Subject: Re: [PATCH v18 11/20] cxl: Define a driver interface for DPA allocation

From: Jonathan Cameron <[email-masked]>
Date: Thu, 18 Sep 2025 15:52:22 +0100

On Thu, 18 Sep 2025 10:17:37 +0100
[email-masked] wrote:

> From: Alejandro Lucero <[email-masked]>
> 
> 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/[email-masked]/
> 
> Signed-off-by: Alejandro Lucero <[email-masked]>

> +
> +/**
> + * cxl_request_dpa - search and reserve DPA given input constraints
> + * @cxlmd: memdev with an endpoint port with available decoders
> + * @mode: DPA operation mode (ram vs pmem)
> + * @alloc: dpa size required
> + *
> + * Returns a pointer to a cxl_endpoint_decoder struct or an error
> + *
> + * 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 it could not be 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)
> +{
> +	struct cxl_endpoint_decoder *cxled __free(put_cxled) =
> +					cxl_find_free_decoder(cxlmd);
I've no idea where this style comes from. Local style in this file seems to be
the more common (I think?)
	struct cxl_endpoint_decoder *cxled __free(put_cxled) =
		cxl_find_free_decoder(cxlmd);

> +	int rc;
> +
> +	if (!IS_ALIGNED(alloc, SZ_256M))
> +		return ERR_PTR(-EINVAL);
> +
> +	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);
> +}
> +EXPORT_SYMBOL_NS_GPL(cxl_request_dpa, "CXL");