Re: [PATCH v23 14/22] cxl: Define a driver interface for DPA allocation

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

On 2/11/26 22:12, Cheatham, Benjamin wrote:
> On 2/1/2026 9:54 AM, alejandro.lucero-palau@amd.com wrote:
>> From: Alejandro Lucero <alucerop@amd.com>
>>
>> Region creation involves finding available DPA (device-physical-address)
>> capacity to map into HPA (host-physical-address) space.
>>
>> In order to support CXL Type2 devices, define an API, cxl_request_dpa(),
>> that tries to allocate the DPA memory the driver requires to operate.The
>> memory requested should not be bigger than the max available HPA obtained
>> previously with cxl_get_hpa_freespace().
>>
>> Based on https://lore.kernel.org/linux-cxl/168592158743.1948938.7622563891193802610.stgit@dwillia2-xfh.jf.intel.com/
>>
>> Signed-off-by: Alejandro Lucero <alucerop@amd.com>
>> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
>> Reviewed-by: Dave Jiang <dave.jiang@intel.com>
>> ---
>>   drivers/cxl/core/hdm.c | 84 ++++++++++++++++++++++++++++++++++++++++++
>>   drivers/cxl/cxl.h      |  1 +
>>   include/cxl/cxl.h      |  5 +++
>>   3 files changed, 90 insertions(+)
>>
>> diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
>> index a172ce4e9b19..d60a697f12cc 100644
>> --- a/drivers/cxl/core/hdm.c
>> +++ b/drivers/cxl/core/hdm.c
>> @@ -3,6 +3,7 @@
>>   #include <linux/seq_file.h>
>>   #include <linux/device.h>
>>   #include <linux/delay.h>
>> +#include <cxl/cxl.h>
>>   
>>   #include "cxlmem.h"
>>   #include "core.h"
>> @@ -546,6 +547,12 @@ bool cxl_resource_contains_addr(const struct resource *res, const resource_size_
>>   	return resource_contains(res, &_addr);
>>   }
>>   
>> +/**
>> + * cxl_dpa_free - release DPA (Device Physical Address)
>> + * @cxled: endpoint decoder linked to the DPA
>> + *
>> + * Returns 0 or error.
>> + */
>>   int cxl_dpa_free(struct cxl_endpoint_decoder *cxled)
>>   {
>>   	struct cxl_port *port = cxled_to_port(cxled);
>> @@ -572,6 +579,7 @@ int cxl_dpa_free(struct cxl_endpoint_decoder *cxled)
>>   	devm_cxl_dpa_release(cxled);
>>   	return 0;
>>   }
>> +EXPORT_SYMBOL_NS_GPL(cxl_dpa_free, "CXL");
>>   
>>   int cxl_dpa_set_part(struct cxl_endpoint_decoder *cxled,
>>   		     enum cxl_partition_mode mode)
>> @@ -603,6 +611,82 @@ int cxl_dpa_set_part(struct cxl_endpoint_decoder *cxled,
>>   	return 0;
>>   }
>>   
>> +static int find_free_decoder(struct device *dev, const void *data)
>> +{
>> +	struct cxl_endpoint_decoder *cxled;
>> +	struct cxl_port *port;
>> +
>> +	if (!is_endpoint_decoder(dev))
>> +		return 0;
>> +
>> +	cxled = to_cxl_endpoint_decoder(dev);
>> +	port = cxled_to_port(cxled);
>> +
>> +	return cxled->cxld.id == (port->hdm_end + 1);
>> +}
>> +
>> +static struct cxl_endpoint_decoder *
>> +cxl_find_free_decoder(struct cxl_memdev *cxlmd)
>> +{
>> +	struct cxl_port *endpoint = cxlmd->endpoint;
>> +	struct device *dev;
>> +
>> +	guard(rwsem_read)(&cxl_rwsem.dpa);
>> +	dev = device_find_child(&endpoint->dev, NULL,
>> +				find_free_decoder);
>> +	if (!dev)
>> +		return NULL;
>> +
>> +	return to_cxl_endpoint_decoder(dev);
>> +}
>> +
>> +/**
>> + * cxl_request_dpa - search and reserve DPA given input constraints
>> + * @cxlmd: memdev with an endpoint port with available decoders
>> + * @mode: CXL partition mode (ram vs pmem)
>> + * @alloc: dpa size required
>> + *
>> + * Returns a pointer to a 'struct cxl_endpoint_decoder' on success or
>> + * an errno encoded pointer on failure.
>> + *
>> + * Given that a region needs to allocate from limited HPA capacity it
>> + * may be the case that a device has more mappable DPA capacity than
>> + * available HPA. The expectation is that @alloc is a driver known
>> + * value based on the device capacity but which could not be fully
>> + * available due to HPA constraints.
>> + *
>> + * Returns a pinned cxl_decoder with at least @alloc bytes of capacity
>> + * reserved, or an error pointer. The caller is also expected to own the
>> + * lifetime of the memdev registration associated with the endpoint to
>> + * pin the decoder registered as well.
>> + */
>> +struct cxl_endpoint_decoder *cxl_request_dpa(struct cxl_memdev *cxlmd,
>> +					     enum cxl_partition_mode mode,
>> +					     resource_size_t alloc)
>> +{
>> +	int rc;
>> +
>> +	if (!IS_ALIGNED(alloc, SZ_256M))
>> +		return ERR_PTR(-EINVAL);
>> +
>> +	struct cxl_endpoint_decoder *cxled __free(put_cxled) =
>> +		cxl_find_free_decoder(cxlmd);
>> +
>> +	if (!cxled)
>> +		return ERR_PTR(-ENODEV);
>> +
>> +	rc = cxl_dpa_set_part(cxled, mode);
>> +	if (rc)
>> +		return ERR_PTR(rc);
>> +
>> +	rc = cxl_dpa_alloc(cxled, alloc);
>> +	if (rc)
>> +		return ERR_PTR(rc);
> Should cxl_dpa_set_part() be unwound here, or does it not matter?


