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Message-ID: <43d957e0-f52b-4ba8-aa87-cfb8472b8b67@infradead.org>
Date: Sat, 10 May 2025 19:18:27 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Gregory Price <gourry@...rry.net>, linux-cxl@...r.kernel.org
Cc: linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
kernel-team@...a.com, dave@...olabs.net, jonathan.cameron@...wei.com,
dave.jiang@...el.com, alison.schofield@...el.com, vishal.l.verma@...el.com,
ira.weiny@...el.com, dan.j.williams@...el.com, corbet@....net
Subject: Re: [RFC PATCH v2 10/18] cxl: docs/linux/dax-driver documentation
On 4/30/25 11:10 AM, Gregory Price wrote:
> Add documentation on how the CXL driver interacts with the DAX driver.
>
> Signed-off-by: Gregory Price <gourry@...rry.net>
> ---
> Documentation/driver-api/cxl/index.rst | 1 +
> .../driver-api/cxl/linux/cxl-driver.rst | 115 ++++++++++++++++--
> .../driver-api/cxl/linux/dax-driver.rst | 43 +++++++
> 3 files changed, 149 insertions(+), 10 deletions(-)
> create mode 100644 Documentation/driver-api/cxl/linux/dax-driver.rst
>
> diff --git a/Documentation/driver-api/cxl/linux/cxl-driver.rst b/Documentation/driver-api/cxl/linux/cxl-driver.rst
> index 486baf8551aa..1a354ea1cda4 100644
> --- a/Documentation/driver-api/cxl/linux/cxl-driver.rst
> +++ b/Documentation/driver-api/cxl/linux/cxl-driver.rst
> @@ -34,6 +34,32 @@ into a single memory region. The memory region has been converted to dax. ::
> decoder1.0 decoder5.0 endpoint5 port1 region0
> decoder2.0 decoder5.1 endpoint6 port2 root0
>
> +
> +.. kernel-render:: DOT
> + :alt: Digraph of CXL fabric describing host-bridge interleaving
> + :caption: Diagraph of CXL fabric with a host-bridge interleave memory region
> +
> + digraph foo {
> + "root0" -> "port1";
> + "root0" -> "port3";
> + "root0" -> "decoder0.0";
> + "port1" -> "endpoint5";
> + "port3" -> "endpoint6";
> + "port1" -> "decoder1.0";
> + "port3" -> "decoder3.0";
> + "endpoint5" -> "decoder5.0";
> + "endpoint6" -> "decoder6.0";
> + "decoder0.0" -> "region0";
> + "decoder0.0" -> "decoder1.0";
> + "decoder0.0" -> "decoder3.0";
> + "decoder1.0" -> "decoder5.0";
> + "decoder3.0" -> "decoder6.0";
> + "decoder5.0" -> "region0";
> + "decoder6.0" -> "region0";
> + "region0" -> "dax_region0";
> + "dax_region0" -> "dax0.0";
> + }
> +
> For this section we'll explore the devices present in this configuration, but
> we'll explore more configurations in-depth in example configurations below.
>
> @@ -41,7 +67,7 @@ Base Devices
> ------------
> Most devices in a CXL fabric are a `port` of some kind (because each
> device mostly routes request from one device to the next, rather than
> -provide a manageable service).
> +provide a direct service).
>
> Root
> ~~~~
> @@ -53,6 +79,8 @@ The Root contains links to:
>
> * `Host Bridge Ports` defined by ACPI CEDT CHBS.
>
> +* `Downstream Ports` typically connected to `Host Bridge Ports`
Add ending '.' for consistency.
> +
> * `Root Decoders` defined by ACPI CEDT CFMWS.
>
> ::
> @@ -150,6 +178,27 @@ device configuration data. ::
> driver label_storage_size pmem serial
> firmware numa_node ram subsystem
>
> +A Memory Device is a discrete base object that is not a port. While it the
> +physical device it belongs to may host an `endpoint`, this relationship is
I have some parsing trouble with the sentence above. Maybe s/it the/the/.
> +not captured in sysfs.
> +
> +Port Relationships
> +~~~~~~~~~~~~~~~~~~
> +In our example described above, there are four host bridges attached to the
> +root, and two of the host bridges have one endpoint attached.
> +
> +.. kernel-render:: DOT
> + :alt: Digraph of CXL fabric describing host-bridge interleaving
> + :caption: Diagraph of CXL fabric with a host-bridge interleave memory region
> +
> + digraph foo {
> + "root0" -> "port1";
> + "root0" -> "port2";
> + "root0" -> "port3";
> + "root0" -> "port4";
> + "port1" -> "endpoint5";
> + "port3" -> "endpoint6";
> + }
>
> Decoders
> --------
> @@ -322,6 +371,29 @@ settings (granularity and ways must be the same).
> Endpoint decoders are created during :code:`cxl_endpoint_port_probe` in the
> :code:`cxl_port` driver, and is created based on a PCI device's DVSEC registers.
>
> +Decoder Relationships
> +~~~~~~~~~~~~~~~~~~~~~
> +In our example described above, there is one root decoder which routes memory
> +accesses over two host bridges. Each host bridge has a decoder which routes
> +access to their singular endpoint targets. Each endpoint has an decoder which
a decoder
> +translates HPA to DPA and services the memory request.
> +
> +The driver validates relationships between ports by decoder programming, so
> +we can think of decoders being related in a similarly hierarchical fashion to
> +ports.
> +
> +.. kernel-render:: DOT
> + :alt: Digraph of hierarchical relationship between root, switch, and endpoint decoders.
