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fwctl/cxl: Add documentation to FWCTL CXL
Add policy and operational documentation for FWCTL CXL. Link: https://patch.msgid.link/r/[email protected] Reviewed-by: Jonathan Cameron <[email protected]> Reviewed-by: Dan Williams <[email protected]> Signed-off-by: Dave Jiang <[email protected]> Signed-off-by: Jason Gunthorpe <[email protected]>
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‎Documentation/userspace-api/fwctl/fwctl-cxl.rst

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.. SPDX-License-Identifier: GPL-2.0
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================
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fwctl cxl driver
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================
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:Author: Dave Jiang
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Overview
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========
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The CXL spec defines a set of commands that can be issued to the mailbox of a
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CXL device or switch. It also left room for vendor specific commands to be
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issued to the mailbox as well. fwctl provides a path to issue a set of allowed
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mailbox commands from user space to the device moderated by the kernel driver.
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The following 3 commands will be used to support CXL Features:
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CXL spec r3.1 8.2.9.6.1 Get Supported Features (Opcode 0500h)
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CXL spec r3.1 8.2.9.6.2 Get Feature (Opcode 0501h)
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CXL spec r3.1 8.2.9.6.3 Set Feature (Opcode 0502h)
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The "Get Supported Features" return data may be filtered by the kernel driver to
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drop any features that are forbidden by the kernel or being exclusively used by
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the kernel. The driver will set the "Set Feature Size" of the "Get Supported
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Features Supported Feature Entry" to 0 to indicate that the Feature cannot be
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modified. The "Get Supported Features" command and the "Get Features" falls
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under the fwctl policy of FWCTL_RPC_CONFIGURATION.
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For "Set Feature" command, the access policy currently is broken down into two
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categories depending on the Set Feature effects reported by the device. If the
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Set Feature will cause immediate change to the device, the fwctl access policy
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must be FWCTL_RPC_DEBUG_WRITE_FULL. The effects for this level are
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"immediate config change", "immediate data change", "immediate policy change",
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or "immediate log change" for the set effects mask. If the effects are "config
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change with cold reset" or "config change with conventional reset", then the
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fwctl access policy must be FWCTL_RPC_DEBUG_WRITE or higher.
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fwctl cxl User API
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==================
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.. kernel-doc:: include/uapi/fwctl/cxl.h
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1. Driver info query
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--------------------
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First step for the app is to issue the ioctl(FWCTL_CMD_INFO). Successful
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invocation of the ioctl implies the Features capability is operational and
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returns an all zeros 32bit payload. A ``struct fwctl_info`` needs to be filled
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out with the ``fwctl_info.out_device_type`` set to ``FWCTL_DEVICE_TYPE_CXL``.
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The return data should be ``struct fwctl_info_cxl`` that contains a reserved
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32bit field that should be all zeros.
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2. Send hardware commands
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-------------------------
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Next step is to send the 'Get Supported Features' command to the driver from
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user space via ioctl(FWCTL_RPC). A ``struct fwctl_rpc_cxl`` is pointed to
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by ``fwctl_rpc.in``. ``struct fwctl_rpc_cxl.in_payload`` points to
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the hardware input structure that is defined by the CXL spec. ``fwctl_rpc.out``
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points to the buffer that contains a ``struct fwctl_rpc_cxl_out`` that includes
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the hardware output data inlined as ``fwctl_rpc_cxl_out.payload``. This command
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is called twice. First time to retrieve the number of features supported.
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A second time to retrieve the specific feature details as the output data.
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After getting the specific feature details, a Get/Set Feature command can be
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appropriately programmed and sent. For a "Set Feature" command, the retrieved
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feature info contains an effects field that details the resulting
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"Set Feature" command will trigger. That will inform the user whether
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the system is configured to allowed the "Set Feature" command or not.
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Code example of a Get Feature
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. code-block:: c
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static int cxl_fwctl_rpc_get_test_feature(int fd, struct test_feature *feat_ctx,
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const uint32_t expected_data)
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{
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struct cxl_mbox_get_feat_in *feat_in;
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struct fwctl_rpc_cxl_out *out;
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struct fwctl_rpc rpc = {0};
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struct fwctl_rpc_cxl *in;
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size_t out_size, in_size;
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uint32_t val;
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void *data;
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int rc;
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in_size = sizeof(*in) + sizeof(*feat_in);
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rc = posix_memalign((void **)&in, 16, in_size);
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if (rc)
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return -ENOMEM;
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memset(in, 0, in_size);
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feat_in = &in->get_feat_in;
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uuid_copy(feat_in->uuid, feat_ctx->uuid);
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feat_in->count = feat_ctx->get_size;
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out_size = sizeof(*out) + feat_ctx->get_size;
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rc = posix_memalign((void **)&out, 16, out_size);
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if (rc)
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goto free_in;
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memset(out, 0, out_size);
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in->opcode = CXL_MBOX_OPCODE_GET_FEATURE;
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in->op_size = sizeof(*feat_in);
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rpc.size = sizeof(rpc);
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rpc.scope = FWCTL_RPC_CONFIGURATION;
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rpc.in_len = in_size;
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rpc.out_len = out_size;
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rpc.in = (uint64_t)(uint64_t *)in;
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rpc.out = (uint64_t)(uint64_t *)out;
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rc = send_command(fd, &rpc, out);
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if (rc)
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goto free_all;
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data = out->payload;
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val = le32toh(*(__le32 *)data);
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if (memcmp(&val, &expected_data, sizeof(val)) != 0) {
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rc = -ENXIO;
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goto free_all;
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}
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free_all:
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free(out);
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free_in:
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free(in);
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return rc;
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}
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Take a look at CXL CLI test directory
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<https://github.com/pmem/ndctl/tree/main/test/fwctl.c> for a detailed user code
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for examples on how to exercise this path.
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fwctl cxl Kernel API
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====================
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.. kernel-doc:: drivers/cxl/core/features.c
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:export:
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.. kernel-doc:: include/cxl/features.h

‎Documentation/userspace-api/fwctl/index.rst

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:maxdepth: 1
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fwctl
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fwctl-cxl

‎MAINTAINERS

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L: [email protected]
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S: Maintained
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F: Documentation/driver-api/cxl
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F: Documentation/userspace-api/fwctl/fwctl-cxl.rst
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F: drivers/cxl/
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F: include/cxl/
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F: include/uapi/linux/cxl_mem.h

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