Re: [PATCH] cxl: Fix cxl_endpoint_get_perf_coordinate() support for RCH

[Dan Williams <dan.j.williams@intel.com>]

Dave Jiang wrote:
> Robert reported the following when booting a CXL host with Restricted CXL
> Host (RCH) topology:
>  [   39.815379] cxl_acpi ACPI0017:00: not a cxl_port device
>  [   39.827123] WARNING: CPU: 46 PID: 1754 at drivers/cxl/core/port.c:592 to_cxl_port+0x56/0x70 [cxl_core]
> 
> ... plus some related subsequent NULL pointer dereference:
> 
>  [   40.718708] BUG: kernel NULL pointer dereference, address: 00000000000002d8
> 
> The iterator to walk the PCIe path did not account for RCH topology.
> However RCH does not support hotplug and the memory exported by the
> Restricted CXL Device (RCD) should be covered by HMAT and therefore no
> access_coordinate is needed. Add check to see if the endpoint device is
> RCD and skip calculation.
> 
> Also add a call to cxl_endpoint_get_perf_coordinates() in cxl_test in order
> to exercise the topology iterator. The dev_is_pci() check added is to help
> with this test and should be harmless for normal operation.
> 
> Reported-by: Robert Richter <rrichter@amd.com>
> Closes: https://lore.kernel.org/all/Ziv8GfSMSbvlBB0h@rric.localdomain/
> Fixes: 592780b8391f ("cxl: Fix retrieving of access_coordinates in PCIe path")
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> ---
> 
> Hi Robert,
> Can you please try this patch and see if it addresses the issue you saw
> on your RCH platform? Thanks!

Definitely wait for that test result, but as for the patch and the
approach it looks good to me. It matches other RCH-topology-walk skips
like the one in devm_cxl_enumerate_ports().

Reviewed-by: Dan Williams <dan.j.williams@intel.com>

Now, that said, if someone can think of why the driver needs to do
dynamic performance enumeration for RCH (i.e. a platform BIOS does not
populate HMAT), they should speak up now.