Re: [PATCH v6 15/28] cxl: define a driver interface for HPA free space enumeration

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

On 12/6/24 19:48, Fan Ni wrote:
> On Mon, Dec 02, 2024 at 05:12:09PM +0000, 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 create 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.
>>
>> 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>
>> Co-developed-by: Dan Williams <dan.j.williams@intel.com>
>> ---
>>   drivers/cxl/core/region.c | 145 ++++++++++++++++++++++++++++++++++++++
>>   drivers/cxl/cxl.h         |   3 +
>>   include/cxl/cxl.h         |   8 +++
>>   3 files changed, 156 insertions(+)
>>
> ...
>> +	/*
>> +	 * 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;
> Should it be
>      free = res->start - (prev->end + 1);
> ?
>> +			max = max(free, max);
>> +		}
>> +		if (next && res->end + 1 < next->start) {
>> +			free = next->start - res->end + 1;
> free = next->start - (res->end + 1);
>
> Fan


I do not think it is necessary except for legibility.

The only case would be res->end or prev->end above being from a resource 
defined with zero size, implying being all 1s as it would be initialized 
with 0 - 1 for an unsigned variable.

But I think we can be sure no resource with size 0 will be in the list 
walked ... But maybe we could make a sanity check here.

I'll add that.

Thanks!


>> +			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), "%s, found %pa bytes of free space\n",
>> +		__func__, &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), "%s, found %pa bytes of free space\n",
>> +			__func__, &max);
>> +	}
>> +	return 0;
>> +}
>> +
>> +/**
>> + * cxl_get_hpa_freespace - find a root decoder with free capacity per constraints
>> + * @endpoint: an endpoint that is mapped by the returned decoder
>> + * @flags: CXL_DECODER_F flags for selecting RAM vs PMEM, and HDM-H vs HDM-D[B]
>> + * @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 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_get_hpa_freespace(struct cxl_memdev *cxlmd,
>> +					       unsigned long flags,
>> +					       resource_size_t *max_avail_contig)
>> +{
>> +	struct cxl_port *endpoint = cxlmd->endpoint;
>> +	struct cxlrd_max_context ctx = {
>> +		.host_bridge = 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;
>> +	down_read(&cxl_region_rwsem);
>> +	device_for_each_child(&root_port->dev, &ctx, find_max_hpa);
>> +	up_read(&cxl_region_rwsem);
>> +
>> +	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);
>> +
>>   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 22e787748d79..57d6dda3fb4a 100644
>> --- a/drivers/cxl/cxl.h
>> +++ b/drivers/cxl/cxl.h
>> @@ -785,6 +785,9 @@ static inline void cxl_dport_init_ras_reporting(struct cxl_dport *dport,
>>   struct cxl_decoder *to_cxl_decoder(struct device *dev);
>>   struct cxl_root_decoder *to_cxl_root_decoder(struct device *dev);
>>   struct cxl_switch_decoder *to_cxl_switch_decoder(struct device *dev);
>> +
>> +#define CXLRD_DEV(cxlrd) (&(cxlrd)->cxlsd.cxld.dev)
>> +
>>   struct cxl_endpoint_decoder *to_cxl_endpoint_decoder(struct device *dev);
>>   bool is_root_decoder(struct device *dev);
>>   bool is_switch_decoder(struct device *dev);
>> diff --git a/include/cxl/cxl.h b/include/cxl/cxl.h
>> index 26d7735b5f31..eacd5e5e6fe8 100644
>> --- a/include/cxl/cxl.h
>> +++ b/include/cxl/cxl.h
>> @@ -7,6 +7,10 @@
>>   #include <linux/ioport.h>
>>   #include <linux/pci.h>
>>   
>> +#define CXL_DECODER_F_RAM   BIT(0)
>> +#define CXL_DECODER_F_PMEM  BIT(1)
>> +#define CXL_DECODER_F_TYPE2 BIT(2)
>> +
>>   enum cxl_resource {
>>   	CXL_RES_DPA,
>>   	CXL_RES_RAM,
>> @@ -47,4 +51,8 @@ int cxl_release_resource(struct cxl_dev_state *cxlds, enum cxl_resource type);
>>   void cxl_set_media_ready(struct cxl_dev_state *cxlds);
>>   struct cxl_memdev *devm_cxl_add_memdev(struct device *host,
>>   				       struct cxl_dev_state *cxlds);
>> +struct cxl_port;
>> +struct cxl_root_decoder *cxl_get_hpa_freespace(struct cxl_memdev *cxlmd,
>> +					       unsigned long flags,
>> +					       resource_size_t *max);
>>   #endif
>> -- 
>> 2.17.1
>>