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

[Dave Jiang]

On 3/18/26 6:00 AM, 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@xxxxxxxxxxxx>
> ---
>  drivers/cxl/cxl.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> index 9b947286eb9b..466b8eeefed7 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -77,7 +77,12 @@ 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;
> +	if (!val)
> +		return 1;
> +	else if (val <= 8)
> +		return val * 2;
> +	else
> +		return 4 * (val - 4);

That looks a bit messy. How about:

switch (val) {
case 0:
	return 1;
case 1..8:
	return val * 2;
case 9..12:
	return val 4 * (val - 4);
default:
	return -ENXIO;
}

DJ

>  }
>  
>  /* Encode defined in CXL 2.0 8.2.5.12.7 HDM Decoder Control Register */
> 
> ---
> base-commit: f338e77383789c0cae23ca3d48adcc5e9e137e3c
> change-id: 20260318-add_support_for_32_decoders-4a7c77949a41
> 
> Best regards,