[Intel-gfx] [PATCH] drm/i915/gem: Drop relocation support on all new hardware (v4)

[Matthew Auld]

On Fri, 12 Mar 2021 at 11:47, Tvrtko Ursulin
<tvrtko.ursulin at linux.intel.com> wrote:
>
>
> On 12/03/2021 10:56, Matthew Auld wrote:
> > On Fri, 12 Mar 2021 at 09:50, Tvrtko Ursulin
> > <tvrtko.ursulin at linux.intel.com> wrote:
> >>
> >>
> >> On 11/03/2021 18:17, 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.
> >>>
> >>> There is one piece of hardware enabled by default in i915: RKL which was
> >>> enabled by e22fa6f0a976 which has not yet landed in drm-next so this
> >>> almost but not really a userspace API change for RKL.  If it becomes a
> >>> problem, we can always add !IS_ROCKETLAKE(eb->i915) to the condition.
> >>>
> >>> Rejecting relocations starting with newer Gen12 platforms 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
> >>>
> >>> v4 (Jason Ekstrand):
> >>>    - Call out Rocket Lake in the commit message
> >>>
> >>> Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> >>> Acked-by: Keith Packard <keithp at keithp.com>
> >>> Cc: Dave Airlie <airlied at redhat.com>
> >>> Cc: Daniel Vetter <daniel.vetter at intel.com>
> >>> ---
> >>>    drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 15 ++++++++++++---
> >>>    1 file changed, 12 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> >>> index 99772f37bff60..b02dbd16bfa03 100644
> >>> --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> >>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> >>> @@ -1764,7 +1764,8 @@ eb_relocate_vma_slow(struct i915_execbuffer *eb, struct eb_vma *ev)
> >>>        return err;
> >>>    }
> >>>
> >>> -static int check_relocations(const struct drm_i915_gem_exec_object2 *entry)
> >>> +static int check_relocations(const struct i915_execbuffer *eb,
> >>> +                          const struct drm_i915_gem_exec_object2 *entry)
> >>>    {
> >>>        const char __user *addr, *end;
> >>>        unsigned long size;
> >>> @@ -1774,6 +1775,14 @@ static int check_relocations(const struct drm_i915_gem_exec_object2 *entry)
> >>>        if (size == 0)
> >>>                return 0;
> >>>
> >>> +     /* Relocations are disallowed for all platforms after TGL-LP */
> >>> +     if (INTEL_GEN(eb->i915) >= 12 && !IS_TIGERLAKE(eb->i915))
> >>> +             return -EINVAL;
> >>
> >> I still recommend ENODEV as more inline with our established error
> >> codes. (Platform does not support vs dear userspace you messed up your
> >> flags, modes, whatever.)
> >>
> >>> +
> >>> +     /* All discrete memory platforms are Gen12 or above */
> >>> +     if (WARN_ON(HAS_LMEM(eb->i915)))
> >>> +             return -EINVAL;
> >>
> >> What was the conclusion on value of supporting fake lmem?
> >
> >>From the previous thread, nothing is currently using it, we did have a
> > dedicated machine in CI but that has been gone for some months it
> > seems, so it might already be broken. Also its use was limited only to
> > the live selftests, which can't even hit this path. The plan was to
> > eventually remove it, since supporting both real and fake lmem in the
> > same tree is likely more effort than it's worth.
>
> If I understand correctly you are saying it is safe to not have this
> check even if fake lmem is removed later? Presumably since there is no
> way to place an object into lmem in upstream from userspace, hence
> execbuf cannot use any?

The current usage is gated behind setting i915_selftest.live < 0 (when
loading the driver, run the live selfests and then exit module probe).
So for this and pread/pwrite, or any uAPI stuff we shouldn't have to
worry about fake lmem.

>
> Regards,
>
> Tvrtko