No, I do not think that is necessary. The CXL initialization fails, and 
the result is the modified struct will be released sooner or later.


> If it doesn't matter:
> Reviewed-by: Ben Cheatham <benjamin.cheatham@amd.com>


Thank you


>> +
>> +	return no_free_ptr(cxled);
>> +}
>> +EXPORT_SYMBOL_NS_GPL(cxl_request_dpa, "CXL");
>> +
>>   static int __cxl_dpa_alloc(struct cxl_endpoint_decoder *cxled, u64 size)
>>   {
>>   	struct cxl_memdev *cxlmd = cxled_to_memdev(cxled);
>> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
>> index d1b010e5e1d0..2b1f7d687a0e 100644
>> --- a/drivers/cxl/cxl.h
>> +++ b/drivers/cxl/cxl.h
>> @@ -667,6 +667,7 @@ struct cxl_root *find_cxl_root(struct cxl_port *port);
>>   
>>   DEFINE_FREE(put_cxl_root, struct cxl_root *, if (_T) put_device(&_T->port.dev))
>>   DEFINE_FREE(put_cxl_port, struct cxl_port *, if (!IS_ERR_OR_NULL(_T)) put_device(&_T->dev))
>> +DEFINE_FREE(put_cxled, struct cxl_endpoint_decoder *, if (!IS_ERR_OR_NULL(_T)) put_device(&_T->cxld.dev))
>>   DEFINE_FREE(put_cxl_root_decoder, struct cxl_root_decoder *, if (!IS_ERR_OR_NULL(_T)) put_device(&_T->cxlsd.cxld.dev))
>>   DEFINE_FREE(put_cxl_region, struct cxl_region *, if (!IS_ERR_OR_NULL(_T)) put_device(&_T->dev))
>>   
>> diff --git a/include/cxl/cxl.h b/include/cxl/cxl.h
>> index 783ad570a6eb..4802371db00e 100644
>> --- a/include/cxl/cxl.h
>> +++ b/include/cxl/cxl.h
>> @@ -7,6 +7,7 @@
>>   
>>   #include <linux/node.h>
>>   #include <linux/ioport.h>
>> +#include <linux/range.h>
>>   #include <cxl/mailbox.h>
>>   
>>   /**
>> @@ -276,4 +277,8 @@ struct cxl_root_decoder *cxl_get_hpa_freespace(struct cxl_memdev *cxlmd,
>>   					       unsigned long flags,
>>   					       resource_size_t *max);
>>   void cxl_put_root_decoder(struct cxl_root_decoder *cxlrd);
>> +struct cxl_endpoint_decoder *cxl_request_dpa(struct cxl_memdev *cxlmd,
>> +					     enum cxl_partition_mode mode,
>> +					     resource_size_t alloc);
>> +int cxl_dpa_free(struct cxl_endpoint_decoder *cxled);
>>   #endif /* __CXL_CXL_H__ */