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+====================================
+Coherent Accelerator Interface (CXL)
+====================================
+
+Introduction
+============
+
+    The coherent accelerator interface is designed to allow the
+    coherent connection of accelerators (FPGAs and other devices) to a
+    POWER system. These devices need to adhere to the Coherent
+    Accelerator Interface Architecture (CAIA).
+
+    IBM refers to this as the Coherent Accelerator Processor Interface
+    or CAPI. In the kernel it's referred to by the name CXL to avoid
+    confusion with the ISDN CAPI subsystem.
+
+    Coherent in this context means that the accelerator and CPUs can
+    both access system memory directly and with the same effective
+    addresses.
+
+
+Hardware overview
+=================
+
+    ::
+
+         POWER8/9             FPGA
+       +----------+        +---------+
+       |          |        |         |
+       |   CPU    |        |   AFU   |
+       |          |        |         |
+       |          |        |         |
+       |          |        |         |
+       +----------+        +---------+
+       |   PHB    |        |         |
+       |   +------+        |   PSL   |
+       |   | CAPP |<------>|         |
+       +---+------+  PCIE  +---------+
+
+    The POWER8/9 chip has a Coherently Attached Processor Proxy (CAPP)
+    unit which is part of the PCIe Host Bridge (PHB). This is managed
+    by Linux by calls into OPAL. Linux doesn't directly program the
+    CAPP.
+
+    The FPGA (or coherently attached device) consists of two parts.
+    The POWER Service Layer (PSL) and the Accelerator Function Unit
+    (AFU). The AFU is used to implement specific functionality behind
+    the PSL. The PSL, among other things, provides memory address
+    translation services to allow each AFU direct access to userspace
+    memory.
+
+    The AFU is the core part of the accelerator (eg. the compression,
+    crypto etc function). The kernel has no knowledge of the function
+    of the AFU. Only userspace interacts directly with the AFU.
+
+    The PSL provides the translation and interrupt services that the
+    AFU needs. This is what the kernel interacts with. For example, if
+    the AFU needs to read a particular effective address, it sends
+    that address to the PSL, the PSL then translates it, fetches the
+    data from memory and returns it to the AFU. If the PSL has a
+    translation miss, it interrupts the kernel and the kernel services
+    the fault. The context to which this fault is serviced is based on
+    who owns that acceleration function.
+
+    - POWER8 and PSL Version 8 are compliant to the CAIA Version 1.0.
+    - POWER9 and PSL Version 9 are compliant to the CAIA Version 2.0.
+
+    This PSL Version 9 provides new features such as:
+
+    * Interaction with the nest MMU on the P9 chip.
+    * Native DMA support.
+    * Supports sending ASB_Notify messages for host thread wakeup.
+    * Supports Atomic operations.
+    * etc.
+
+    Cards with a PSL9 won't work on a POWER8 system and cards with a
+    PSL8 won't work on a POWER9 system.
+
+AFU Modes
+=========
+
+    There are two programming modes supported by the AFU. Dedicated
+    and AFU directed. AFU may support one or both modes.
+
+    When using dedicated mode only one MMU context is supported. In
+    this mode, only one userspace process can use the accelerator at
+    time.
+
+    When using AFU directed mode, up to 16K simultaneous contexts can
+    be supported. This means up to 16K simultaneous userspace
+    applications may use the accelerator (although specific AFUs may
+    support fewer). In this mode, the AFU sends a 16 bit context ID
+    with each of its requests. This tells the PSL which context is
+    associated with each operation. If the PSL can't translate an
+    operation, the ID can also be accessed by the kernel so it can
+    determine the userspace context associated with an operation.
+
+
+MMIO space
+==========
+
+    A portion of the accelerator MMIO space can be directly mapped
+    from the AFU to userspace. Either the whole space can be mapped or
+    just a per context portion. The hardware is self describing, hence
+    the kernel can determine the offset and size of the per context
+    portion.
+
+
+Interrupts
+==========
+
+    AFUs may generate interrupts that are destined for userspace. These
+    are received by the kernel as hardware interrupts and passed onto
+    userspace by a read syscall documented below.
+
+    Data storage faults and error interrupts are handled by the kernel
+    driver.
+
+
+Work Element Descriptor (WED)
+=============================
+
+    The WED is a 64-bit parameter passed to the AFU when a context is
+    started. Its format is up to the AFU hence the kernel has no
+    knowledge of what it represents. Typically it will be the
+    effective address of a work queue or status block where the AFU
+    and userspace can share control and status information.
+
+
+
+
+User API
+========
+
+1. AFU character devices
+
+    For AFUs operating in AFU directed mode, two character device
+    files will be created. /dev/cxl/afu0.0m will correspond to a
+    master context and /dev/cxl/afu0.0s will correspond to a slave
+    context. Master contexts have access to the full MMIO space an
+    AFU provides. Slave contexts have access to only the per process
+    MMIO space an AFU provides.
