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Message-ID: <20250930154719.000000fd@huawei.com>
Date: Tue, 30 Sep 2025 15:47:19 +0100
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: Alejandro Lucero Palau <alucerop@....com>
CC: <alejandro.lucero-palau@....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 Wed, 24 Sep 2025 17:16:14 +0100
Alejandro Lucero Palau <alucerop@....com> wrote:
> On 9/18/25 15:35, Jonathan Cameron wrote:
> > On Thu, 18 Sep 2025 10:17:35 +0100
> > <alejandro.lucero-palau@....com> wrote:
> >
> >> From: Alejandro Lucero <alucerop@....com>
> >>
> >> 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/168592159290.1948938.13522227102445462976.stgit@dwillia2-xfh.jf.intel.com/
> >>
> >> Signed-off-by: Alejandro Lucero <alucerop@....com>
> >> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> > 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.
> >
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