Re: [PATCH v5 6/7] cxl/region: Support mixed-granularity user created regions

[Robert Richter <rrichter@amd.com>]

On 03.09.26 16:23:48, Alison Schofield wrote:
> User region creation currently requires an interleaving root to have the
> same granularity as the region.
> 
> That rejects mixed-granularity layouts, those where the region granularity
> is finer than the granularity of the interleaving root decoder. A region
> granularity coarser than the root remains invalid because it produces
> incorrect DPA translations.
> 
> Allow a region granularity finer than an interleaving root and continue to
> reject a coarser granularity. Validate the complete ways/granularity
> relationship when the region endpoints are attached.
> 
> For example, CXL 4.0 Section 9.13.1.1 Table 9-7 describes a 6-way region at
> IGB across three host bridges as a 3-way root interleave at 2 * IGB with a
> 2-way interleave at IGB below it.
> 
> Signed-off-by: Alison Schofield <alison.schofield@intel.com>
> Reviewed-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>
> ---
>  drivers/cxl/core/region.c | 15 ++++++---------
>  1 file changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 5b7104a55fdd..374d8c2c32d1 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -572,16 +572,13 @@ static int set_interleave_granularity(struct cxl_region *cxlr, int val)
>  	if (rc)
>  		return rc;
>  
> -	/*
> -	 * When the host-bridge is interleaved, disallow region granularity !=
> -	 * root granularity. Regions with a granularity less than the root
> -	 * interleave result in needing multiple endpoints to support a single
> -	 * slot in the interleave (possible to support in the future). Regions
> -	 * with a granularity greater than the root interleave result in invalid
> -	 * DPA translations (invalid to support).
> -	 */
> -	if (cxld->interleave_ways > 1 && val != cxld->interleave_granularity)
> +	/* Region granularity must not be coarser than an interleaving root's */
> +	if (cxld->interleave_ways > 1 && val > cxld->interleave_granularity) {
> +		dev_dbg(&cxlr->dev,
> +			"granularity %d exceeds root decoder granularity %d\n",
> +			val, cxld->interleave_granularity);

The spec only restricts that for Mod3 configs. Else, there is no
driver support. Could that added as a comment?

>  		return -EINVAL;
> +	}
>  
>  	lockdep_assert_held_write(&cxl_rwsem.region);
>  
> -- 
> 2.37.3
>