Re: [PATCH v14 11/22] cxl: define a driver interface for HPA free space enumeration

[Alejandro Lucero Palau <alucerop@amd.com>]

On 5/8/25 02:13, Alison Schofield wrote:
> On Thu, Apr 17, 2025 at 10:29:14PM +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.
> This commit message lacks a why.
>
> It would be useful to state whether or not it makes any functional
> changes to the existing cxl driver hpa handling. Seems not.
>

I have had to think about the why and I'm not sure I have the right 
answer, so maybe other should comment on this.


I think with Type2 support, regions can be created from the drivers now, 
what requires more awareness and to find the proper HPA/cxl root port. 
Until now regions are created from user space, and those sysfs files 
hide the already established link to the cxl root port ... but I can not 
tell now for sure how this is being performed at decoder init time where 
a region is created for committed decoders (by the BIOS).


A comment from Dan will be helpful here.


>> 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 | 164 ++++++++++++++++++++++++++++++++++++++
>>   drivers/cxl/cxl.h         |   3 +
>>   include/cxl/cxl.h         |  11 +++
>>   3 files changed, 178 insertions(+)
>>
>> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
>> index 80caaf14d08a..0a9eab4f8e2e 100644
>> --- a/drivers/cxl/core/region.c
>> +++ b/drivers/cxl/core/region.c
>> @@ -695,6 +695,170 @@ 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 max;
>> +	int found = 0;
>> +
>> +	if (!is_root_decoder(dev))
>> +		return 0;
>> +
>> +	cxlrd = to_cxl_root_decoder(dev);
>> +	cxlsd = &cxlrd->cxlsd;
>> +	cxld = &cxlsd->cxld;
>> +
>> +	/*
>> +	 * None flags are declared as bitmaps but for the sake of better code
>> +	 * used here as such, restricting the bitmap size to those bits used by
>> +	 * any Type2 device driver requester.
>> +	 */
>> +	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;
>> +	}
>> +
>> +	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);
>> +	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);
>> +		}
>> +	}
>> +
>> +	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;
>> +}
>> +
>> +/**
>> + * cxl_get_hpa_freespace - find a root decoder with free capacity per constraints
>> + * @cxlmd: the CXL memory device with an endpoint that is mapped by the returned
>> + *	    decoder
>> + * @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
>> + *
>> + * 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 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 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 cxl_port *endpoint = cxlmd->endpoint;
>> +	struct cxlrd_max_context ctx = {
>> +		.host_bridges = &endpoint->host_bridge,
>> +		.flags = flags,
>> +	};
>> +	struct cxl_port *root_port;
>> +	struct cxl_root *root __free(put_cxl_root) = find_cxl_root(endpoint);
>> +
>> +	if (!is_cxl_endpoint(endpoint)) {
>> +		dev_dbg(&endpoint->dev, "hpa requestor is not an endpoint\n");
>> +		return ERR_PTR(-EINVAL);
>> +	}
>> +
>> +	if (!root) {
>> +		dev_dbg(&endpoint->dev, "endpoint can not be related to a root port\n");
>> +		return ERR_PTR(-ENXIO);
>> +	}
>> +
>> +	root_port = &root->port;
>> +	scoped_guard(rwsem_read, &cxl_region_rwsem)
>> +		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 4523864eebd2..c35620c24c8f 100644
>> --- a/drivers/cxl/cxl.h
>> +++ b/drivers/cxl/cxl.h
>> @@ -672,6 +672,9 @@ 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 9c0f097ca6be..e9ae7eff2393 100644
>> --- a/include/cxl/cxl.h
>> +++ b/include/cxl/cxl.h
>> @@ -26,6 +26,11 @@ enum cxl_devtype {
>>   
>>   struct device;
>>   
>> +#define CXL_DECODER_F_RAM   BIT(0)
>> +#define CXL_DECODER_F_PMEM  BIT(1)
>> +#define CXL_DECODER_F_TYPE2 BIT(2)
>> +#define CXL_DECODER_F_MAX 3
>> +
>>   /*
>>    * Capabilities as defined for:
>>    *
>> @@ -250,4 +255,10 @@ void cxl_mem_dpa_init(struct cxl_dpa_info *info, u64 volatile_bytes,
>>   int cxl_dpa_setup(struct cxl_dev_state *cxlds, const struct cxl_dpa_info *info);
>>   struct cxl_memdev *devm_cxl_add_memdev(struct device *host,
>>   				       struct cxl_dev_state *cxlmds);
>> +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__ */
>> -- 
>> 2.34.1
>>
>>