+
+    For AFUs operating in dedicated process mode, the driver will
+    only create a single character device per AFU called
+    /dev/cxl/afu0.0d. This will have access to the entire MMIO space
+    that the AFU provides (like master contexts in AFU directed).
+
+    The types described below are defined in include/uapi/misc/cxl.h
+
+    The following file operations are supported on both slave and
+    master devices.
+
+    A userspace library libcxl is available here:
+
+        https://github.com/ibm-capi/libcxl
+
+    This provides a C interface to this kernel API.
+
+open
+----
+
+    Opens the device and allocates a file descriptor to be used with
+    the rest of the API.
+
+    A dedicated mode AFU only has one context and only allows the
+    device to be opened once.
+
+    An AFU directed mode AFU can have many contexts, the device can be
+    opened once for each context that is available.
+
+    When all available contexts are allocated the open call will fail
+    and return -ENOSPC.
+
+    Note:
+          IRQs need to be allocated for each context, which may limit
+          the number of contexts that can be created, and therefore
+          how many times the device can be opened. The POWER8 CAPP
+          supports 2040 IRQs and 3 are used by the kernel, so 2037 are
+          left. If 1 IRQ is needed per context, then only 2037
+          contexts can be allocated. If 4 IRQs are needed per context,
+          then only 2037/4 = 509 contexts can be allocated.
+
+
+ioctl
+-----
+
+    CXL_IOCTL_START_WORK:
+        Starts the AFU context and associates it with the current
+        process. Once this ioctl is successfully executed, all memory
+        mapped into this process is accessible to this AFU context
+        using the same effective addresses. No additional calls are
+        required to map/unmap memory. The AFU memory context will be
+        updated as userspace allocates and frees memory. This ioctl
+        returns once the AFU context is started.
+
+        Takes a pointer to a struct cxl_ioctl_start_work
+
+            ::
+
+                struct cxl_ioctl_start_work {
+                        __u64 flags;
+                        __u64 work_element_descriptor;
+                        __u64 amr;
+                        __s16 num_interrupts;
+                        __s16 reserved1;
+                        __s32 reserved2;
+                        __u64 reserved3;
+                        __u64 reserved4;
+                        __u64 reserved5;
+                        __u64 reserved6;
+                };
+
+            flags:
+                Indicates which optional fields in the structure are
+                valid.
+
+            work_element_descriptor:
+                The Work Element Descriptor (WED) is a 64-bit argument
+                defined by the AFU. Typically this is an effective
+                address pointing to an AFU specific structure
+                describing what work to perform.
+
+            amr:
+                Authority Mask Register (AMR), same as the powerpc
+                AMR. This field is only used by the kernel when the
+                corresponding CXL_START_WORK_AMR value is specified in
+                flags. If not specified the kernel will use a default
+                value of 0.
+
+            num_interrupts:
+                Number of userspace interrupts to request. This field
+                is only used by the kernel when the corresponding
+                CXL_START_WORK_NUM_IRQS value is specified in flags.
+                If not specified the minimum number required by the
+                AFU will be allocated. The min and max number can be
+                obtained from sysfs.
+
+            reserved fields:
+                For ABI padding and future extensions
+
+    CXL_IOCTL_GET_PROCESS_ELEMENT:
+        Get the current context id, also known as the process element.
+        The value is returned from the kernel as a __u32.
+
+
+mmap
+----
+
+    An AFU may have an MMIO space to facilitate communication with the
+    AFU. If it does, the MMIO space can be accessed via mmap. The size
+    and contents of this area are specific to the particular AFU. The
+    size can be discovered via sysfs.
+
+    In AFU directed mode, master contexts are allowed to map all of
+    the MMIO space and slave contexts are allowed to only map the per
+    process MMIO space associated with the context. In dedicated
+    process mode the entire MMIO space can always be mapped.
+
+    This mmap call must be done after the START_WORK ioctl.
+
+    Care should be taken when accessing MMIO space. Only 32 and 64-bit
+    accesses are supported by POWER8. Also, the AFU will be designed
+    with a specific endianness, so all MMIO accesses should consider
+    endianness (recommend endian(3) variants like: le64toh(),
+    be64toh() etc). These endian issues equally apply to shared memory
+    queues the WED may describe.
+
+
+read
+----
+
+    Reads events from the AFU. Blocks if no events are pending
+    (unless O_NONBLOCK is supplied). Returns -EIO in the case of an
+    unrecoverable error or if the card is removed.
+
+    read() will always return an integral number of events.
+
+    The buffer passed to read() must be at least 4K bytes.
