[ndctl PATCH v2] test/cxl-region-sysfs.sh: test a repeat write of a region target

[Alison Schofield <alison.schofield@intel.com>]

A CXL driver fix making its way upstream[1], makes a repeat write of an
endpoint decoder to the region targetN attribute that already holds it
succeed without changing the region. Without the fix, that write fails,
leaves the region half detached from the endpoint's ports, and a later
write can crash the kernel.

Add a test case to backstop that fix. Write target0 twice on a partially
populated region and check that the second write succeeds and leaves the
region's decoders attached.

[1] https://lore.kernel.org/linux-cxl/920d80735a449f93a1f17a8bca4c25e4b302cb42.1792989098.git.alison.schofield@intel.com/ 

Signed-off-by: Alison Schofield <alison.schofield@intel.com>
---

Changes since v1:
- Expect a repeat write to succeed, as the v2 kernel fix makes it a no-op


 test/cxl-region-sysfs.sh | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/test/cxl-region-sysfs.sh b/test/cxl-region-sysfs.sh
index db1a163efb30..cb5e6ba53e71 100644
--- a/test/cxl-region-sysfs.sh
+++ b/test/cxl-region-sysfs.sh
@@ -87,9 +87,19 @@ do
 						     select(.mode == \"pmem\") | .decoder")
 done
 
-# attach all endpoint decoders to the region
-pos=0
-for i in ${endpoint[@]}
+# a repeat write of an attached endpoint decoder must succeed and leave
+# every decoder between it and the root attached
+echo "${endpoint[0]}" > "/sys/bus/cxl/devices/$region/target0"
+nr_attached=$(cat /sys/bus/cxl/devices/decoder*/region | grep -cx "$region")
+if ! echo "${endpoint[0]}" > "/sys/bus/cxl/devices/$region/target0"; then
+	err "$LINENO: repeat write of ${endpoint[0]} to target0 failed"
+fi
+nr=$(cat /sys/bus/cxl/devices/decoder*/region | grep -cx "$region")
+[ "$nr" -ne "$nr_attached" ] && err "$LINENO: $nr of $nr_attached decoders still attached"
+
+# attach the remaining endpoint decoders to the region
+pos=1
+for i in ${endpoint[@]:1}
 do
 	echo $i > /sys/bus/cxl/devices/$region/target$pos
 	pos=$((pos+1))
-- 
2.37.3