From: Kenneth Lee <[email protected]>
Uacce (Unified/User-space-access-intended Accelerator Framework) is
a kernel module targets to provide Shared Virtual Addressing (SVA)
between the accelerator and process.
This patch add document to explain how it works.
Signed-off-by: Kenneth Lee <[email protected]>
Signed-off-by: Zaibo Xu <[email protected]>
Signed-off-by: Zhou Wang <[email protected]>
Signed-off-by: Zhangfei Gao <[email protected]>
---
Documentation/misc-devices/uacce.rst | 159 +++++++++++++++++++++++++++++++++++
1 file changed, 159 insertions(+)
create mode 100644 Documentation/misc-devices/uacce.rst
diff --git a/Documentation/misc-devices/uacce.rst b/Documentation/misc-devices/uacce.rst
new file mode 100644
index 0000000..733521a
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+++ b/Documentation/misc-devices/uacce.rst
@@ -0,0 +1,159 @@
+.. 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_register(struct device *parent,
+ struct uacce_interface *interface);
+void uacce_unregister(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.
--
2.7.4