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diff --git a/Documentation/misc-devices/uacce.rst b/Documentation/misc-devices/uacce.rst new file mode 100644 index 000000000..1db412e9b --- /dev/null +++ b/Documentation/misc-devices/uacce.rst @@ -0,0 +1,176 @@ +.. SPDX-License-Identifier: GPL-2.0 + +Introduction of Uacce +--------------------- + +Uacce (Unified/User-space-access-intended Accelerator Framework) targets to +provide Shared Virtual Addressing (SVA) between accelerators and processes. +So accelerator can access any data structure of the main cpu. +This differs from the data sharing between cpu and io device, which share +only data content rather than address. +Because of the unified address, hardware and user space of process can +share the same virtual address in the communication. +Uacce takes the hardware accelerator as a heterogeneous processor, while +IOMMU share the same CPU page tables and as a result the same translation +from va to pa. + +:: + + __________________________ __________________________ + | | | | + | User application (CPU) | | Hardware Accelerator | + |__________________________| |__________________________| + + | | + | va | va + V V + __________ __________ + | | | | + | MMU | | IOMMU | + |__________| |__________| + | | + | | + V pa V pa + _______________________________________ + | | + | Memory | + |_______________________________________| + + + +Architecture +------------ + +Uacce is the kernel module, taking charge of iommu and address sharing. +The user drivers and libraries are called WarpDrive. + +The uacce device, built around the IOMMU SVA API, can access multiple +address spaces, including the one without PASID. + +A virtual concept, queue, is used for the communication. It provides a +FIFO-like interface. And it maintains a unified address space between the +application and all involved hardware. + +:: + + ___________________ ________________ + | | user API | | + | WarpDrive library | ------------> | user driver | + |___________________| |________________| + | | + | | + | queue fd | + | | + | | + v | + ___________________ _________ | + | | | | | mmap memory + | Other framework | | uacce | | r/w interface + | crypto/nic/others | |_________| | + |___________________| | + | | | + | register | register | + | | | + | | | + | _________________ __________ | + | | | | | | + ------------- | Device Driver | | IOMMU | | + |_________________| |__________| | + | | + | V + | ___________________ + | | | + -------------------------- | Device(Hardware) | + |___________________| + + +How does it work +---------------- + +Uacce uses mmap and IOMMU to play the trick. + +Uacce creates a chrdev for every device registered to it. New queue is +created when user application open the chrdev. The file descriptor is used +as the user handle of the queue. +The accelerator device present itself as an Uacce object, which exports as +a chrdev to the user space. The user application communicates with the +hardware by ioctl (as control path) or share memory (as data path). + +The control path to the hardware is via file operation, while data path is +via mmap space of the queue fd. + +The queue file address space: + +:: + + /** + * enum uacce_qfrt: qfrt type + * @UACCE_QFRT_MMIO: device mmio region + * @UACCE_QFRT_DUS: device user share region + */ + enum uacce_qfrt { + UACCE_QFRT_MMIO = 0, + UACCE_QFRT_DUS = 1, + }; + +All regions are optional and differ from device type to type. +Each region can be mmapped only once, otherwise -EEXIST returns. + +The device mmio region is mapped to the hardware mmio space. It is generally +used for doorbell or other notification to the hardware. It is not fast enough +as data channel. + +The device user share region is used for share data buffer between user process +and device. + + +The Uacce register API +---------------------- + +The register API is defined in uacce.h. + +:: + + struct uacce_interface { + char name[UACCE_MAX_NAME_SIZE]; + unsigned int flags; + const struct uacce_ops *ops; + }; + +According to the IOMMU capability, uacce_interface flags can be: + +:: + + /** + * UACCE Device flags: + * UACCE_DEV_SVA: Shared Virtual Addresses + * Support PASID + * Support device page faults (PCI PRI or SMMU Stall) + */ + #define UACCE_DEV_SVA BIT(0) + + struct uacce_device *uacce_alloc(struct device *parent, + struct uacce_interface *interface); + int uacce_register(struct uacce_device *uacce); + void uacce_remove(struct uacce_device *uacce); + +uacce_register results can be: + +a. If uacce module is not compiled, ERR_PTR(-ENODEV) + +b. Succeed with the desired flags + +c. Succeed with the negotiated flags, for example + + uacce_interface.flags = UACCE_DEV_SVA but uacce->flags = ~UACCE_DEV_SVA + + So user driver need check return value as well as the negotiated uacce->flags. + + +The user driver +--------------- + +The queue file mmap space will need a user driver to wrap the communication +protocol. Uacce provides some attributes in sysfs for the user driver to +match the right accelerator accordingly. +More details in Documentation/ABI/testing/sysfs-driver-uacce. |