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

Dave Jiang
2025-10-15T20:07:12Z

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");
>