Re: [PATCH v19 13/22] cxl: Define a driver interface for DPA allocation

[Dave Jiang <dave.jiang@intel.com>]

On 10/7/25 6:52 AM, Jonathan Cameron wrote:
> On Mon, 6 Oct 2025 11:01:21 +0100
> <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>
> 
> A few minor things inline.  Depending on how much this changed
> from the 'Based on' it might be appropriate to keep a SoB / author
> set to Dan, but I'll let him request that if he feels appropriate
> (or you can make that decision if Dan is busy).
> 
> A few things inline.  All trivial
> 
> Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
> 
>>  int cxl_dpa_set_part(struct cxl_endpoint_decoder *cxled,
>>  		     enum cxl_partition_mode mode)
>> @@ -613,6 +622,82 @@ 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 to_cxl_endpoint_decoder(dev);
>> +
>> +	return NULL;
> Trivial but I'd prefer to see the 'error' like thing out of line
> 
> 	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);
>> +
>> +	return no_free_ptr(cxled);
> 
> return_ptr() (it's exactly the same implementation).

I don't care either way. But as I recall Dan preferred the 'return no_free_ptr()' version to make it obvious.

DJ

> 
>> +}
>> +EXPORT_SYMBOL_NS_GPL(cxl_request_dpa, "CXL");
>