cxl/region: Flag partially torn down regions as unusable

cxl_region_decode_reset() walks all the decoders associated with a given
region and disables them. Due to decoder ordering rules it is possible
that a switch in the topology notices that a given decoder can not be
shutdown before another region with a higher HPA is shutdown first. That
can leave the region in a partially committed state.

Capture that state in a new CXL_REGION_F_NEEDS_RESET flag and require
that a successful cxl_region_decode_reset() attempt must be completed
before cxl_region_probe() accepts the region.

This is a corollary for the bug that Jonathan identified in "CXL/region
:  commit reset of out of order region appears to succeed." [1].

Cc: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Link: http://lore.kernel.org/r/20230316171441.0000205b@Huawei.com [1]
Fixes: 176baef ("cxl/hdm: Commit decoder state to hardware")
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/168696507423.3590522.16254212607926684429.stgit@dwillia2-xfh.jf.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

Author: djbw

drivers/cxl/core/region.c

@@ -182,14 +182,19 @@ static int cxl_region_decode_reset(struct cxl_region *cxlr, int count)
 				rc = cxld->reset(cxld);
 			if (rc)
 				return rc;
+			set_bit(CXL_REGION_F_NEEDS_RESET, &cxlr->flags);
 		}
 
 endpoint_reset:
 		rc = cxled->cxld.reset(&cxled->cxld);
 		if (rc)
 			return rc;
+		set_bit(CXL_REGION_F_NEEDS_RESET, &cxlr->flags);
 	}
 
+	/* all decoders associated with this region have been torn down */
+	clear_bit(CXL_REGION_F_NEEDS_RESET, &cxlr->flags);
+
 	return 0;
 }
 
@@ -2864,6 +2869,13 @@ static int cxl_region_probe(struct device *dev)
 		goto out;
 	}
 
+	if (test_bit(CXL_REGION_F_NEEDS_RESET, &cxlr->flags)) {
+		dev_err(&cxlr->dev,
+			"failed to activate, re-commit region and retry\n");
+		rc = -ENXIO;
+		goto out;
+	}
+
 	/*
 	 * From this point on any path that changes the region's state away from
 	 * CXL_CONFIG_COMMIT is also responsible for releasing the driver.

drivers/cxl/cxl.h

@@ -469,6 +469,14 @@ struct cxl_region_params {
  */
 #define CXL_REGION_F_AUTO 0
 
+/*
+ * Require that a committed region successfully complete a teardown once
+ * any of its associated decoders have been torn down. This maintains
+ * the commit state for the region since there are committed decoders,
+ * but blocks cxl_region_probe().
+ */
+#define CXL_REGION_F_NEEDS_RESET 1
+
 /**
  * struct cxl_region - CXL region
  * @dev: This region's device