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

[Alejandro Lucero Palau <alucerop@amd.com>]

On 12/24/24 17:53, Jonathan Cameron wrote:
> 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.


Ok.


>> 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?


Yes, I should be using the function.


>> +			     &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.


Right. I'll fix it.


>> + *
>> + * 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?


This handling makes the code clearer, and yes, you are right about the 
put_device.

I'll fix it.


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


Sure.

Thanks!


>
>> +
>> +	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");