netdev - Re: [PATCH v9 17/27] cxl: define a driver interface for DPA allocation

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Message-ID: <20250102151555.000072fd@huawei.com>
Date: Thu, 2 Jan 2025 15:15:55 +0000
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: <alejandro.lucero-palau@....com>
CC: <linux-cxl@...r.kernel.org>, <netdev@...r.kernel.org>,
	<dan.j.williams@...el.com>, <edward.cree@....com>, <davem@...emloft.net>,
	<kuba@...nel.org>, <pabeni@...hat.com>, <edumazet@...gle.com>,
	<dave.jiang@...el.com>, Alejandro Lucero <alucerop@....com>
Subject: Re: [PATCH v9 17/27] cxl: define a driver interface for DPA
 allocation

On Mon, 30 Dec 2024 21:44:35 +0000
<alejandro.lucero-palau@....com> wrote:

> From: Alejandro Lucero <alucerop@....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@....com>
> Co-developed-by: Dan Williams <dan.j.williams@...el.com>

A couple of really minor things inline. Either way

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>

> +/**
> + * 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: HPA capacity available
> + *
> + * 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

is the limit? Not sure what the "is based" means


> + * 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)
> +		return ERR_PTR(-ENXIO);
> +
> +	cxled = to_cxl_endpoint_decoder(cxled_dev);
> +
> +	if (!cxled) {
> +		put_device(cxled_dev);
> +		return ERR_PTR(-ENODEV);

Ah. My suggestion on v8 missed that there is an error block
below. More consistent with rest of the code as

		rc = -ENODEV;
		goto err;

> +	}
> +
> +	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");