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

From: Jonathan Cameron <[email-masked]>
Date: Tue, 30 Sep 2025 15:47:19 +0100

On Wed, 24 Sep 2025 17:16:14 +0100
Alejandro Lucero Palau <[email-masked]> wrote:

> On 9/18/25 15:35, Jonathan Cameron wrote:
> > 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.  
> 
> 
> OK.
> 
> 
> >  
> >> + * @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.  
> 
> 
> I guess you mean to declare the pointer later on when assigned to the 
> object instead of a default NULL, as you point out later.

yes.

> 
> After reading the cleanup file, it is not clear to me if this is really 
> needed since there is no lock involved in that example for a potential bug.
> 

Do it anyway. It's not about bugs today, but about fragile code patterns that
may break later without it being obvious.

> 
> >> +	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.
> >