > +	 * points at a zero-size decoder, exercising the zero-size
> > +	 * reservation and poison-by-endpoint code paths.
> > +	 */
> > +	if (cxld->id == 1 || cxld->id == 2) {
> > +		size_zero_mock_decoder_ep(cxld, base);
> > +		port->commit_end = cxld->id;
> > +		WARN_ON_ONCE(!cxld_registry_new(cxld));
> > +		return;
> > +	}
> >  	cxld->hpa_range = (struct range) {
> >  		.start = base,
> >  		.end = base + mock_auto_region_size - 1,
> > @@ -1255,9 +1303,11 @@ static void mock_init_hdm_type3_cxled(struct cxl_endpoint_decoder *cxled,
> >  	 */
> >  	iter = port;
> >  	for (i = 0; i < 2; i++) {
> > +		int id = 0;
> > +
> >  		dport = iter->parent_dport;
> >  		iter = dport->port;
> > -		dev = device_find_child(&iter->dev, NULL, first_decoder);
> > +		dev = device_find_child(&iter->dev, &id, match_decoder_by_index);
> >  		/*
> >  		 * Ancestor ports are guaranteed to be enumerated before
> >  		 * @port, and all ports have at least one decoder.
> > @@ -1307,6 +1357,26 @@ static void mock_init_hdm_type3_cxled(struct cxl_endpoint_decoder *cxled,
> >  
> >  		cxld_registry_update(cxld);
> >  		put_device(dev);
> > +
> > +		if (!mock_zero_size_decoders)
> > +			continue;
> > +
> > +		/*
> > +		 * Mirror the endpoint: commit the next two switch decoders
> > +		 * as zero-size + locked so the empty-decoder layout extends
> > +		 * end-to-end through the switch and host bridge.
> > +		 */
> > +		for (id = 1; id <= 2; id++) {
> > +			dev = device_find_child(&iter->dev, &id,
> > +						match_decoder_by_index);
> > +			if (WARN_ON(!dev))
> > +				continue;
> > +			cxld = to_cxl_decoder(dev);
> > +			size_zero_mock_decoder_sw(cxld, base, i);
> > +			iter->commit_end = id;
> > +			cxld_registry_update(cxld);
> > +			put_device(dev);
> > +		}
> >  	}
> >  }
> >  
> > @@ -2085,6 +2155,12 @@ static bool __init have_multiple_modparms(void)
> >  		count++;
> >  	if (type2_test)
> >  		count++;
> > +	if (mock_zero_size_decoders)
> > +		count++;
> > +
> > +	/* Zero-size decoders require decoder[0] to preserve commit order. */
> > +	if (fail_autoassemble && mock_zero_size_decoders)
> > +		return true;
> >  
> >  	return count > 1;
> >  }
> > @@ -2493,6 +2569,9 @@ module_param(fail_autoassemble, bool, 0444);
> >  MODULE_PARM_DESC(fail_autoassemble, "Simulate missing member of an auto-region");
> >  module_param(type2_test, bool, 0444);
> >  MODULE_PARM_DESC(type2_test, "Enable type 2 support testing");
> > +module_param(mock_zero_size_decoders, bool, 0444);
> > +MODULE_PARM_DESC(mock_zero_size_decoders,
> > +		 "Mock committed, locked, empty decoders under host-bridge0");
> >  module_init(cxl_test_init);
> >  module_exit(cxl_test_exit);
> >  MODULE_LICENSE("GPL v2");
> > -- 
> > 2.43.0
> >