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

[Alison Schofield]

On Thu, Oct 15, 2026 at 05:40:59PM +0800, Richard Cheng wrote:
> This patch depends on Alison Schofield's v3 "cxl/region: Refactor
> cxl_port_setup_targets()" series [1], this is inspired from that serie
> 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@xxxxxxxxx/
> Signed-off-by: Richard Cheng <icheng@xxxxxxxxxx>
> ---
> 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.

Nice patch! I've picked it up into v4 of the refactor set.

I tested it with using the cxl unit test cxl-region-sysfs.sh and this
at the end of cxl_port_peer_distance():

dev_dbg(&cxlr->dev, "%s:%s: peer distance %d\n",
        dev_name(port->uport_dev), dev_name(&port->dev), distance);

The x8 region went from 10 calls to 4. The eliminated calls were for
new downstream targets and the remaining calls were for reused
targets where check_last_peer() needs the distance.

The x2 auto region also went from 2 calls to 1.

I tested the peer validation path by swapping endpoints at positions
0 and 2 in cxl-region-sysfs.sh. The mismatched peer was detected and
the region rejected.

-- Alison

> 
> 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)
> -- 
> 2.43.0
>