Re: [PATCH 17/19] cxl/region: Define a driver interface for HPA free space enumeration

[Dave Jiang <dave.jiang@intel.com>]

On 6/4/23 16:33, Dan Williams wrote:
> CXL region creation involves allocating capacity from device DPA
> (device-physical-address space) and assigning it to decode a given HPA
> (host-physical-address space). Before determininig how much DPA to
> allocate the amount of available HPA must be determined. Also, not all
> HPA is created equal, some specifically targets RAM, some target PMEM,
> some is prepared for the device-memory flows like HDM-D and HDM-DB, and
> some is host-only (HDM-H).
> 
> 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.
> 
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>   drivers/cxl/core/region.c |  143 +++++++++++++++++++++++++++++++++++++++++++++
>   drivers/cxl/cxl.h         |    5 ++
>   drivers/cxl/cxlmem.h      |    5 ++
>   3 files changed, 153 insertions(+)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 75c5de627868..a41756249f8d 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -575,6 +575,149 @@ 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 max;
> +	unsigned int seq;
> +	int found;
> +
> +	if (!is_root_decoder(dev))
> +		return 0;
> +
> +	cxlrd = to_cxl_root_decoder(dev);
> +	cxld = &cxlrd->cxlsd.cxld;
> +	if ((cxld->flags & ctx->flags) != ctx->flags)
> +		return 0;
> +
> +	if (cxld->interleave_ways != ctx->interleave_ways)
> +		return 0;
> +
> +	cxlsd = &cxlrd->cxlsd;
> +	do {
> +		found = 0;
> +		seq = read_seqbegin(&cxlsd->target_lock);
> +		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) {
> +					found++;
> +					break;
> +				}
> +	} while (read_seqretry(&cxlsd->target_lock, seq));
> +
> +	if (found != ctx->interleave_ways)
> +		return 0;
> +
> +	/*
> +	 * Walk the root decoder resource range relying on cxl_region_rwsem to
> +	 * preclude sibling arrival/departure and find the largest free space
> +	 * gap.
> +	 */
> +	lockdep_assert_held_read(&cxl_region_rwsem);
> +	max = 0;
> +	res = cxlrd->res->child;
> +	if (!res)
> +		max = resource_size(cxlrd->res);
> +	else
> +		max = 0;
> +	for (prev = NULL; res; prev = res, res = res->sibling) {
> +		struct resource *next = res->sibling;
> +		resource_size_t free = 0;
> +
> +		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);
> +		}

Can skip the extra compare, not sure if it's worth the extra level of 
indent.

		if (!prev) {
			if (res->start > cxlrd->res->start) {
				free = res->start - cxlrd->res->start;
				max = max(free, max)
			}
		} else {
			if (res->start > prev->end + 1) {
				free = res->start - prev->end + 1;
				max = max(free, max);
			}
		}


Same below.

DJ

> +		if (next && res->end + 1 < next->start) {
> +			free = next->start - res->end + 1;
> +			max = max(free, max);
> +		}
> +		if (!next && res->end + 1 < cxlrd->res->end + 1) {
> +			free = cxlrd->res->end + 1 - res->end + 1;
> +			max = max(free, 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;
> +		dev_dbg(cxlrd_dev(cxlrd), "found %pa bytes of free space\n", &max);
> +	}
> +
> +	return 0;
> +}
> +
> +/**
> + * cxl_hpa_freespace - find a root decoder with free capacity per constraints
> + * @endpoint: an endpoint that is mapped by the returned decoder
> + * @host_bridges: array of host-bridges that the decoder must interleave
> + * @interleave_ways: number of entries in @host_bridges
> + * @flags: CXL_DECODER_F flags for selecting RAM vs PMEM, and HDM-H vs HDM-D[B]
> + * @max: output parameter of bytes available in the returned decoder
> + *
> + * The return tuple of a 'struct cxl_root_decoder' and 'bytes available (@max)'
> + * is a point in time snapshot. If by the time the caller goes to use this root
> + * decoder's capacity the 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 put_device(cxlrd_dev(cxlrd)). Locking context is with
> + * cxl_{acquire,release}_endpoint(), that ensures removal of the root decoder
> + * does not race.
> + */
> +struct cxl_root_decoder *cxl_hpa_freespace(struct cxl_port *endpoint,
> +					   struct device *const *host_bridges,
> +					   int interleave_ways,
> +					   unsigned long flags,
> +					   resource_size_t *max)
> +{
> +	struct cxlrd_max_context ctx = {
> +		.host_bridges = host_bridges,
> +		.interleave_ways = interleave_ways,
> +		.flags = flags,
> +	};
> +	struct cxl_port *root;
> +
> +	if (!is_cxl_endpoint(endpoint))
> +		return ERR_PTR(-EINVAL);
> +
> +	root = find_cxl_root(endpoint);
> +	if (!root)
> +		return ERR_PTR(-ENXIO);
> +
> +	down_read(&cxl_region_rwsem);
> +	device_for_each_child(&root->dev, &ctx, find_max_hpa);
> +	up_read(&cxl_region_rwsem);
> +	put_device(&root->dev);
> +
> +	if (!ctx.cxlrd)
> +		return ERR_PTR(-ENOMEM);
> +
> +	*max = ctx.max_hpa;
> +	return ctx.cxlrd;
> +}
> +EXPORT_SYMBOL_NS_GPL(cxl_hpa_freespace, 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 55808697773f..8400af85d99f 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -686,6 +686,11 @@ static inline struct device *cxled_dev(struct cxl_endpoint_decoder *cxled)
>   	return &cxled->cxld.dev;
>   }
>   
> +static inline struct device *cxlrd_dev(struct cxl_root_decoder *cxlrd)
> +{
> +	return &cxlrd->cxlsd.cxld.dev;
> +}
> +
>   bool is_root_decoder(struct device *dev);
>   bool is_switch_decoder(struct device *dev);
>   bool is_endpoint_decoder(struct device *dev);
> diff --git a/drivers/cxl/cxlmem.h b/drivers/cxl/cxlmem.h
> index 8ec5c305d186..69f07186502d 100644
> --- a/drivers/cxl/cxlmem.h
> +++ b/drivers/cxl/cxlmem.h
> @@ -93,6 +93,11 @@ struct cxl_endpoint_decoder *cxl_request_dpa(struct cxl_port *endpoint,
>   					     enum cxl_decoder_mode mode,
>   					     resource_size_t min,
>   					     resource_size_t max);
> +struct cxl_root_decoder *cxl_hpa_freespace(struct cxl_port *endpoint,
> +					   struct device *const *host_bridges,
> +					   int interleave_ways,
> +					   unsigned long flags,
> +					   resource_size_t *max);
>   
>   static inline struct cxl_ep *cxl_ep_load(struct cxl_port *port,
>   					 struct cxl_memdev *cxlmd)
>