Re: [MAINTAINER SUMMIT] User-space requirements for accelerator drivers

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Steven,

On Sat, Sep 11, 2021 at 10:20:07AM -0400, Steven Rostedt wrote:
> On Sat, 11 Sep 2021 03:20:50 +0300 Laurent Pinchart wrote:
> 
> > > Freely accessible documentation is really the key.  
> > 
> > In principle I'd agree, but that assumes such documentation would exist
> > in the first place, with a sufficient level of quality. In many cases an
> > open implementation the exercises all device features is a better form
> > of documentation than what vendors have, even internally. Of course, the
> > opposite is true as well, having seen too much vendor code for my own
> > good, there is such a thing as a working for unreadable implementation.
> > 
> > I fully agree with your point about feature completeness by the way,
> > vendors will always find ways to hide pieces of the API if they really
> > want to, but I think that would be true of documentation as well.
> 
> I would like not only documentation, but also an open source test suite
> that simply tests the interface. Honestly, I believe that all new
> interfaces to the kernel (open or not) should have full documentation
> and a test suite interface before it gets accepted. We have
> tools/selftests that should be updated with all new interfaces into the
> kernel.
> 
> Even if it's just a smoke test, that would be fine. Obviously if
> there's a driver without hardware, it can't be tested. But if you have
> that hardware, perhaps there could be a simple test suite of the
> interface to let you know it is still functional.

It really depends on the device and the interface it requires. A GPU or
camera ISP driver can't be meaningfully tested just at the interface
level. The interface exposed to userspace is usually of the form of an
ioctl that allows passing a large command buffer in a device-specific
format, full of data that is then consumed by hardware or firmware. For
instance, look at the ipu3_uapi_params structure in
drivers/staging/media/ipu3/include/uapi/intel-ipu3.h. You need very
elaborate code to exercise such an API.

If you wanted GPU drivers to have tests in tools/selftests, you'd have
to move Mesa to the kernel :-)

-- 
Regards,

Laurent Pinchart