* [PATCH v3 1/2] cxl/hdm: Allow zero sized HDM decoders
2026-06-07 5:38 [PATCH v3 0/2] Support zero-sized HDM decoders Richard Cheng
@ 2026-06-07 5:38 ` Richard Cheng
2026-06-07 5:51 ` sashiko-bot
2026-06-07 5:38 ` [PATCH v3 2/2] tools/testing/cxl: Enable zero sized decoder under hb0 Richard Cheng
1 sibling, 1 reply; 5+ messages in thread
From: Richard Cheng @ 2026-06-07 5:38 UTC (permalink / raw)
To: dave, jonathan.cameron, dave.jiang, alison.schofield,
vishal.l.verma, ira.weiny, dan.j.williams
Cc: linux-cxl, linux-kernel, newtonl, kristinc, kaihengf, kobak,
vaslot, smadhavan, Richard Cheng
CXL r3.2 8.2.4.20.12 and 14.13.10 permit committing on HDM decoder with
size 0. BIOS commits and locks such decoders to burn slots for a Type 3
device in a TSP-established TCB. init_hdm_decoder() rejected them with
-ENXIO and aborted port enumeration, so "cxl list" showed nothing.
Enumerate the decoder with its HW LOCK state instead. Skip the DPA
reservation it does not need, but advance port->hdm_end so a following
committed decoder still passes the in-order check in
__cxl_dpa_reserve(). commit_end can now point at a decoder with no DPA
resource, so poison_by_decoder() must still scan the unmapped DPA tail.
Signed-off-by: Vishal Aslot <vaslot@nvidia.com>
Signed-off-by: Richard Cheng <icheng@nvidia.com>
---
Changelog:
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() .
- 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 | 25 +++++++++++++++++------
drivers/cxl/core/region.c | 42 +++++++++++++++++++--------------------
2 files changed, 40 insertions(+), 27 deletions(-)
diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
index 0c80b76a5f9b..b5fe2b053bc2 100644
--- a/drivers/cxl/core/hdm.c
+++ b/drivers/cxl/core/hdm.c
@@ -1031,13 +1031,17 @@ 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;
+
+ /*
+ * CXL r3.2 8.2.4.20.12 permits committing an HDM decoder with
+ * size 0. Enumerate it into the topology with its HW-reported
+ * LOCK state instead of aborting the port.
+ */
+ if (size == 0)
+ dev_dbg(&port->dev,
+ "decoder%d.%d: Committed with zero size\n",
+ port->id, cxld->id);
} else {
if (cxled) {
struct cxl_memdev *cxlmd = cxled_to_memdev(cxled);
@@ -1096,6 +1100,15 @@ static int init_hdm_decoder(struct cxl_port *port, struct cxl_decoder *cxld,
if (!committed)
return 0;
+ /*
+ * A committed zero-size decoder reserves no DPA, but still advance
+ * the port's DPA watermark.
+ */
+ if (size == 0) {
+ port->hdm_end = cxld->id;
+ return 0;
+ }
+
dpa_size = div_u64_rem(size, cxld->interleave_ways, &remainder);
if (remainder) {
dev_err(&port->dev,
diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index e50dc716d4e8..a353d8e7489d 100644
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -2907,38 +2907,38 @@ static int poison_by_decoder(struct device *dev, void *arg)
return rc;
cxled = to_cxl_endpoint_decoder(dev);
- if (!cxled->dpa_res)
- return rc;
- cxlmd = cxled_to_memdev(cxled);
- cxlds = cxlmd->cxlds;
- mode = cxlds->part[cxled->part].mode;
+ if (cxled->dpa_res) {
+ 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;
ctx->part = cxled->part;
- return 1;
}
+ /* Iterate until commit_end is reached */
+ if (cxled->cxld.id == ctx->port->commit_end)
+ return 1;
+
return 0;
}
--
2.43.0
^ permalink raw reply related [flat|nested] 5+ messages in thread* [PATCH v3 2/2] tools/testing/cxl: Enable zero sized decoder under hb0
2026-06-07 5:38 [PATCH v3 0/2] Support zero-sized HDM decoders Richard Cheng
2026-06-07 5:38 ` [PATCH v3 1/2] cxl/hdm: Allow zero sized " Richard Cheng
@ 2026-06-07 5:38 ` Richard Cheng
1 sibling, 0 replies; 5+ messages in thread
From: Richard Cheng @ 2026-06-07 5:38 UTC (permalink / raw)
To: dave, jonathan.cameron, dave.jiang, alison.schofield,
vishal.l.verma, ira.weiny, dan.j.williams
Cc: linux-cxl, linux-kernel, newtonl, kristinc, kaihengf, kobak,
vaslot, smadhavan, Richard Cheng
The kernel now allows committed HDM decoders of zero size so BIOS can
burn slots with LOCK, cxl_test needs to exercise the patch.
Add a mock_zero_size_decoders module param, When set, the special
endpoint 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. commit_end then lands on a decoder
with no DPA resource, exercising the new enumeration and
poison-by-endpoint paths.
Signed-off-by: Vishal Aslot <vaslot@nvidia.com>
Signed-off-by: Richard Cheng <icheng@nvidia.com>
---
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 | 83 +++++++++++++++++++++++++++++++++---
1 file changed, 77 insertions(+), 6 deletions(-)
diff --git a/tools/testing/cxl/test/cxl.c b/tools/testing/cxl/test/cxl.c
index 296516eecfd6..190cb18d6932 100644
--- a/tools/testing/cxl/test/cxl.c
+++ b/tools/testing/cxl/test/cxl.c
@@ -17,6 +17,7 @@
static int interleave_arithmetic;
static bool extended_linear_cache;
static bool fail_autoassemble;
+static bool mock_zero_size_decoders;
#define FAKE_QTG_ID 42
@@ -1041,16 +1042,47 @@ 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 BIOS-burnt slot: a committed, locked, zero-size decoder
+ * (CXL r3.2 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_MANUAL;
+ 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;
}
/*
@@ -1131,7 +1163,7 @@ static bool mock_init_hdm_decoder(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)) {
default_mock_decoder(cxld);
return false;
}
@@ -1145,6 +1177,20 @@ static bool mock_init_hdm_decoder(struct cxl_decoder *cxld)
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 BIOS-burnt zero-size + locked slots above the
+ * decoder[0] auto-region (CXL r3.2 8.2.4.20.12). commit_end then
+ * points at a decoder with no DPA resource, exercising the
+ * zero-size enumeration 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 false;
+ }
cxld->hpa_range = (struct range) {
.start = base,
.end = base + mock_auto_region_size - 1,
@@ -1168,9 +1214,11 @@ static bool mock_init_hdm_decoder(struct cxl_decoder *cxld)
*/
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.
@@ -1214,6 +1262,26 @@ static bool mock_init_hdm_decoder(struct cxl_decoder *cxld)
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 burnt-slot 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);
+ }
}
return false;
@@ -2030,6 +2098,9 @@ module_param(extended_linear_cache, bool, 0444);
MODULE_PARM_DESC(extended_linear_cache, "Enable extended linear cache support");
module_param(fail_autoassemble, bool, 0444);
MODULE_PARM_DESC(fail_autoassemble, "Simulate missing member of an auto-region");
+module_param(mock_zero_size_decoders, bool, 0444);
+MODULE_PARM_DESC(mock_zero_size_decoders,
+ "Mock BIOS-burnt committed zero-size locked decoders under host-bridge0");
module_init(cxl_test_init);
module_exit(cxl_test_exit);
MODULE_LICENSE("GPL v2");
--
2.43.0
^ permalink raw reply related [flat|nested] 5+ messages in thread