[RFC PATCH v1 0/8] Arm Core Local Accelerator Driver

[Ryan Roberts]

Hi All,

This RFC introduces a driver for the Arm Core Local Accelerator (CLA), a
CPU-local interface for programming attached accelerators. While the interface
is agnostic to the accelerator type, the initial (and currently only) target is
a compute engine.

The RFC aims to engage the community and get feedback on some key aspects. This
will help plan our approach for eventual upstreaming. The patches implement a
bare bones driver to aid discussion. Arm plans to publish the CLA spec in
future, but for now I hope the documentation included with patch 1 suffices.
Note that the CLA is not part of the Arm Architecture.

Patch 1 documents the hardware and driver design, and adds a driver skeleton.
Patches 2-4 initialize and probe the device. Patches 5-8 add context management
for switching the device between different users.

Aspects I'm seeking feedback for:

* The general structure of the driver: The current shape addresses the
  performance requirements we have for the use cases, and is therefore our
  preferred approach. But there are some unusual aspects due to the HW design.

* Driver interaction with architectural support: I've opted to treat the CLA as
  a device rather than a CPU extension, so it's implemented as a (mostly)
  self-contained driver. It has some unavoidable coupling to the arch code since
  it needs to share page tables and ASIDs (arm64_mm_context_[put|get]()).

* The location of the driver: although most accelerators will likely be compute
  ones, CLA is a generic MMIO interface that could handle any kind of
  accelerator. It's currently implemented as a misc driver. accel is another
  potential option, but given the current SVA approach, we would not use any of
  the services (memory-management or otherwise) that accel provides.

* User space availability: The kernel driver exposes the capabilities of the
  hardware to user space. Arm plans to open source a user space driver, but does
  not yet have any committed date. I'd like to understand if the availability of
  this component will be a prerequisite for upstream acceptance of the kernel
  driver; either way, I'm hoping we can at least progress with some discussion
  in its absence.

I'm deliberately constraining the scope to bare-metal support for now.
Virtualization is something we are considering (and have prototyped), but plan
to post a separate RFC for that as follow-up, once we have agreement on
direction for the bare-metal driver.

Source, along with some tests, is available at [1]. Patches based on v7.2-rc3.

[1] https://gitlab.arm.com/linux-arm/linux-rr/-/tree/features/cla-driver-v1-rfc-tests

Thanks,
Ryan


Jean-Philippe Brucker (5):
  misc/arm-cla: Add driver skeleton and documentation
  misc/arm-cla: Add launch operation helpers
  misc/arm-cla: Probe firmware-described devices
  misc/arm-cla: Initialize devices on CPU bringup
  misc/arm-cla: Accelerator context save and restore

Ryan Roberts (3):
  misc/arm-cla: Set up memory translation context
  misc/arm-cla: Manage domain contexts
  misc/arm-cla: Add userspace interface

 Documentation/misc-devices/arm-cla.rst | 206 ++++++++++
 drivers/misc/Kconfig                   |   1 +
 drivers/misc/Makefile                  |   1 +
 drivers/misc/arm-cla/Kconfig           |  10 +
 drivers/misc/arm-cla/Makefile          |  13 +
 drivers/misc/arm-cla/arm-cla-regs.h    | 179 +++++++++
 drivers/misc/arm-cla/arm-cla.h         | 296 +++++++++++++++
 drivers/misc/arm-cla/cla-ctx.c         | 142 +++++++
 drivers/misc/arm-cla/cla-init.c        | 503 +++++++++++++++++++++++++
 drivers/misc/arm-cla/cla-mtc.c         | 139 +++++++
 drivers/misc/arm-cla/cla-ops.c         | 342 +++++++++++++++++
 drivers/misc/arm-cla/cla-regs.c        | 263 +++++++++++++
 drivers/misc/arm-cla/cla-sched.c       | 474 +++++++++++++++++++++++
 drivers/misc/arm-cla/cla-topology.c    | 187 +++++++++
 drivers/misc/arm-cla/cla-user.c        | 351 +++++++++++++++++
 include/uapi/linux/arm-cla.h           | 207 ++++++++++
 16 files changed, 3314 insertions(+)
 create mode 100644 Documentation/misc-devices/arm-cla.rst
 create mode 100644 drivers/misc/arm-cla/Kconfig
 create mode 100644 drivers/misc/arm-cla/Makefile
 create mode 100644 drivers/misc/arm-cla/arm-cla-regs.h
 create mode 100644 drivers/misc/arm-cla/arm-cla.h
 create mode 100644 drivers/misc/arm-cla/cla-ctx.c
 create mode 100644 drivers/misc/arm-cla/cla-init.c
 create mode 100644 drivers/misc/arm-cla/cla-mtc.c
 create mode 100644 drivers/misc/arm-cla/cla-ops.c
 create mode 100644 drivers/misc/arm-cla/cla-regs.c
 create mode 100644 drivers/misc/arm-cla/cla-sched.c
 create mode 100644 drivers/misc/arm-cla/cla-topology.c
 create mode 100644 drivers/misc/arm-cla/cla-user.c
 create mode 100644 include/uapi/linux/arm-cla.h

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2.43.0