Re: [RFC PATCH] cxl/region: Calculate peer distance only for reused targets

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 15 Oct 2026 17:40:59 +0800
Richard Cheng <icheng@nvidia.com> wrote:

> This patch depends on Alison Schofield's v3 "cxl/region: Refactor
> cxl_port_setup_targets()" series [1], this is inspired from that serie

series is a an odd word in English. Singular and plural are the same.

> as well, since her serie contains no functional change, I pull this one
> out as a standalone patch.
> 
> cxl_switch_port_setup_targets() calculates peer distance before
> searching the existing targets. When the endpoint needs a new target,
> the calculated distance is never used.
> 
> Move cxl_port_peer_distance() into the matching-target branch,
> immediately before check_last_peer(). This avoids unnecessary ancestor
> walks and per-port region lookups when adding distinct targets.
> 
> [1]:
> https://lore.kernel.org/linux-cxl/cover.1791860305.git.alison.schofield@intel.com/

Use a link tag.

> Signed-off-by: Richard Cheng <icheng@nvidia.com>
> ---
> I don't have available HW for multi-way interleaving setup, so I can't
> exercise the case optimized here. I would appreciate if someone is
> williing to validate the actual benefit of this.

Maybe give it a spin on qemu.  But it looks like it's just a code reorder
effectively so It would be very odd if it is a problem.

I'd just put this as a comment on Alison's series as I don't think
that has merged yet.

Seems like a nice little optimization.


> 
> Best regards,
> Richard Cheng.
> ---
>  drivers/cxl/core/region.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 50f4830c225d..efd173c57dad 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -1666,10 +1666,10 @@ static int cxl_switch_port_setup_targets(struct cxl_port *port, struct cxl_regio
>  
>  	cxlsd = to_cxl_switch_decoder(&cxld->dev);
>  	if (cxl_rr->nr_targets_set) {
> -		int distance = cxl_port_peer_distance(port, cxlr);
> -
>  		for (int i = 0; i < cxl_rr->nr_targets_set; i++)
>  			if (ep->dport == cxlsd->target[i]) {
> +				int distance = cxl_port_peer_distance(port, cxlr);
> +
>  				rc = check_last_peer(cxled, ep, cxl_rr,
>  						     distance);
>  				if (rc)