IOCTL feature detection (Was: Re: [Intel-gfx] [PATCH 8/8] drm/i915: Allow user to set cache at BO creation)

[Jordan Justen]

On 2023-04-26 04:52:43, Daniel Vetter wrote:
> 
> Joonas asked me to put my thoughts here:
> 
> - in general the "feature discovery by trying it out" approach is most
>   robust and hence preferred, but it's also not something that's required
>   when there's good reasons against it

More robust in what sense?

Userspace has to set up some scenario to use the interface, which
hopefully is not too complex. It's difficult to predict what it may
look like in the future since we are talking about undefined
extensions.

Userspace has to rely on the kernel making creating and destroying
this thing essentially free. For
I915_GEM_CREATE_EXT_PROTECTED_CONTENT, that did not work out. For
I915_GEM_CREATE_EXT_SET_PAT, just wondering, since the PAT indices are
platform specific, what might happen if we tried to choose some common
index value to detect the extension in a generic manner? Could it
potentially lead to unexpected behavior, or maybe just an error. I
guess it's really extension specific what kind of issues potentially
could arise.

> tldr; prefer discovery, don't sweat it if not, definitely don't
> overengineer this with some magic "give me all extensions" thing because
> that results in guaranteed cross-component backporting pains sooner or
> later. Or inconsistency, which defeats the point.

I guess I don't know the full context of your concerns here.

For returning the gem-create extensions, isn't this just a matter of
returning the valid indices to the create_extensions array in
i915_gem_create.c? Is that an over-engineered query?

It seems weird that there's a reasonably well defined "extension"
mechanism here, but no way for the kernel to just tell us what
extensions it knows about.

-Jordan