Subject: Re: [PATCH v2 10/15] cxl: define a driver interface for DPA allocation

From: Alejandro Lucero Palau <[email-masked]>
Date: Mon, 19 Aug 2024 16:52:34 +0100

On 8/4/24 19:07, Jonathan Cameron wrote:
> On Mon, 15 Jul 2024 18:28:30 +0100
> [email-masked] wrote:
>
>> From: Alejandro Lucero <[email-masked]>
>>
>> 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/[email-masked]/T/#m4271ee49a91615c8af54e3ab20679f8be3099393
>>
> Use the permalink link under these to get shorter links.
> https://lore.kernel.org/linux-cxl/[email-masked]/
> goes to the same patch.

I'll do.

>
>> Signed-off-by: Alejandro Lucero <[email-masked]>
>> Co-developed-by: Dan Williams <[email-masked]>
>
>> +
>> +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);
> Some cleanup.h magic would help here by allowing early returns.
> Needs the scoped lock though to ensure it's released before the
> devm_add_action_or_reset() as I'd guess we will deadlock otherwise
> if that fails.

Yes, I'll try to use it making cleaner code.

>> +	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",
>> +			     &avail);
>>   		rc = -ENOSPC;
>>   		goto out;
>>   	}
>> @@ -550,6 +570,99 @@ 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);
>>   }
>>   
>> +static int find_free_decoder(struct device *dev, void *data)
>> +{
>> +	struct cxl_endpoint_decoder *cxled;
>> +	struct cxl_port *port;
>> +
>> +	if (!is_endpoint_decoder(dev))
>> +		return 0;
>> +
>> +	cxled = to_cxl_endpoint_decoder(dev);
>> +	port = cxled_to_port(cxled);
>> +
>> +	if (cxled->cxld.id != port->hdm_end + 1) {
>> +		return 0;
> No brackets

Sure.

>> +	}
>> +	return 1;
>> +}
>> +
>> +/**
>> + * cxl_request_dpa - search and reserve DPA given input constraints
>> + * @endpoint: an endpoint port with available decoders
>> + * @mode: 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.
> We are going to need a policy control on the max value.
> Otherwise, if you have two devices that support huge capacity and
> not enough space, who gets it will just be a race.
>
> Not a problem for now though!

I agree. If CXL ends up being what we hope, these races will need to be 
better handled.

Thanks!

>> + *
>> + * 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.
>> + */
>
>