Re: [PATCH v2 1/5] drivers/accel: Introduce subsystem

Andrew Donnellan
2019-01-27T04:38:39Z

[+ linuxppc-dev, because cxl/ocxl are handled through powerpc - please
cc on future versions of this series]

On 26/1/19 8:13 am, Olof Johansson wrote:
> We're starting to see more of these kind of devices, the current
> upcoming wave will likely be around machine learning and inference
> engines. A few drivers have been added to drivers/misc for this, but
> it's timely to make it into a separate group of drivers/subsystem, to
> make it easier to find them, and to encourage collaboration between
> contributors.
>
> Over time, we expect to build shared frameworks that the drivers will
> make use of, but how that framework needs to look like to fill the needs
> is still unclear, and the best way to gain that knowledge is to give the
> disparate implementations a shared location.
>
> There has been some controversy around expectations for userspace
> stacks being open. The clear preference is to see that happen, and any
> driver and platform stack that is delivered like that will be given
> preferential treatment, and at some point in the future it might
> become the requirement. Until then, the bare minimum we need is an
> open low-level userspace such that the driver and HW interfaces can be
> exercised if someone is modifying the driver, even if the full details
> of the workload are not always available.
>
> Bootstrapping this with myself and Greg as maintainers (since the current
> drivers will be moving out of drivers/misc). Looking forward to expanding
> that group over time.
>

[snip]

> +
> +Hardware offload accelerator subsystem
> +======================================
> +
> +This is a brief overview of the subsystem (grouping) of hardware
> +accelerators kept under drivers/accel
> +
> +Types of hardware supported
> +---------------------------
> +
> + The general types of hardware supported are hardware devices that has
> + general interactions of sending commands and buffers to the hardware,
> + returning completions and possible filled buffers back, together
> + with the usual driver pieces around hardware control, setup, error
> + handling, etc.
> +
> + Drivers that fit into other subsystems are expected to be merged
> + there, and use the appropriate userspace interfaces of said functional
> + areas. We don't expect to see drivers for network, storage, graphics
> + and similar hardware implemented by drivers here.
> +
> +Expectations for contributions
> +------------------------------
> +
> + - Platforms and hardware that has fully open stacks, from Firmware to
> + Userspace, are always going to be given preferential treatment. These
> + platforms give the best insight for behavior and interaction of all
> + layers, including ability to improve implementation across the stack
> + over time.
> +
> + - If a platform is partially proprietary, it is still expected that the
> + portions that interact the driver can be shared in a form that allows
> + for exercising the hardware/driver and evolution of the interface over
> + time. This could be separated into a shared library and test/sample
> + programs, for example.
> +
> + - Over time, there is an expectation to converge drivers over to shared
> + frameworks and interfaces. Until then, the general rule is that no
> + more than one driver per vendor will be acceptable. For vendors that
> + aren't participating in the work towards shared frameworks over time,
> + we reserve the right to phase out support for the hardware.
How exactly do generic drivers for interconnect protocols, such as
cxl/ocxl, fit in here?

cxl and ocxl are not drivers for a specific device, they are generic
drivers which can be used with any device implementing the CAPI or
OpenCAPI protocol respectively - many of which will be FPGA boards
flashed with customer-designed accelerator cores for specific workloads,
some will be accelerators using ASICs or using FPGA images supplied by
vendors, some will be driven from userspace, others using the cxl/ocxl
kernel API, etc.

--
Andrew Donnellan OzLabs, ADL Canberra
andrew.donnellan@au1.ibm.com IBM Australia Limited