[Intel-gfx] [PATCH] drm/i915: Align engine->uabi_class/instance with i915_drm.h

[Chris Wilson]

Quoting Tvrtko Ursulin (2020-01-15 15:28:19)
> 
> On 15/01/2020 15:24, Tvrtko Ursulin wrote:
> > From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > 
> > In our ABI we have defined I915_ENGINE_CLASS_INVALID_NONE and
> > I915_ENGINE_CLASS_INVALID_VIRTUAL as negative values which creates
> > implicit coupling with type widths used in, also ABI, struct
> > i915_engine_class_instance.
> > 
> > When for instance we export engine->uabi_class
> > I915_ENGINE_CLASS_INVALID_VIRTUAL from our our tracepoints, because the
> > type of the former is u8 in contrast to u16 defined in the ABI, 254 will
> > be returned instead of 65534 which userspace would legitimately expect.
> > 
> > Therefore we need to align the type used to store engine ABI class and
> > instance.
> > 
> > I did not find any other user visible inconsistency apart from the
> > tracepoints so I think importance of the fix is low.
> 
> Alternatives:
> 
> 1.
> Embed struct i915_engine_class_instance in struct intel_engine_cs, but 
> downside is more churn.

Could do. It would seem to make sense.
 
> 2.
> Only tweak the tracepoints to cast back and forth, but is it possible to 
> cast from unsigned to signed and get a negative number?
> 
> 3.
> Do nothing, does anyone cares?

It actually changes the value reported by GET_ENGINES for a virtual
engine, right?

engine->uabi_instance is u8, so u16 ci.engine_instance =
engine->uabi_instance is zero extended, not sign extended.

And we did say that is expected to be (u16)-2 already.

So cc:stable

> > Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > Cc: Chris Wilson <chris at chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

> > ---
> >   drivers/gpu/drm/i915/gem/i915_gem_busy.c     | 12 ++++++------
> >   drivers/gpu/drm/i915/gt/intel_engine_types.h |  4 ++--
> >   2 files changed, 8 insertions(+), 8 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_busy.c b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
> > index 3d4f5775a4ba..25235ef630c1 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_busy.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
> > @@ -9,16 +9,16 @@
> >   #include "i915_gem_ioctls.h"
> >   #include "i915_gem_object.h"
> >   
> > -static __always_inline u32 __busy_read_flag(u8 id)
> > +static __always_inline u32 __busy_read_flag(u16 id)
> >   {
> > -     if (id == (u8)I915_ENGINE_CLASS_INVALID)
> > +     if (id == (u16)I915_ENGINE_CLASS_INVALID)
> >               return 0xffff0000u;
> >   
> >       GEM_BUG_ON(id >= 16);
> >       return 0x10000u << id;
> >   }
> >   
> > -static __always_inline u32 __busy_write_id(u8 id)
> > +static __always_inline u32 __busy_write_id(u16 id)
> >   {
> >       /*
> >        * The uABI guarantees an active writer is also amongst the read
> > @@ -29,14 +29,14 @@ static __always_inline u32 __busy_write_id(u8 id)
> >        * last_read - hence we always set both read and write busy for
> >        * last_write.
> >        */
> > -     if (id == (u8)I915_ENGINE_CLASS_INVALID)
> > +     if (id == (u16)I915_ENGINE_CLASS_INVALID)
> >               return 0xffffffffu;
> >   
> >       return (id + 1) | __busy_read_flag(id);
> >   }
> >   
> >   static __always_inline unsigned int
> > -__busy_set_if_active(const struct dma_fence *fence, u32 (*flag)(u8 id))
> > +__busy_set_if_active(const struct dma_fence *fence, u32 (*flag)(u16 id))
> >   {
> >       const struct i915_request *rq;
> >   
> > @@ -57,7 +57,7 @@ __busy_set_if_active(const struct dma_fence *fence, u32 (*flag)(u8 id))
> >               return 0;
> >   
> >       /* Beware type-expansion follies! */
> > -     BUILD_BUG_ON(!typecheck(u8, rq->engine->uabi_class));
> > +     BUILD_BUG_ON(!typecheck(u16, rq->engine->uabi_class));
> >       return flag(rq->engine->uabi_class);
> >   }
> >   
> > diff --git a/drivers/gpu/drm/i915/gt/intel_engine_types.h b/drivers/gpu/drm/i915/gt/intel_engine_types.h
> > index 00287515e7af..350da59e605b 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_engine_types.h
> > +++ b/drivers/gpu/drm/i915/gt/intel_engine_types.h
> > @@ -278,8 +278,8 @@ struct intel_engine_cs {
> >       u8 class;
> >       u8 instance;
> >   
> > -     u8 uabi_class;
> > -     u8 uabi_instance;
> > +     u16 uabi_class;
> > +     u16 uabi_instance;

Bah, doesn't this leave us with a u16 hole!
-Chris