Subject: Re: [PATCH v23 12/22] cxl: Define a driver interface for HPA free space enumeration

From: Dave Jiang <[email-masked]>
Date: Fri, 20 Feb 2026 08:42:46 -0700

On 2/1/26 8:54 AM, [email-masked] wrote:
> From: Alejandro Lucero <[email-masked]>
> 
> CXL region creation involves allocating capacity from Device Physical
> Address (DPA) and assigning it to decode a given Host Physical Address
> (HPA). Before determining how much DPA to allocate the amount of available
> HPA must be determined. Also, not all HPA is created equal, some HPA
> targets RAM, some targets PMEM, some is prepared for device-memory flows
> like HDM-D and HDM-DB, and some is HDM-H (host-only).
> 
> In order to support Type2 CXL devices, wrap all of those concerns into
> an API that retrieves a root decoder (platform CXL window) that fits the
> specified constraints and the capacity available for a new region.
> 
> Add a complementary function for releasing the reference to such root
> decoder.
> 
> Based on https://lore.kernel.org/linux-cxl/[email-masked]/
> 
> Signed-off-by: Alejandro Lucero <[email-masked]>
> Reviewed-by: Jonathan Cameron <[email-masked]>
> ---
>  drivers/cxl/core/region.c | 164 ++++++++++++++++++++++++++++++++++++++
>  drivers/cxl/cxl.h         |   3 +
>  include/cxl/cxl.h         |   6 ++
>  3 files changed, 173 insertions(+)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 954b8fcdbac6..bdefd088f5f1 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -705,6 +705,170 @@ static int free_hpa(struct cxl_region *cxlr)
>  	return 0;
>  }
>  
> +struct cxlrd_max_context {
> +	struct device * const *host_bridges;
> +	int interleave_ways;
> +	unsigned long flags;
> +	resource_size_t max_hpa;
> +	struct cxl_root_decoder *cxlrd;
> +};
> +
> +static int find_max_hpa(struct device *dev, void *data)
> +{
> +	struct cxlrd_max_context *ctx = data;
> +	struct cxl_switch_decoder *cxlsd;
> +	struct cxl_root_decoder *cxlrd;
> +	struct resource *res, *prev;
> +	struct cxl_decoder *cxld;
> +	resource_size_t free = 0;
> +	resource_size_t max;
> +	int found = 0;
> +
> +	if (!is_root_decoder(dev))
> +		return 0;
> +
> +	cxlrd = to_cxl_root_decoder(dev);
> +	cxlsd = &cxlrd->cxlsd;
> +	cxld = &cxlsd->cxld;
> +
> +	if ((cxld->flags & ctx->flags) != ctx->flags) {
> +		dev_dbg(dev, "flags not matching: %08lx vs %08lx\n",
> +			cxld->flags, ctx->flags);
> +		return 0;
> +	}
> +
> +	for (int i = 0; i < ctx->interleave_ways; i++) {
> +		for (int j = 0; j < ctx->interleave_ways; j++) {
> +			if (ctx->host_bridges[i] == cxlsd->target[j]->dport_dev) {
> +				found++;
> +				break;
> +			}
> +		}
> +	}
> +
> +	if (found != ctx->interleave_ways) {
> +		dev_dbg(dev,
> +			"Not enough host bridges. Found %d for %d interleave ways requested\n",
> +			found, ctx->interleave_ways);
> +		return 0;
> +	}
> +
> +	/*
> +	 * Walk the root decoder resource range relying on cxl_rwsem.region to
> +	 * preclude sibling arrival/departure and find the largest free space
> +	 * gap.
> +	 */
> +	lockdep_assert_held_read(&cxl_rwsem.region);
> +	res = cxlrd->res->child;
> +
> +	/* With no resource child the whole parent resource is available */
> +	if (!res)
> +		max = resource_size(cxlrd->res);
> +	else
> +		max = 0;
> +
> +	for (prev = NULL; res; prev = res, res = res->sibling) {
> +		if (!prev && res->start == cxlrd->res->start &&
> +		    res->end == cxlrd->res->end) {
> +			max = resource_size(cxlrd->res);
> +			break;
> +		}

Can this block be pulled out of the for loop so it only needs to run once?

> +		/*
> +		 * Sanity check for preventing arithmetic problems below as a
> +		 * resource with size 0 could imply using the end field below
> +		 * when set to unsigned zero - 1 or all f in hex.
> +		 */
> +		if (prev && !resource_size(prev))
> +			continue;
> +
> +		if (!prev && res->start > cxlrd->res->start) {
> +			free = res->start - cxlrd->res->start;
> +			max = max(free, max);
> +		}
> +		if (prev && res->start > prev->end + 1) {
> +			free = res->start - prev->end + 1;
> +			max = max(free, max);
> +		}
> +	}
> +
> +	if (prev && prev->end + 1 < cxlrd->res->end + 1) {
> +		free = cxlrd->res->end + 1 - prev->end + 1;
> +		max = max(free, max);
> +	}
> +
> +	dev_dbg(cxlrd_dev(cxlrd), "found %pa bytes of free space\n", &max);
> +	if (max > ctx->max_hpa) {
> +		if (ctx->cxlrd)
> +			put_device(cxlrd_dev(ctx->cxlrd));
> +		get_device(cxlrd_dev(cxlrd));
> +		ctx->cxlrd = cxlrd;
> +		ctx->max_hpa = max;

