[Intel-gfx] [PATCH 3/9] drm/i915: Wrap drm_gem_object_lookup in i915_gem_object_lookup

[Chris Wilson]

On Wed, Jul 20, 2016 at 12:28:16PM +0100, Dave Gordon wrote:
> On 20/07/16 09:59, Chris Wilson wrote:
> >For symmetry with a forthcoming i915_gem_object_get() and
> >i915_gem_object_pu().
> >
> >Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >---
> >  drivers/gpu/drm/i915/i915_drv.h        | 20 +++++++++++-
> >  drivers/gpu/drm/i915/i915_gem.c        | 58 +++++++++++++++++-----------------
> >  drivers/gpu/drm/i915/i915_gem_tiling.c |  8 ++---
> >  drivers/gpu/drm/i915/intel_display.c   |  4 +--
> >  drivers/gpu/drm/i915/intel_overlay.c   |  5 ++-
> >  5 files changed, 56 insertions(+), 39 deletions(-)
> >
> >diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> >index 2131e7f1e57a..47f244f9c64e 100644
> >--- a/drivers/gpu/drm/i915/i915_drv.h
> >+++ b/drivers/gpu/drm/i915/i915_drv.h
> >@@ -2286,7 +2286,25 @@ struct drm_i915_gem_object {
> >  		} userptr;
> >  	};
> >  };
> >-#define to_intel_bo(x) container_of(x, struct drm_i915_gem_object, base)
> >+
> >+static inline struct drm_i915_gem_object *
> >+to_intel_bo(struct drm_gem_object *gem)
> >+{
> >+	/* Assert that to_intel_bo(NULL) == NULL */
> >+	BUILD_BUG_ON(offsetof(struct drm_i915_gem_object, base));
> >+
> >+	return container_of(gem, struct drm_i915_gem_object, base);
> >+}
> 
> Yes, I think this is a much better way of doing the conversion, 'cos
> it's type-safe and we get that all-important check that 'base' is
> still at offset 0.
> 
> In my variant on this, I also introduced the reverse mapping,
> to_gem_object(), so that we could completely abstract away the
> conversion back-and-forth between the two classes. Do you think that
> would be worth adding here too?

The convention is that the downcast is tidied away, but as the upcast is
just &foo->bar that is easy enough on the eyes. There is also the
problem of a limited namespace, so picking who should be to_foo() and
to_bar() is tricky. (Without delving into huge polymorphic macros.)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre