Re: [PATCH v2 1/1] cxl/hdm: Add support for 32 switch decoders

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

On 3/20/26 11:14 PM, Li Ming wrote:
> Per CXL r4.0 section 8.2.4.20.1. CXL host bridge and switch ports can
> support 32 HDM decoders. Current implementation misses some decoders on
> CXL host bridge and switch in the case that the value of Decoder Count
> field in CXL HDM decoder Capability Register is greater than or equal to
> 9.
> 
> Update calculation implementation to ensure the decoder count calculation
> is correct for CXL host bridge/switch ports.
> 
> Signed-off-by: Li Ming <ming.li@zohomail.com>

Reviewed-by: Dave Jiang <dave.jiang@intel.com>


> ---
> Changes from v1:
> - Return -ENXIO if the value violates SPEC. (Dave & Alison)
> ---
>  drivers/cxl/core/hdm.c |  2 +-
>  drivers/cxl/cxl.h      | 11 ++++++++++-
>  drivers/cxl/cxlmem.h   |  2 +-
>  3 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
> index c222e98ae736..3930e130d6b6 100644
> --- a/drivers/cxl/core/hdm.c
> +++ b/drivers/cxl/core/hdm.c
> @@ -177,7 +177,7 @@ static struct cxl_hdm *devm_cxl_setup_hdm(struct cxl_port *port,
>  	}
>  
>  	parse_hdm_decoder_caps(cxlhdm);
> -	if (cxlhdm->decoder_count == 0) {
> +	if (cxlhdm->decoder_count < 0) {
>  		dev_err(dev, "Spec violation. Caps invalid\n");
>  		return ERR_PTR(-ENXIO);
>  	}
> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> index 9b947286eb9b..0a5301049cf3 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -77,7 +77,16 @@ static inline int cxl_hdm_decoder_count(u32 cap_hdr)
>  {
>  	int val = FIELD_GET(CXL_HDM_DECODER_COUNT_MASK, cap_hdr);
>  
> -	return val ? val * 2 : 1;
> +	switch (val) {
> +	case 0:
> +		return 1;
> +	case 1 ... 8:
> +		return val * 2;
> +	case 9 ... 12:
> +		return (val - 4) * 4;
> +	default:
> +		return -ENXIO;
> +	}
>  }
>  
>  /* Encode defined in CXL 2.0 8.2.5.12.7 HDM Decoder Control Register */
> diff --git a/drivers/cxl/cxlmem.h b/drivers/cxl/cxlmem.h
> index e21d744d639b..399b150b404c 100644
> --- a/drivers/cxl/cxlmem.h
> +++ b/drivers/cxl/cxlmem.h
> @@ -923,7 +923,7 @@ int cxl_mem_sanitize(struct cxl_memdev *cxlmd, u16 cmd);
>   */
>  struct cxl_hdm {
>  	struct cxl_component_regs regs;
> -	unsigned int decoder_count;
> +	int decoder_count;
>  	unsigned int target_count;
>  	unsigned int interleave_mask;
>  	unsigned long iw_cap_mask;