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

From: Jonathan Cameron <[email-masked]>
Date: Thu, 18 Sep 2025 15:35:03 +0100

On Thu, 18 Sep 2025 10:17:35 +0100
<[email-masked]> wrote:

> From: Alejandro Lucero <[email-masked]>
> 
> CXL region creation involves allocating capacity from Device Physical Address
> (DPA) and assigning it to decode a given Host Physical Address (HPA). Before
> determining how much DPA to allocate the amount of available HPA must be
> determined. Also, not all HPA is created equal, some HPA targets RAM, some
> targets PMEM, some is prepared for device-memory flows like HDM-D and HDM-DB,
> and some is HDM-H (host-only).
> 
> In order to support Type2 CXL devices, wrap all of those concerns into
> an API that retrieves a root decoder (platform CXL window) that fits the
> specified constraints and the capacity available for a new region.
> 
> Add a complementary function for releasing the reference to such root
> decoder.
> 
> Based on https://lore.kernel.org/linux-cxl/[email-masked]/
> 
> Signed-off-by: Alejandro Lucero <[email-masked]>
> Reviewed-by: Jonathan Cameron <[email-masked]>
Either I was half asleep or a few things have snuck in.

See below.

> +
> +/**
> + * cxl_get_hpa_freespace - find a root decoder with free capacity per constraints
> + * @endpoint: the endpoint requiring the HPA

The parameter seems to have changed.  Make sure to point scripts/kernel-doc at each
file to check for stuff like this.

> + * @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
> + * 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;

Nope to this.  See the stuff in cleanup.h on why not.

> +	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);
> +	}
> +
> +	root  = find_cxl_root(endpoint);

extra space, but should be 
	struct cxl_root *root __free(put_cxl_root) = find_cxl_root(endpoint);
anyway.

> +	if (!root) {
> +		dev_dbg(&endpoint->dev, "endpoint can not be related to a root port\n");
> +		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);
> +
> +	if (!ctx.cxlrd)
> +		return ERR_PTR(-ENOMEM);
> +
> +	*max_avail_contig = ctx.max_hpa;
> +	return ctx.cxlrd;
> +}
> +EXPORT_SYMBOL_NS_GPL(cxl_get_hpa_freespace, "CXL");
> +
> +/*
> + * TODO: those references released here should avoid the decoder to be
> + * unregistered.

That is an ominous sounding TODO for a v18.  Perhaps add more on why this
is still here. 

> + */
> +void cxl_put_root_decoder(struct cxl_root_decoder *cxlrd)
> +{
> +	put_device(cxlrd_dev(cxlrd));
> +}
> +EXPORT_SYMBOL_NS_GPL(cxl_put_root_decoder, "CXL");