Re: [Intel-gfx] [PATCH 2/2] drm/i915/gem: Migrate to system at dma-buf attach time (v5)

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Jul 13, 2021 at 04:06:13PM +0100, Matthew Auld wrote:
> On Tue, 13 Jul 2021 at 15:44, Daniel Vetter <daniel@ffwll.ch> wrote:
> >
> > On Mon, Jul 12, 2021 at 06:12:34PM -0500, Jason Ekstrand wrote:
> > > From: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> > >
> > > Until we support p2p dma or as a complement to that, migrate data
> > > to system memory at dma-buf attach time if possible.
> > >
> > > v2:
> > > - Rebase on dynamic exporter. Update the igt_dmabuf_import_same_driver
> > >   selftest to migrate if we are LMEM capable.
> > > v3:
> > > - Migrate also in the pin() callback.
> > > v4:
> > > - Migrate in attach
> > > v5: (jason)
> > > - Lock around the migration
> > >
> > > Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> > > Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
> > > Reported-by: kernel test robot <lkp@intel.com>
> > > Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> > > Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
> > > ---
> > >  drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c    | 25 ++++++++++++++++++-
> > >  .../drm/i915/gem/selftests/i915_gem_dmabuf.c  |  4 ++-
> > >  2 files changed, 27 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> > > index 9a655f69a0671..3163f00554476 100644
> > > --- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> > > +++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> > > @@ -170,8 +170,31 @@ static int i915_gem_dmabuf_attach(struct dma_buf *dmabuf,
> > >                                 struct dma_buf_attachment *attach)
> > >  {
> > >       struct drm_i915_gem_object *obj = dma_buf_to_obj(dmabuf);
> > > +     struct i915_gem_ww_ctx ww;
> > > +     int err;
> > > +
> > > +     for_i915_gem_ww(&ww, err, true) {
> > > +             err = i915_gem_object_lock(obj, &ww);
> > > +             if (err)
> > > +                     continue;
> > > +
> > > +             if (!i915_gem_object_can_migrate(obj, INTEL_REGION_SMEM)) {
> > > +                     err = -EOPNOTSUPP;
> > > +                     continue;
> > > +             }
> > > +
> > > +             err = i915_gem_object_migrate(obj, &ww, INTEL_REGION_SMEM);
> > > +             if (err)
> > > +                     continue;
> > >
> > > -     return i915_gem_object_pin_pages_unlocked(obj);
> > > +             err = i915_gem_object_wait_migration(obj, 0);
> > > +             if (err)
> > > +                     continue;
> > > +
> > > +             err = i915_gem_object_pin_pages(obj);
> > > +     }
> > > +
> > > +     return err;
> > >  }
> > >
> > >  static void i915_gem_dmabuf_detach(struct dma_buf *dmabuf,
> > > diff --git a/drivers/gpu/drm/i915/gem/selftests/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/gem/selftests/i915_gem_dmabuf.c
> > > index 3dc0f8b3cdab0..4f7e77b1c0152 100644
> > > --- a/drivers/gpu/drm/i915/gem/selftests/i915_gem_dmabuf.c
> > > +++ b/drivers/gpu/drm/i915/gem/selftests/i915_gem_dmabuf.c
> > > @@ -106,7 +106,9 @@ static int igt_dmabuf_import_same_driver(void *arg)
> > >       int err;
> > >
> > >       force_different_devices = true;
> > > -     obj = i915_gem_object_create_shmem(i915, PAGE_SIZE);
> > > +     obj = i915_gem_object_create_lmem(i915, PAGE_SIZE, 0);
> >
> > I'm wondering (and couldn't answer) whether this creates an lmem+smem
> > buffer, since if we create an lmem-only buffer then the migration above
> > should fail.
> 
> It's lmem-only, but it's also a kernel internal object, so the
> migration path will still happily migrate it if asked. On the other
> hand if it's a userspace object then we always have to respect the
> placements.
> 
> I think for now the only usecase for that is in the selftests.

Yeah I've read the kerneldoc, it's all nicely documented but feels a bit
dangerous. What I proposed on irc:
- i915_gem_object_migrate does the placement check, i.e. as strict as
  can_migrate.
- A new __i915_gem_object_migrate is for selftest that do special stuff.
- In the import selftest we check that lmem-only fails (because we can't
  pin it into smem) for a non-dynamic importer, but lmem+smem works and
  gets migrated.
- Once we have dynamic dma-buf for p2p pci, then we'll have another
  selftest which checks that things work for lmem only if and only if the
  importer is dynamic and has set the allow_p2p flag.

We could also add the can_migrate check everywhere (including
dma_buf->attach), but that feels like the less save api.
-Daniel


> 
> >
> > Which I'm also not sure we have a testcase for that testcase either ...
> >
> > I tried to read some code here, but got a bit lost. Ideas?
> > -Daniel
> >
> > > +     if (IS_ERR(obj))
> > > +             obj = i915_gem_object_create_shmem(i915, PAGE_SIZE);
> > >       if (IS_ERR(obj))
> > >               goto out_ret;
> > >
> > > --
> > > 2.31.1
> > >
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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