cxl/hdm: Fix skip allocations vs multiple pmem allocations

Vishal notes that when attempting to define a second pmem region on a
device the DPA allocation fails with a message of the form:

    decoder11.1: failed to reserve skipped space

Recall that the skip setting is used when there is a pmem allocation in
the presence of free ram DPA space. The first pmem allocation skips over
the free ram and subsequent pmem allocations do not require a skip. The
bug is that a skip is still attempted and the DPA reservation code
flags the double skip allocation conflict.

Fixes: cf880423 ("cxl/hdm: Add support for allocating DPA to an endpoint decoder")
Reported-by: Vishal Verma <vishal.l.verma@intel.com>
Tested-by: Vishal Verma <vishal.l.verma@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/165973754730.1558392.15466392461645857658.stgit@dwillia2-xfh.jf.intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/cxl/core/hdm.c

@@ -445,7 +445,16 @@ int cxl_dpa_alloc(struct cxl_endpoint_decoder *cxled, unsigned long long size)
 		start = free_pmem_start;
 		avail = cxlds->pmem_res.end - start + 1;
 		skip_start = free_ram_start;
-		skip_end = start - 1;
+
+		/*
+		 * If some pmem is already allocated, then that allocation
+		 * already handled the skip.
+		 */
+		if (cxlds->pmem_res.child &&
+		    skip_start == cxlds->pmem_res.child->start)
+			skip_end = skip_start - 1;
+		else
+			skip_end = start - 1;
 		skip = skip_end - skip_start + 1;
 	} else {
 		dev_dbg(dev, "mode not set\n");