Is there any chance that ctx->cxlrd == cxlrd? Maybe you can do:

if (ctx->cxlrd && ctx->cxlrd != cxlrd) {
	put_device(cxlrd_dev(ctx->cxlrd));
	get_device(cxlrd_dev(cxlrd));
	ctx->cxlrd = cxlrd;
}
ctx->max_hpa = max;

DJ

> +	}
> +	return 0;
> +}
> +
> +/**
> + * cxl_get_hpa_freespace - find a root decoder with free capacity per constraints
> + * @cxlmd: the mem device requiring the HPA
> + * @interleave_ways: number of entries in @host_bridges
> + * @flags: CXL_DECODER_F flags for selecting RAM vs PMEM, and Type2 device
> + * @max_avail_contig: output parameter of max contiguous bytes available in the
> + *		      returned decoder
> + *
> + * Returns a pointer to a struct cxl_root_decoder
> + *
> + * The return tuple of a 'struct cxl_root_decoder' and 'bytes available given
> + * in (@max_avail_contig))' is a point in time snapshot. If by the time the
> + * caller goes to use this decoder and its capacity is reduced then caller needs
> + * to loop and retry.
> + *
> + * The returned root decoder has an elevated reference count that needs to be
> + * put with cxl_put_root_decoder(cxlrd).
> + */
> +struct cxl_root_decoder *cxl_get_hpa_freespace(struct cxl_memdev *cxlmd,
> +					       int interleave_ways,
> +					       unsigned long flags,
> +					       resource_size_t *max_avail_contig)
> +{
> +	struct cxlrd_max_context ctx = {
> +		.flags = flags,
> +		.interleave_ways = interleave_ways,
> +	};
> +	struct cxl_port *root_port;
> +	struct cxl_port *endpoint;
> +
> +	endpoint = cxlmd->endpoint;
> +	if (!endpoint) {
> +		dev_dbg(&cxlmd->dev, "endpoint not linked to memdev\n");
> +		return ERR_PTR(-ENXIO);
> +	}
> +
> +	ctx.host_bridges = &endpoint->host_bridge;
> +
> +	struct cxl_root *root __free(put_cxl_root) = find_cxl_root(endpoint);
> +	if (!root) {
> +		dev_dbg(&endpoint->dev, "endpoint is not related to a root port\n");
> +		return ERR_PTR(-ENXIO);
> +	}
> +
> +	root_port = &root->port;
> +	scoped_guard(rwsem_read, &cxl_rwsem.region)
> +		device_for_each_child(&root_port->dev, &ctx, find_max_hpa);
> +
> +	if (!ctx.cxlrd)
> +		return ERR_PTR(-ENOMEM);
> +
> +	*max_avail_contig = ctx.max_hpa;
> +	return ctx.cxlrd;
> +}
> +EXPORT_SYMBOL_NS_GPL(cxl_get_hpa_freespace, "CXL");
> +
> +void cxl_put_root_decoder(struct cxl_root_decoder *cxlrd)
> +{
> +	put_device(cxlrd_dev(cxlrd));
> +}
> +EXPORT_SYMBOL_NS_GPL(cxl_put_root_decoder, "CXL");
> +
>  static ssize_t size_store(struct device *dev, struct device_attribute *attr,
>  			  const char *buf, size_t len)
>  {
> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> index 944c5d1ccceb..c7d9b2c2908f 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -706,6 +706,9 @@ struct cxl_root_decoder *to_cxl_root_decoder(struct device *dev);
>  struct cxl_switch_decoder *to_cxl_switch_decoder(struct device *dev);
>  struct cxl_endpoint_decoder *to_cxl_endpoint_decoder(struct device *dev);
>  bool is_root_decoder(struct device *dev);
> +
> +#define cxlrd_dev(cxlrd) (&(cxlrd)->cxlsd.cxld.dev)
> +
>  bool is_switch_decoder(struct device *dev);
>  bool is_endpoint_decoder(struct device *dev);
>  struct cxl_root_decoder *cxl_root_decoder_alloc(struct cxl_port *port,
> diff --git a/include/cxl/cxl.h b/include/cxl/cxl.h
> index 92880c26b2d5..834dc7e78934 100644
> --- a/include/cxl/cxl.h
> +++ b/include/cxl/cxl.h
> @@ -255,4 +255,10 @@ struct cxl_endpoint_decoder *cxl_get_committed_decoder(struct cxl_memdev *cxlmd,
>  struct range;
>  int cxl_get_region_range(struct cxl_region *region, struct range *range);
>  void cxl_unregister_region(struct cxl_region *cxlr);
> +struct cxl_port;
> +struct cxl_root_decoder *cxl_get_hpa_freespace(struct cxl_memdev *cxlmd,
> +					       int interleave_ways,
> +					       unsigned long flags,
> +					       resource_size_t *max);
> +void cxl_put_root_decoder(struct cxl_root_decoder *cxlrd);
>  #endif /* __CXL_CXL_H__ */