+
+    The result of the read will be a buffer of one or more events,
+    each event is of type struct cxl_event, of varying size::
+
+            struct cxl_event {
+                    struct cxl_event_header header;
+                    union {
+                            struct cxl_event_afu_interrupt irq;
+                            struct cxl_event_data_storage fault;
+                            struct cxl_event_afu_error afu_error;
+                    };
+            };
+
+    The struct cxl_event_header is defined as
+
+        ::
+
+            struct cxl_event_header {
+                    __u16 type;
+                    __u16 size;
+                    __u16 process_element;
+                    __u16 reserved1;
+            };
+
+        type:
+            This defines the type of event. The type determines how
+            the rest of the event is structured. These types are
+            described below and defined by enum cxl_event_type.
+
+        size:
+            This is the size of the event in bytes including the
+            struct cxl_event_header. The start of the next event can
+            be found at this offset from the start of the current
+            event.
+
+        process_element:
+            Context ID of the event.
+
+        reserved field:
+            For future extensions and padding.
+
+    If the event type is CXL_EVENT_AFU_INTERRUPT then the event
+    structure is defined as
+
+        ::
+
+            struct cxl_event_afu_interrupt {
+                    __u16 flags;
+                    __u16 irq; /* Raised AFU interrupt number */
+                    __u32 reserved1;
+            };
+
+        flags:
+            These flags indicate which optional fields are present
+            in this struct. Currently all fields are mandatory.
+
+        irq:
+            The IRQ number sent by the AFU.
+
+        reserved field:
+            For future extensions and padding.
+
+    If the event type is CXL_EVENT_DATA_STORAGE then the event
+    structure is defined as
+
+        ::
+
+            struct cxl_event_data_storage {
+                    __u16 flags;
+                    __u16 reserved1;
+                    __u32 reserved2;
+                    __u64 addr;
+                    __u64 dsisr;
+                    __u64 reserved3;
+            };
+
+        flags:
+            These flags indicate which optional fields are present in
+            this struct. Currently all fields are mandatory.
+
+        address:
+            The address that the AFU unsuccessfully attempted to
+            access. Valid accesses will be handled transparently by the
+            kernel but invalid accesses will generate this event.
+
+        dsisr:
+            This field gives information on the type of fault. It is a
+            copy of the DSISR from the PSL hardware when the address
+            fault occurred. The form of the DSISR is as defined in the
+            CAIA.
+
+        reserved fields:
+            For future extensions
+
+    If the event type is CXL_EVENT_AFU_ERROR then the event structure
+    is defined as
+
+        ::
+
+            struct cxl_event_afu_error {
+                    __u16 flags;
+                    __u16 reserved1;
+                    __u32 reserved2;
+                    __u64 error;
+            };
+
+        flags:
+            These flags indicate which optional fields are present in
+            this struct. Currently all fields are Mandatory.
+
+        error:
+            Error status from the AFU. Defined by the AFU.
+
+        reserved fields:
+            For future extensions and padding
+
+
+2. Card character device (powerVM guest only)
+
+    In a powerVM guest, an extra character device is created for the
+    card. The device is only used to write (flash) a new image on the
+    FPGA accelerator. Once the image is written and verified, the
+    device tree is updated and the card is reset to reload the updated
+    image.
+
+open
+----
+
+    Opens the device and allocates a file descriptor to be used with
+    the rest of the API. The device can only be opened once.
+
+ioctl
+-----
+
+CXL_IOCTL_DOWNLOAD_IMAGE / CXL_IOCTL_VALIDATE_IMAGE:
+    Starts and controls flashing a new FPGA image. Partial
+    reconfiguration is not supported (yet), so the image must contain
+    a copy of the PSL and AFU(s). Since an image can be quite large,
+    the caller may have to iterate, splitting the image in smaller
+    chunks.
+
+    Takes a pointer to a struct cxl_adapter_image::
+
+        struct cxl_adapter_image {
+            __u64 flags;
+            __u64 data;
+            __u64 len_data;
+            __u64 len_image;
+            __u64 reserved1;
+            __u64 reserved2;
+            __u64 reserved3;
+            __u64 reserved4;
+        };
+
+    flags:
+        These flags indicate which optional fields are present in
+        this struct. Currently all fields are mandatory.
+
+    data:
+        Pointer to a buffer with part of the image to write to the
+        card.
+
+    len_data:
+        Size of the buffer pointed to by data.
+
+    len_image:
+        Full size of the image.
+
+
+Sysfs Class
+===========
+
+    A cxl sysfs class is added under /sys/class/cxl to facilitate
+    enumeration and tuning of the accelerators. Its layout is
+    described in Documentation/ABI/testing/sysfs-class-cxl
+
+
+Udev rules
+==========
+
+    The following udev rules could be used to create a symlink to the
+    most logical chardev to use in any programming mode (afuX.Yd for
+    dedicated, afuX.Ys for afu directed), since the API is virtually
+    identical for each::
+
+        SUBSYSTEM=="cxl", ATTRS{mode}=="dedicated_process", SYMLINK="cxl/%b"
+        SUBSYSTEM=="cxl", ATTRS{mode}=="afu_directed", \
+                          KERNEL=="afu[0-9]*.[0-9]*s", SYMLINK="cxl/%b"