Re: [PATCH v17 11/22] cxl: Define a driver interface for HPA free space enumeration

[<dan.j.williams@intel.com>]

alejandro.lucero-palau@ 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).
> 
> 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/168592159290.1948938.13522227102445462976.stgit@dwillia2-xfh.jf.intel.com/
> 
> Signed-off-by: Alejandro Lucero <alucerop@amd.com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> ---
>  drivers/cxl/core/region.c | 169 ++++++++++++++++++++++++++++++++++++++
>  drivers/cxl/cxl.h         |   3 +
>  include/cxl/cxl.h         |  11 +++
>  3 files changed, 183 insertions(+)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index c3f4dc244df7..03e058ab697e 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -695,6 +695,175 @@ 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;
> +	int found = 0;
> +
> +	if (!is_root_decoder(dev))
> +		return 0;
> +
> +	cxlrd = to_cxl_root_decoder(dev);
> +	cxlsd = &cxlrd->cxlsd;
> +	cxld = &cxlsd->cxld;
> +
> +	/*
> +	 * Flags are single unsigned longs. As CXL_DECODER_F_MAX is less than
> +	 * 32 bits, the bitmap functions can be used.
> +	 */

Comments are supposed to explain the code, not repeat the code in
natural language.

> +	if (!bitmap_subset(&ctx->flags, &cxld->flags, CXL_DECODER_F_MAX)) {
> +		dev_dbg(dev, "flags not matching: %08lx vs %08lx\n",
> +			cxld->flags, ctx->flags);
> +		return 0;
> +	}

How is this easier to read than:

	if ((cxld->flags & ctx->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_region_rwsem to
> +	 * preclude sibling arrival/departure and find the largest free space
> +	 * gap.
> +	 */
> +	lockdep_assert_held_read(&cxl_region_rwsem);
> +	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) {
> +		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);
> +		}
> +	}

With the benefit of time to reflect, and looking at this again after all
this time it strikes me that it is simply duplicating
get_free_mem_region() and in a way that can still fail later.

Does it simplify the implementation if this just attempts to
allocate the capacity in each window that might support the mapping
constraints and then pass that allocation to the region construction
routine?

Otherwise, this completes a survey of the capacity that is not
guaranteed to be present when the region finally gets allocated.