Re: [Intel-gfx] [PATCH 2/3] drm/i915/gem: Drop relocation support on all new hardware (v5)

[Jason Ekstrand <jason@jlekstrand.net>]

On Wed, Mar 17, 2021 at 2:55 AM Petri Latvala <petri.latvala@intel.com> wrote:
>
> On Mon, Mar 15, 2021 at 10:29:20PM -0700, Ashutosh Dixit wrote:
> > From: Jason Ekstrand <jason@jlekstrand.net>
> >
> > 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
> >
> > v5 (Jason Ekstrand):
> >  - Drop the HAS_LMEM check as it's already covered by the version check
> >
> > Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> > Reviewed-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
> > Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 13 ++++++++++---
> >  1 file changed, 10 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..f66cff2943baa 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 +1775,12 @@ 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.  This
> > +      * also covers all platforms with local memory.
> > +      */
> > +     if (INTEL_GEN(eb->i915) >= 12 && !IS_TIGERLAKE(eb->i915))
> > +             return -EINVAL;
> > +
> >       if (size > N_RELOC(ULONG_MAX))
> >               return -EINVAL;
> >
> > @@ -1807,7 +1814,7 @@ static int eb_copy_relocations(const struct i915_execbuffer *eb)
> >               if (nreloc == 0)
> >                       continue;
> >
> > -             err = check_relocations(&eb->exec[i]);
> > +             err = check_relocations(eb, &eb->exec[i]);
> >               if (err)
> >                       goto err;
> >
> > @@ -1880,7 +1887,7 @@ static int eb_prefault_relocations(const struct i915_execbuffer *eb)
> >       for (i = 0; i < count; i++) {
> >               int err;
> >
> > -             err = check_relocations(&eb->exec[i]);
> > +             err = check_relocations(eb, &eb->exec[i]);
> >               if (err)
> >                       return err;
> >       }
>
>
> Disclaimer: I don't know deeply how any of this works.
>
> check_relocations() is only called if eb_relocate_parse goes on its
> slowpath, fast path doesn't check for gen. Should it, or am I
> misunderstanding something?

Good eye!  Yeah, this doesn't work for the reason you just said.  I
was trying to avoid an up-front list walk of the object list but if we
want to be very sure it's correct, we probably have to.

--Jason
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