Re: [PATCH v12 11/23] cxl: define a driver interface for HPA free space enumeration

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Mon, 31 Mar 2025 15:45:43 +0100
alejandro.lucero-palau@amd.com wrote:

> From: Alejandro Lucero <alucerop@amd.com>
> 
> 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 determining 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 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.
> 
> Add a complementary function for releasing the reference to such root
> decoder.
> 
> Based on https://lore.kernel.org/linux-cxl/168592159290.1948938.13522227102445462976.stgit@dwillia2-xfh.jf.intel.com/
> 
> Signed-off-by: Alejandro Lucero <alucerop@amd.com>
> ---
>  drivers/cxl/core/region.c | 160 ++++++++++++++++++++++++++++++++++++++
>  drivers/cxl/cxl.h         |   3 +
>  include/cxl/cxl.h         |  11 ++-
>  3 files changed, 173 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index c3f4dc244df7..59fb51ff8922 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -695,6 +695,166 @@ static int free_hpa(struct cxl_region *cxlr)
>  	return 0;
>  }

> +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;
> +	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) {

Hmm. Is this a subset test?
if (!(cxld->flags & ~ctx->flags)) or something like that?
Or just use bitmap_subset() on it if they are both unsigned longs.

> +		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 interleave ways requested (%d)\n",
> +			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;
> +
> +	/* With no resource child the whole parent resource is available */
> +	if (!res)
> +		max = resource_size(cxlrd->res);
> +	else
> +		max = 0;

max is already 0 in this path.


> +
> +	for (prev = NULL; res; prev = res, res = res->sibling) {
> +		struct resource *next = res->sibling;
> +		resource_size_t free = 0;
> +
> +		/*
> +		 * 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 (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);
> +		}
> +	}
> +
> +	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;
> +	}
> +	return 0;
> +}