Re: [PATCH v4 2/2] tools/testing/cxl: Enable zero sized decoder under hb0

[Alison Schofield]

On Mon, Jun 22, 2026 at 06:19:00PM +0800, Richard Cheng wrote:
> On Fri, Jun 12, 2026 at 11:16:00AM +0800, Alison Schofield wrote:
> > On Sun, Jun 07, 2026 at 04:13:45PM +0800, Richard Cheng wrote:
> > > The kernel now allows committed HDM decoders of zero size so BIOS can
> > > burn slots with LOCK; cxl_test needs to exercise that path.
> > > 
> > > Add a mock_zero_size_decoders module parameter (default off). When set,
> > > the special endpoints under host-bridge0 (cxl_mem.0 and cxl_mem.4) commit
> > > decoders 1 and 2 as zero-size + locked above the decoder[0] auto-region,
> > > mirrored on the parent switch and host bridge. commit_end then lands on a
> > > decoder with no DPA resource, exercising the new enumeration and
> > > poison-by-endpoint paths.
> > > 
> > > Gating keeps the default topology, shared by the rest of the cxl suite,
> > > unchanged.
> > 
> > Can you add a unit test case for this?  I think it makes sense in
> > cxl-topology.sh. Perhaps before or after the work on the default
> > cxl-test topology, load this new topo up and validate the zero size
> > decoders are as expected.
> >
> 
> Hi Alison,
> 
> No problem for that, but I want to make sure you are talking about
> adding test in ndctl right?
> 
> I'll send v5 within this week, want to make sure they are ready
> together.

Yes, I was thinking updates to cxl-topology.sh and now looking more
at this version, cxl-poison.sh.

I only suggest appending to cxl-topology.sh because I don't see 
any other 'work' that you want to do and verify with the zero size
decoders. If there is more work to be done, than add a dedicated
unit test.

The verification for the tail mappings appends nicely in cxl-poison.sh
but frankly, I did that before I looked at changes suggest for v5.
So take the appended diff suggestion as possibly useful, but maybe
out of date:

diff --git a/test/cxl-poison.sh b/test/cxl-poison.sh
index 49aa1b68c5c1..b40c896a0903 100644
--- a/test/cxl-poison.sh
+++ b/test/cxl-poison.sh
@@ -361,6 +361,16 @@ if check_min_kver "7.0"; then
        run_unaligned_poison_test
 fi

+# Assert poison in the unmapped DPA tail is still collected when a
+# committed zero-size decoder sits at port->commit_end.
+if modinfo cxl_test | grep -q '^parm:.*mock_zero_size_decoders'; then
+       modprobe -r cxl_test
+       modprobe cxl_test mock_zero_size_decoders=1
+
+       rc=1
+       test_poison_by_memdev_by_dpa
+fi
+
 check_dmesg "$LINENO"

 modprobe -r cxl_test
diff --git a/test/cxl-topology.sh b/test/cxl-topology.sh
index 170c9caf840b..aee675d3b7fc 100644
--- a/test/cxl-topology.sh
+++ b/test/cxl-topology.sh
@@ -269,10 +269,46 @@ do
        ((count == 4)) || err "$LINENO"
 done

-
 # validate that the bus can be disabled without issue
 $CXL disable-bus $root -f

+# With mock_zero_size_decoders=1 the host-bridge0 endpoints that back the
+# auto-region commit two extra decoders (ids 1 and 2) above decoder0 as
+# zero-size and locked, mocking BIOS-burnt slots. Validate that enumeration
+# does not abort the port and that those slots appear with size 0 and the
+# hardware LOCK bit set.
+test_zero_size_decoders()
+{
+       local region target_decoder base burnt size locked
+
+       region=$("$CXL" list -R | jq -r ".[0].region")
+       [[ -n "$region" && "$region" != "null" ]] || err "$LINENO"
+
+       # Each auto-region target is a special host-bridge0 endpoint. Its
+       # decoder0 backs the region; decoders 1 and 2 are the burnt slots.
+       for target_decoder in $("$CXL" list -r "$region" --targets |
+               jq -r ".[0].mappings[].decoder"); do
+               [[ "$target_decoder" =~ ^(decoder[0-9]+)\.0$ ]] || err "$LINENO"
+               base="${BASH_REMATCH[1]}"
+
+               for id in 1 2; do
+                       burnt="/sys/bus/cxl/devices/${base}.${id}"
+                       [ -d "$burnt" ] || err "$LINENO"
+                       size=$(cat "$burnt/size")
+                       locked=$(cat "$burnt/locked")
+                       ((size == 0)) || err "$LINENO"
+                       ((locked == 1)) || err "$LINENO"
+               done
+       done
+}
+
+if modinfo cxl_test | grep -q '^parm:.*mock_zero_size_decoders'; then
+       modprobe -r cxl_test
+       modprobe cxl_test mock_zero_size_decoders=1
+
+       test_zero_size_decoders
+fi
+
 check_dmesg "$LINENO"

 modprobe -r cxl_test

> 
> Best regards,
> Richard Cheng.