Re: [PATCH] cxl/port: Enable the HDM decoder capability for switch ports

[Ira Weiny <ira.weiny@intel.com>]

Dan Williams wrote:
> Derick noticed, when testing hot plug, that hot-add behaves nominally
> after a removal. However, if the hot-add is done without a prior
> removal, CXL.mem accesses fail. It turns out that the original
> implementation of the port driver and region programming wrongly assumed
> that platform-firmware always enables the host-bridge HDM decoder
> capability. Add support turning on switch-level HDM decoders in the case
> where platform-firmware has not.
> 
> The implementation is careful to only arrange for the enable to be
> undone if the current instance of the driver was the one that did the
> enable. This is to interoperate with platform-firmware that may expect
> CXL.mem to remain active after the driver is shutdown. This comes at the
> cost of potentially not shutting down the enable on kexec flows, but it
> is mitigated by the fact that the related HDM decoders still need to be
> enabled on an individual basis.
> 
> Cc: <stable@vger.kernel.org>
> Reported-by: Derick Marks <derick.w.marks@intel.com>
> Fixes: 54cdbf845cf7 ("cxl/port: Add a driver for 'struct cxl_port' objects")
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>

Reviewed-by: Ira Weiny <ira.weiny@intel.com>