netdev - Re: [PATCH v17 13/22] cxl: Define a driver interface for DPA allocation

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Message-ID: <00624fd6-8b30-4f86-99b7-f7df2cedf401@amd.com>
Date: Fri, 4 Jul 2025 16:18:07 +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, edward.cree@....com, davem@...emloft.net,
 kuba@...nel.org, pabeni@...hat.com, edumazet@...gle.com,
 dave.jiang@...el.com, Ben Cheatham <benjamin.cheatham@....com>
Subject: Re: [PATCH v17 13/22] cxl: Define a driver interface for DPA
 allocation


On 6/27/25 10:06, Jonathan Cameron wrote:
> On Tue, 24 Jun 2025 15:13:46 +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.
>>
>> In order to support CXL Type2 devices, define an API, cxl_request_dpa(),
>> that tries to allocate the DPA memory the driver requires to operate.The
>> memory requested should not be bigger than the max available HPA obtained
>> previously with cxl_get_hpa_freespace.
>>
>> Based on https://lore.kernel.org/linux-cxl/168592158743.1948938.7622563891193802610.stgit@dwillia2-xfh.jf.intel.com/
>>
>> Signed-off-by: Alejandro Lucero <alucerop@....com>
>> Reviewed-by: Ben Cheatham <benjamin.cheatham@....com>
>> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Hmm. I wouldn't trust this last guy not to have missed a few things.
> See below.
>

My mistake. The patch changed after applying Dan's suggestions, so I 
should have removed the tags.


>> +static struct cxl_endpoint_decoder *
>> +cxl_find_free_decoder(struct cxl_memdev *cxlmd)
>> +{
>> +	struct cxl_port *endpoint = cxlmd->endpoint;
>> +	struct device *dev;
>> +
>> +	scoped_guard(rwsem_read, &cxl_dpa_rwsem) {
>> +		dev = device_find_child(&endpoint->dev, NULL,
>> +					find_free_decoder);
>> +	}
>> +	if (dev)
>> +		return to_cxl_endpoint_decoder(dev);
>> +
>> +	return NULL;
> If this code isn't going to get modified later, could be simpler as
>
> 	guard(rwsem_read)(&cxl_dpa_rwsem) {
> 	dev = device_find_child(&endpoint->dev, NULL, find_free_decoder);
> 	if (!dev)
> 		return NULL
>
> 	return to_cxl_endpoint_decoder(dev);
> 		


Yes, it makes sense.


>> +}
>> +
>> +/**
>> + * cxl_request_dpa - search and reserve DPA given input constraints
>> + * @cxlmd: memdev with an endpoint port with available decoders
>> + * @mode: DPA operation mode (ram vs pmem)
>> + * @alloc: dpa size required
>> + *
>> + * Returns a pointer to a cxl_endpoint_decoder struct or an error
>> + *
>> + * 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. The expectation is that @alloc is a driver known
>> + * value based on the device capacity but it could not be available
>> + * due to HPA constraints.
>> + *
>> + * Returns a pinned cxl_decoder with at least @alloc 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,
>> +					     enum cxl_partition_mode mode,
>> +					     resource_size_t alloc)
>> +{
>> +	struct cxl_endpoint_decoder *cxled __free(put_cxled) =
>> +				cxl_find_free_decoder(cxlmd);
>> +	struct device *cxled_dev;
>> +	int rc;
>> +
>> +	if (!IS_ALIGNED(alloc, SZ_256M))
>> +		return ERR_PTR(-EINVAL);
>> +
>> +	if (!cxled) {
>> +		rc = -ENODEV;
>> +		goto err;
>> +	}
>> +
>> +	rc = cxl_dpa_set_part(cxled, mode);
>> +	if (rc)
>> +		goto err;
>> +
>> +	rc = cxl_dpa_alloc(cxled, alloc);
>> +	if (rc)
>> +		goto err;
>> +
>> +	return cxled;
> I was kind of expecting us to disable the put above wuth a return_ptr()
> here.  If there is a reason why not, add a comment as it is not obvious
> to me anyway!


It seems I did a mess applying Dan's suggestions. You are right here and 
the put not invoked.


>
>> +err:
>> +	put_device(cxled_dev);
> It's not been assigned.  I'm surprised if none of the standard tooling
> (sparse, smatch etc screamed about this one).
> For complex series like this it's worth running them on each patch just to
> avoid possible bot warnings later!


This is bad, of course. I did not see it, and I realize now Dan's 
changes make this harder to handle.

I'll fix it.

Thanks!



>
>> +	return ERR_PTR(rc);
>> +}
>> +EXPORT_SYMBOL_NS_GPL(cxl_request_dpa, "CXL");
> __CXL_CXL_H__ */
>