cxl/region: Clarify when a cxld->commit() callback is mandatory

Both cxl_switch_decoders() and cxl_endpoint_decoders() are considered by
cxl_region_decode_commit(). Flag cases where cxl_switch_decoders with
multiple targets, or cxl_endpoint_decoders do not have a commit callback
set. The switch case is unlikely to happen since switches are only
enumerated by the CXL core, but the endpoint case may support decoders
defined by drivers outside of drivers/cxl, like accerator drivers.
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/167124081824.1626103.1555704405392757219.stgit@dwillia2-xfh.jf.intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/cxl/core/region.c

@@ -156,6 +156,22 @@ static int cxl_region_decode_reset(struct cxl_region *cxlr, int count)
 	return 0;
 }
 
+static int commit_decoder(struct cxl_decoder *cxld)
+{
+	struct cxl_switch_decoder *cxlsd = NULL;
+
+	if (cxld->commit)
+		return cxld->commit(cxld);
+
+	if (is_switch_decoder(&cxld->dev))
+		cxlsd = to_cxl_switch_decoder(&cxld->dev);
+
+	if (dev_WARN_ONCE(&cxld->dev, !cxlsd || cxlsd->nr_targets > 1,
+			  "->commit() is required\n"))
+		return -ENXIO;
+	return 0;
+}
+
 static int cxl_region_decode_commit(struct cxl_region *cxlr)
 {
 	struct cxl_region_params *p = &cxlr->params;
@@ -174,8 +190,7 @@ static int cxl_region_decode_commit(struct cxl_region *cxlr)
 		     iter = to_cxl_port(iter->dev.parent)) {
 			cxl_rr = cxl_rr_load(iter, cxlr);
 			cxld = cxl_rr->decoder;
-			if (cxld->commit)
-				rc = cxld->commit(cxld);
+			rc = commit_decoder(cxld);
 			if (rc)
 				break;
 		}