[PATCH] i915: Drop relocation support on all new hardware (v3)

[Chris Wilson]

Quoting Zbigniew Kempczyński (2021-03-11 11:44:32)
> On Wed, Mar 10, 2021 at 03:50:07PM -0600, Jason Ekstrand wrote:
> > The Vulkan driver in Mesa for Intel hardware never uses relocations if
> > it's running on a version of i915 that supports at least softpin which
> > all versions of i915 supporting Gen12 do.  On the OpenGL side, Gen12+ is
> > only supported by iris which never uses relocations.  The older i965
> > driver in Mesa does use relocations but it only supports Intel hardware
> > through Gen11 and has been deprecated for all hardware Gen9+.  The
> > compute driver also never uses relocations.  This only leaves the media
> > driver which is supposed to be switching to softpin going forward.
> > Making softpin a requirement for all future hardware seems reasonable.
> > 
> > Rejecting relocations starting with Gen12 has the benefit that we don't
> > have to bother supporting it on platforms with local memory.  Given how
> > much CPU touching of memory is required for relocations, not having to
> > do so on platforms where not all memory is directly CPU-accessible
> > carries significant advantages.
> > 
> > v2 (Jason Ekstrand):
> >  - Allow TGL-LP platforms as they've already shipped
> > 
> > v3 (Jason Ekstrand):
> >  - WARN_ON platforms with LMEM support in case the check is wrong
> 
> I was asked to review of this patch. It works along with expected
> IGT check https://patchwork.freedesktop.org/patch/423361/?series=82954&rev=25
> 
> Before I'll give you r-b - isn't i915_gem_execbuffer2_ioctl() better place
> to do for loop just after copy_from_user() and check relocation_count?
> We have an access to exec2_list there, we know the gen so we're able to say
> relocations are not supported immediate, without entering i915_gem_do_execbuffer().

There's a NORELOC flag you can enforce as mandatory. That's trivial for
userspace to set, really makes sure they are aware of the change afoot,
and i915_gem_ceck_execbuffer() will perform the validation upfront with
the other flag checks.
-Chris