Re: [PATCH v3 13/20] cxl: define a driver interface for DPA allocation

Jonathan Cameron
2024-09-13T17:59:11Z

On Sat, 7 Sep 2024 09:18:29 +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. Given the HPA
> capacity constraint, define an API, cxl_request_dpa(), that has the
> flexibility to map the minimum amount of memory the driver needs to
> operate vs the total possible that can be mapped given HPA availability.
>
> Factor out the core of cxl_dpa_alloc, that does free space scanning,
> into a cxl_dpa_freespace() helper, and use that to balance the capacity
> available to map vs the @min and @max arguments to cxl_request_dpa.
>
> 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>
> Co-developed-by: Dan Williams <dan.j.williams@intel.com>
Trivial comment below.

> +
> +/**
> + * cxl_request_dpa - search and reserve DPA given input constraints
> + * @endpoint: an endpoint port with available decoders
> + * @is_ram: DPA operation mode (ram vs pmem)
> + * @min: the minimum amount of capacity the call needs
> + * @max: extra capacity to allocate after min is satisfied
> + *
> + * 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. So, the expectation is that @min is a driver known
> + * value for how much capacity is needed, and @max is based the limit of
> + * how much HPA space is available for a new region.
> + *
> + * Returns a pinned cxl_decoder with at least @min 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_port *endpoint,
> + bool is_ram,
> + resource_size_t min,
> + resource_size_t max)
> +{
> + struct cxl_endpoint_decoder *cxled;
> + enum cxl_decoder_mode mode;
> + struct device *cxled_dev;
> + resource_size_t alloc;
> + int rc;
> +
> + if (!IS_ALIGNED(min | max, SZ_256M))
> + return ERR_PTR(-EINVAL);
> +
> + down_read(&cxl_dpa_rwsem);
> +
> + cxled_dev = device_find_child(&endpoint->dev, NULL, find_free_decoder);
> + if (!cxled_dev)
> + cxled = ERR_PTR(-ENXIO);
> + else
> + cxled = to_cxl_endpoint_decoder(cxled_dev);
Does this need to be under the rwsem? If not cleaner to just
check cxled_dev outside the lock and return the error directly.

Also, in theory this could return NULL - in practice not but
checking it for IS_ERR() is perhaps going to lead to a bug
in the distant future.

> +
> + up_read(&cxl_dpa_rwsem);
> +
> + if (IS_ERR(cxled))
> + return cxled;
> +
> + if (is_ram)
> + mode = CXL_DECODER_RAM;
> + else
> + mode = CXL_DECODER_PMEM;
> +
> + rc = cxl_dpa_set_mode(cxled, mode);
> + if (rc)
> + goto err;
> +
> + down_read(&cxl_dpa_rwsem);
> + alloc = cxl_dpa_freespace(cxled, NULL, NULL);
> + up_read(&cxl_dpa_rwsem);
> +
> + if (max)
> + alloc = min(max, alloc);
> + if (alloc < min) {
> + rc = -ENOMEM;
> + goto err;
> + }
> +
> + rc = cxl_dpa_alloc(cxled, alloc);
> + if (rc)
> + goto err;
> +
> + return cxled;
> +err:
> + put_device(cxled_dev);
> + return ERR_PTR(rc);
> +}
> +EXPORT_SYMBOL_NS_GPL(cxl_request_dpa, CXL);