> + :caption: Diagraph of CXL root, switch, and endpoint decoders.
> +
> + digraph foo {
> + "root0" -> "decoder0.0";
> + "decoder0.0" -> "decoder1.0";
> + "decoder0.0" -> "decoder3.0";
> + "decoder1.0" -> "decoder5.0";
> + "decoder3.0" -> "decoder6.0";
> + }
> +
> Regions
> -------
>
> @@ -348,6 +420,17 @@ The interleave settings in a `Memory Region` describe the configuration of the
> `Interleave Set` - and are what can be expected to be seen in the endpoint
> interleave settings.
>
> +.. kernel-render:: DOT
> + :alt: Digraph of CXL memory region relationships between root and endpoint decoders.
> + :caption: Regions are created based on root decoder configurations. Endpoint decoders
> + must be programmed with the same interleave settings as the region.
> +
> + digraph foo {
> + "root0" -> "decoder0.0";
> + "decoder0.0" -> "region0";
> + "region0" -> "decoder5.0";
> + "region0" -> "decoder6.0";
> + }
>
> DAX Region
> ~~~~~~~~~~
> @@ -360,7 +443,6 @@ for more details. ::
> dax0.0 devtype modalias uevent
> dax_region driver subsystem
>
> -
> Mailbox Interfaces
> ------------------
> A mailbox command interface for each device is exposed in ::
> @@ -418,17 +500,30 @@ the relationships between a decoder and it's parent.
>
> For example, in a `Cross-Link First` interleave setup with 16 endpoints
> attached to 4 host bridges, linux expects the following ways/granularity
> -across the root, host bridge, and endpoints respectively. ::
> +across the root, host bridge, and endpoints respectively.
> +
> +.. flat-table:: 4x4 cross-link first interleave settings
> +
> + * - decoder
> + - ways
> + - granularity
>
> - ways granularity
> - root 4 256
> - host bridge 4 1024
> - endpoint 16 256
> + * - root
> + - 4
> + - 256
> +
> + * - host bridge
> + - 4
> + - 1024
> +
> + * - endpoint
> + - 16
> + - 256
>
> At the root, every a given access will be routed to the
> :code:`((HPA / 256) % 4)th` target host bridge. Within a host bridge, every
> -:code:`((HPA / 1024) % 4)th` target endpoint. Each endpoint will translate
> -the access based on the entire 16 device interleave set.
> +:code:`((HPA / 1024) % 4)th` target endpoint. Each endpoint translates based
> +on the entire 16 device interleave set.
>
> Unbalanced interleave sets are not supported - decoders at a similar point
> in the hierarchy (e.g. all host bridge decoders) must have the same ways and
> @@ -467,7 +562,7 @@ In this example, the CFMWS defines two discrete non-interleaved 4GB regions
> for each host bridge, and one interleaved 8GB region that targets both. This
> would result in 3 root decoders presenting in the root. ::
>
> - # ls /sys/bus/cxl/devices/root0
> + # ls /sys/bus/cxl/devices/root0/decoder*
> decoder0.0 decoder0.1 decoder0.2
>
> # cat /sys/bus/cxl/devices/decoder0.0/target_list start size
> diff --git a/Documentation/driver-api/cxl/linux/dax-driver.rst b/Documentation/driver-api/cxl/linux/dax-driver.rst
> new file mode 100644
> index 000000000000..5063d2b675b4
> --- /dev/null
> +++ b/Documentation/driver-api/cxl/linux/dax-driver.rst
> @@ -0,0 +1,43 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +====================
> +DAX Driver Operation
> +====================
> +The `Direct Access Device` driver was originally designed to provide a
> +memory-like access mechanism to memory-like block-devices. It was
> +extended to support CXL Memory Devices, which provide user-configured
> +memory devices.
> +
> +The CXL subsystem depends on the DAX subsystem to generate either:
to either:
> +
> +- A file-like interface to userland via :code:`/dev/daxN.Y`, or
- Generate a file-like interface ...
> +- Engaging the memory-hotplug interface to add CXL memory to page allocator.
- Engage the ...
> +
> +The DAX subsystem exposes this ability through the `cxl_dax_region` driver.
> +A `dax_region` provides the translation between a CXL `memory_region` and
> +a `DAX Device`.
> +
> +DAX Device
> +==========
> +A `DAX Device` is a file-like interface exposed in :code:`/dev/daxN.Y`. A
> +memory region exposed via dax device can be accessed via userland software
> +via the :code:`mmap()` system-call. The result is direct mappings to the
> +CXL capacity in the task's page tables.
> +
> +Users wishing to manually handle allocation of CXL memory should use this
> +interface.
> +
> +kmem conversion
> +===============
> +The :code:`dax_kmem` driver converts a `DAX Device` into a series of `hotplug
> +memory blocks` managed by :code:`kernel/memory-hotplug.c`. This capacity
> +will be exposed to the kernel page allocator in the user-selected memory
> +zone.
> +
> +The :code:`memmap_on_memory` setting (both global and DAX device local) dictate
dictates
> +where the kernell will allocate the :code:`struct folio` descriptors for this
kernel
> +memory will come from. If :code:`memmap_on_memory` is set, memory hotplug
> +will set aside a portion of the memory block capacity to allocate folios. If
> +unset, the memory is allocated via a normal :code:`GFP_KERNEL` allocation -
> +and as a result will most likely land on the local NUM node of the cpu executing
s/cpu/CPU/ preferably.
> +the hotplug operation.
--
~Randy
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