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

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Message-ID: <76860e75-d0b4-d47f-9051-43ba84d43bf1@amd.com>
Date: Mon, 16 Sep 2024 15:26:07 +0100
From: Alejandro Lucero Palau <alucerop@....com>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>,
 alejandro.lucero-palau@....com
Cc: linux-cxl@...r.kernel.org, netdev@...r.kernel.org,
 dan.j.williams@...el.com, martin.habets@...inx.com, edward.cree@....com,
 davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, edumazet@...gle.com
Subject: Re: [PATCH v3 13/20] cxl: define a driver interface for DPA
 allocation


On 9/13/24 18:59, Jonathan Cameron wrote:
> On Sat, 7 Sep 2024 09:18:29 +0100
> 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>
> 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.


We got a get_device inside device_find_child, so it should be safe to 
use to_cxl_endpoint_decoder without the sem.

I'll follow your suggestion.


> 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.
>

Right. I'll fix it.

Thanks


>> +
>> +	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);