[Intel-gfx] [PATCH 1/5] drm/i915: Move intel_engine_free_request_pool to i915_request.c

[Jason Ekstrand]

On Thu, Jun 10, 2021 at 5:04 AM Tvrtko Ursulin
<tvrtko.ursulin at linux.intel.com> wrote:
>
> On 09/06/2021 22:29, Jason Ekstrand wrote:
> > This appears to break encapsulation by moving an intel_engine_cs
> > function to a i915_request file.  However, this function is
> > intrinsically tied to the lifetime rules and allocation scheme of
> > i915_request and having it in intel_engine_cs.c leaks details of
> > i915_request.  We have an abstraction leak either way.  Since
> > i915_request's allocation scheme is far more subtle than the simple
> > pointer that is intel_engine_cs.request_pool, it's probably better to
> > keep i915_request's details to itself.
> >
> > Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> > Cc: Jon Bloomfield <jon.bloomfield at intel.com>
> > Cc: Daniel Vetter <daniel.vetter at intel.com>
> > Cc: Matthew Auld <matthew.auld at intel.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> > ---
> >   drivers/gpu/drm/i915/gt/intel_engine_cs.c | 8 --------
> >   drivers/gpu/drm/i915/i915_request.c       | 7 +++++--
> >   drivers/gpu/drm/i915/i915_request.h       | 2 --
> >   3 files changed, 5 insertions(+), 12 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gt/intel_engine_cs.c b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
> > index 9ceddfbb1687d..df6b80ec84199 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_engine_cs.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
> > @@ -422,14 +422,6 @@ void intel_engines_release(struct intel_gt *gt)
> >       }
> >   }
> >
> > -void intel_engine_free_request_pool(struct intel_engine_cs *engine)
> > -{
> > -     if (!engine->request_pool)
> > -             return;
> > -
> > -     kmem_cache_free(i915_request_slab_cache(), engine->request_pool);
>
> Argument that the slab cache shouldn't be exported from i915_request.c
> sounds good to me.
>
> But I think step better than simply reversing the break of encapsulation
> (And it's even worse because it leaks much higher level object!) could
> be to export a freeing helper from i915_request.c, engine pool would
> then use:
>
> void __i915_request_free(...)
> {
>         kmem_cache_free(...);
> }

That was what I did at first.  However, the semantics of how the
pointer is touched/modified are really also part of i915_request.  In
particular, the use of xchg and cmpxchg.  So I pulled the one other
access (besides NULL initializing) into i915_request.c which meant
pulling in intel_engine_free_request_pool.

Really, if we wanted proper encapsulation here, we'd have

struct i915_request_cache {
    struct i915_request *rq;
};

void i915_request_cache_init(struct i915_request_cache *cache);
void i915_request_cache_finish(struct i915_request_cache *cache);

all in i915_request.h and have all the gory details inside
i915_request.c.  Then all intel_engine_cs knows is that it has a
request cache.

If we really want to go that far, we can, I suppose.

--Jason

> Regards,
>
> Tvrtko
>
> > -}
> > -
> >   void intel_engines_free(struct intel_gt *gt)
> >   {
> >       struct intel_engine_cs *engine;
> > diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> > index 1014c71cf7f52..48c5f8527854b 100644
> > --- a/drivers/gpu/drm/i915/i915_request.c
> > +++ b/drivers/gpu/drm/i915/i915_request.c
> > @@ -106,9 +106,12 @@ static signed long i915_fence_wait(struct dma_fence *fence,
> >                                timeout);
> >   }
> >
> > -struct kmem_cache *i915_request_slab_cache(void)
> > +void intel_engine_free_request_pool(struct intel_engine_cs *engine)
> >   {
> > -     return global.slab_requests;
> > +     if (!engine->request_pool)
> > +             return;
> > +
> > +     kmem_cache_free(global.slab_requests, engine->request_pool);
> >   }
> >
> >   static void i915_fence_release(struct dma_fence *fence)
> > diff --git a/drivers/gpu/drm/i915/i915_request.h b/drivers/gpu/drm/i915/i915_request.h
> > index 270f6cd37650c..f84c38d29f988 100644
> > --- a/drivers/gpu/drm/i915/i915_request.h
> > +++ b/drivers/gpu/drm/i915/i915_request.h
> > @@ -300,8 +300,6 @@ static inline bool dma_fence_is_i915(const struct dma_fence *fence)
> >       return fence->ops == &i915_fence_ops;
> >   }
> >
> > -struct kmem_cache *i915_request_slab_cache(void);
> > -
> >   struct i915_request * __must_check
> >   __i915_request_create(struct intel_context *ce, gfp_t gfp);
> >   struct i915_request * __must_check
> >