[PATCH v3 1/2] cxl/hdm: Allow zero sized HDM decoders

[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@xxxxxxxxxx>
Signed-off-by: Richard Cheng <icheng@xxxxxxxxxx>
---
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