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

[Jason Ekstrand]

On Thu, Mar 11, 2021 at 10:31 AM Chris Wilson <chris at chris-wilson.co.uk> wrote:
>
> 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.

NORELOC doesn't quite ensure that there are no relocations; it just
makes things optional if the kernel hasn't moved anything.  I guess we
could require userspace to set it but it also doesn't do anything if
there are no relocations to begin with.  I think I'd personally err on
the side of not requiring pointless flags.

--Jason