[Intel-xe] [PATCH 19/21] drm/xe/uapi: Drop OA_IOCTL_VERSION

[Dixit, Ashutosh]

On Tue, 03 Oct 2023 19:37:58 -0700, Umesh Nerlige Ramappa wrote:
>
> On Tue, Sep 19, 2023 at 10:02:53AM -0700, Dixit, Ashutosh wrote:
> > On Tue, 19 Sep 2023 09:10:47 -0700, Ashutosh Dixit wrote:
> >>
> >> OA version was previously used to track which OA properties were introduced
> >> at which version. However OA version is an outlier in that a similar
> >> version is not used anywhere else in the kernel.
> >
> > This is not strictly true. E.g. AMD's include/uapi/linux/kfd_ioctl.h
> > contains KFD_IOCTL_MAJOR_VERSION/KFD_IOCTL_MINOR_VERSION.
> >
> >> For XE, we will track addition of new properties by means of
> >> xe_user_extension. Userland can either maintain a mapping of OA properties
> >> against the kernel version, or rely on return codes (e.g. ENOTSUPP) to
> >> "discover" OA properties.
> >
> > But let's see if we need a version for OA or the kernel version itself is
> > sufficient.
> >
> >>
> >> Suggested-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
> >> Signed-off-by: Ashutosh Dixit <ashutosh.dixit at intel.com>
>
> ok, if there is precedence for a version, no harm adding it, but I agree
> that we should see if there are ways to do this with the generic OA
> query. For features that are added with extensions, it's taken care of
> inherently.

Afaiu, in this case userland is still relying on return codes from the
kernel to discover available features, whereas a version or capability
flags allow userland to know available features a priori without
"discovery" by means of return codes.

> Sometimes there are features that are internal to the implementation that
> the user might want to know. Those may need to be exposed via
> capabilities/flags in the generic oa/perf query.

I agree that capability flags do appear to be a better option than a
version. But in i915 we could update the version without modifying the uapi
header, which we cannot do if we have capability flags (we'd e.g. at least
have to expose a new capability bit). Is modifying the uapi header a
concern?

Both the version and capability flags can be easily included in the oa_info
struct we are considering now.

Thanks.
--
Ashutosh