Re: [PATCH v8 17/27] cxl: define a driver interface for DPA allocation

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Mon, 16 Dec 2024 16:10:32 +0000
<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

Bonus space before map.

> 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>
Comments inline.

> ---
>  drivers/cxl/core/hdm.c | 154 +++++++++++++++++++++++++++++++++++------
>  include/cxl/cxl.h      |   5 ++
>  2 files changed, 138 insertions(+), 21 deletions(-)
> 

> +int cxl_dpa_alloc(struct cxl_endpoint_decoder *cxled, unsigned long long size)
> +{
> +	struct cxl_port *port = cxled_to_port(cxled);
> +	struct device *dev = &cxled->cxld.dev;
> +	resource_size_t start, avail, skip;
> +	int rc;
> +
> +	down_write(&cxl_dpa_rwsem);
> +	if (cxled->cxld.region) {
> +		dev_dbg(dev, "EBUSY, decoder attached to %s\n",
> +			dev_name(&cxled->cxld.region->dev));
> +		rc = -EBUSY;
> +		goto out;
> +	}
> +
> +	if (cxled->cxld.flags & CXL_DECODER_F_ENABLE) {
> +		dev_dbg(dev, "EBUSY, decoder enabled\n");
> +		rc = -EBUSY;
>  		goto out;
>  	}
>  
> +	avail = cxl_dpa_freespace(cxled, &start, &skip);
> +
>  	if (size > avail) {
>  		dev_dbg(dev, "%pa exceeds available %s capacity: %pa\n", &size,
> -			cxl_decoder_mode_name(cxled->mode), &avail);
> +			     cxled->mode == CXL_DECODER_RAM ? "ram" : "pmem",
This is reverting an earlier change. I guess accidental?

> +			     &avail);
>  		rc = -ENOSPC;
>  		goto out;
>  	}
> @@ -538,6 +557,99 @@ int cxl_dpa_alloc(struct cxl_endpoint_decoder *cxled, unsigned long long size)
>  	return devm_add_action_or_reset(&port->dev, cxl_dpa_release, cxled);
>  }

> +/**
> + * cxl_request_dpa - search and reserve DPA given input constraints
> + * @cxlmd: memdev with 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

Includes the extra capacity. Otherwise capacity allocated as documented
is min + max which seems unlikely.

> + *
> + * 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_memdev *cxlmd,
> +					     bool is_ram,
> +					     resource_size_t min,
> +					     resource_size_t max)
> +{
> +	struct cxl_port *endpoint = cxlmd->endpoint;
> +	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);
> +	up_read(&cxl_dpa_rwsem);
> +
> +	if (!cxled_dev)
> +		cxled = ERR_PTR(-ENXIO);
	if (!cxled_dev)
		return ERR_PTR(-ENXIO);

	cxled = to...
	if (!cxled) //assuming this has any way to fail in which 
case I think you would need to put the device...
		put_device(cxled_dev);
		return NULL;

Though do you actualy want to return an error in this case?

> +	else
> +		cxled = to_cxl_endpoint_decoder(cxled_dev);
> +
> +	if (!cxled || IS_ERR(cxled))
> +		return cxled;
Drop this with changes above.

> +
> +	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");