Re: [PATCH v4 4/6] cxl/region: Support mixed-granularity user created regions

On Thu, 20 Aug 2026 16:31:22 -0700
Alison Schofield <[email protected]> wrote:

> User region creation currently requires an interleaving root to have
> the same granularity as the region.
> 
> That rejects valid mixed-granularity layouts where the region
> granularity is finer than the root granularity. 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 <[email protected]>

Reviewed-by: Jonathan Cameron <[email protected]>

> ---
>  drivers/cxl/core/region.c | 11 ++---------
>  1 file changed, 2 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 637d1b60a570..4bbfb7cd7e01 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -571,15 +571,8 @@ 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)
>  		return -EINVAL;
>  
>  	lockdep_assert_held_write(&cxl_rwsem.region);