Re: [PATCH v3 07/11] cxl: Introduce callback to translate a decoder's HPA to the next parent port

[Dave Jiang <dave.jiang@intel.com>]

On 9/12/25 7:45 AM, Robert Richter wrote:
> To enable address translation, the endpoint decoder's HPA range must
> be translated when crossing memory domains to the next parent port's
> address ranges up to the root port. The root port's HPA range is
> equivalent to the system's SPA range.
> 
> Introduce a callback to translate an address of the decoder's HPA
> range to the address range of the parent port. The callback can be set
> for ports that need to handle address translation.
> 
> Reviewed-by: Gregory Price <gourry@gourry.net>
> Signed-off-by: Robert Richter <rrichter@amd.com>
> ---
>  drivers/cxl/cxl.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> index f182982f1c14..eb837867d932 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -429,6 +429,17 @@ struct cxl_rd_ops {
>  	u64 (*spa_to_hpa)(struct cxl_root_decoder *cxlrd, u64 spa);
>  };
>  
> +/**
> + * cxl_to_hpa_fn - type of a callback function to translate an HPA
> + * @cxld: cxl_decoder to translate from
> + * @hpa: HPA of the @cxld decoder's address range
> + *
> + * The callback translates a decoder's HPA to the address range of the
> + * decoder's parent port. The return value is the translated HPA on
> + * success or ULLONG_MAX otherwise.
> + */
> +typedef u64 (*cxl_to_hpa_fn)(struct cxl_decoder *cxld, u64 hpa);
> +
>  /**
>   * struct cxl_root_decoder - Static platform CXL address decoder
>   * @res: host / parent resource for region allocations
> @@ -599,6 +610,7 @@ struct cxl_dax_region {
>   * @parent_dport: dport that points to this port in the parent
>   * @decoder_ida: allocator for decoder ids
>   * @reg_map: component and ras register mapping parameters
> + * @to_hpa: Callback to translate a child port's decoder address to the port's HPA address range
>   * @nr_dports: number of entries in @dports
>   * @hdm_end: track last allocated HDM decoder instance for allocation ordering
>   * @commit_end: cursor to track highest committed decoder for commit ordering
> @@ -619,6 +631,7 @@ struct cxl_port {
>  	struct cxl_dport *parent_dport;
>  	struct ida decoder_ida;
>  	struct cxl_register_map reg_map;
> +	cxl_to_hpa_fn to_hpa;

The more I look at this, the more I feel the callback should be part of 'struct cxl_rd_ops' and not under each port. While this provides flexibility in a general case if there is a need to translate at each level, the actual use case in the field today only requires translation at the top as far as I can tell. And the translate functionality should be part of a decoder and not a port. And in this case, the root decoder would suffice.

DJ 

>  	int nr_dports;
>  	int hdm_end;
>  	int commit_end;