netdev - Re: [PATCH v18 09/20] cxl: Define a driver interface for HPA free space enumeration

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Message-ID: <a5783611-b837-4882-8d52-dfb5f4094f2c@amd.com>
Date: Wed, 24 Sep 2025 17:53:54 +0100
From: Alejandro Lucero Palau <alucerop@....com>
To: "Cheatham, Benjamin" <benjamin.cheatham@....com>,
 alejandro.lucero-palau@....com
Cc: Jonathan Cameron <Jonathan.Cameron@...wei.com>,
 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
Subject: Re: [PATCH v18 09/20] cxl: Define a driver interface for HPA free
 space enumeration


On 9/22/25 22:09, Cheatham, Benjamin wrote:
> [snip]
>
>> +
>> +/**
>> + * cxl_get_hpa_freespace - find a root decoder with free capacity per constraints
>> + * @endpoint: the endpoint requiring the HPA
>> + * @interleave_ways: number of entries in @host_bridges
>> + * @flags: CXL_DECODER_F flags for selecting RAM vs PMEM, and Type2 device
>> + * @max_avail_contig: output parameter of max contiguous bytes available in the
>> + *		      returned decoder
>> + *
>> + * Returns a pointer to a struct cxl_root_decoder
>> + *
>> + * The return tuple of a 'struct cxl_root_decoder' and 'bytes available given
>> + * in (@max_avail_contig))' is a point in time snapshot. If by the time the
>> + * caller goes to use this root decoder's capacity the capacity is reduced then
> s/capacity the/capacity and the
>
> Or you could cleanup to: "If by the time the caller goes to use this root decoder
> and its capacity is reduced then...".


OK.


>> + * caller needs to loop and retry.
>> + *
>> + * The returned root decoder has an elevated reference count that needs to be
>> + * put with cxl_put_root_decoder(cxlrd).
>> + */
>> +struct cxl_root_decoder *cxl_get_hpa_freespace(struct cxl_memdev *cxlmd,
>> +					       int interleave_ways,
>> +					       unsigned long flags,
>> +					       resource_size_t *max_avail_contig)
>> +{
>> +	struct cxl_root *root __free(put_cxl_root) = NULL;
>> +	struct cxl_port *endpoint = cxlmd->endpoint;
>> +	struct cxlrd_max_context ctx = {
>> +		.host_bridges = &endpoint->host_bridge,
>> +		.flags = flags,
>> +	};
>> +	struct cxl_port *root_port;
>> +
>> +	if (!endpoint) {
>> +		dev_dbg(&cxlmd->dev, "endpoint not linked to memdev\n");
>> +		return ERR_PTR(-ENXIO);
>> +	}
> I'm 99% sure you'll crash in this case. The endpoint pointer is dereferenced when assigning to ctx above.


Yes, I need to delay the host_bridges assignment.

>> +
>> +	root  = find_cxl_root(endpoint);
>> +	if (!root) {
>> +		dev_dbg(&endpoint->dev, "endpoint can not be related to a root port\n");
> s/can not/is not


OK


>
>> +		return ERR_PTR(-ENXIO);
>> +	}
>> +
>> +	root_port = &root->port;
>> +	scoped_guard(rwsem_read, &cxl_rwsem.region)
>> +		device_for_each_child(&root_port->dev, &ctx, find_max_hpa);
> Do you need the brackets for the scoped guard? I'm not sure how the macro expands when they aren't
> there.


I can see other uses like this in core/region.c so I guess they are not 
needed.


Thanks!