cxl/region: Fix null pointer dereference for resetting decoder

Not all decoders have a reset callback.

The CXL specification allows a host bridge with a single root port to
have no explicit HDM decoders. Currently the region driver assumes there
are none.  As such the CXL core creates a special pass through decoder
instance without a commit/reset callback.

Prior to this patch, the ->reset() callback was called unconditionally when
calling cxl_region_decode_reset. Thus a configuration with 1 Host Bridge,
1 Root Port, and one directly attached CXL type 3 device or multiple CXL
type 3 devices attached to downstream ports of a switch can cause a null
pointer dereference.

Before the fix, a kernel crash was observed when we destroy the region, and
a pass through decoder is reset.

The issue can be reproduced as below,
    1) create a region with a CXL setup which includes a HB with a
    single root port under which a memdev is attached directly.
    2) destroy the region with cxl destroy-region regionX -f.

Fixes: 176baefb2eb5 ("cxl/hdm: Commit decoder state to hardware")
Cc: <[email protected]>
Signed-off-by: Fan Ni <[email protected]>
Reviewed-by: Davidlohr Bueso <[email protected]>
Reviewed-by: Dave Jiang <[email protected]>
Reviewed-by: Jonathan Cameron <[email protected]>
Tested-by: Gregory Price <[email protected]>
Reviewed-by: Gregory Price <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Dan Williams <[email protected]>

0 files changed, 0 insertions, 0 deletions