Re: [PATCH] drm/i915: Drop stealing of bits from i915_sw_fence function pointer

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 22/09/2021 16:21, Tvrtko Ursulin wrote:
> On 22/09/2021 15:57, Matthew Brost wrote:
> > Rather than stealing bits from i915_sw_fence function pointer use
> > seperate fields for function pointer and flags. If using two different
> > fields, the 4 byte alignment for the i915_sw_fence function pointer can
> > also be dropped.
> >
> > v2:
> >   (CI)
> >    - Set new function field rather than flags in __i915_sw_fence_init
> >
> > Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> > ---
> >   drivers/gpu/drm/i915/display/intel_display.c  |  2 +-
> >   drivers/gpu/drm/i915/gem/i915_gem_context.c   |  2 +-
> >   drivers/gpu/drm/i915/i915_request.c           |  4 ++--
> >   drivers/gpu/drm/i915/i915_sw_fence.c          | 12 +++++------
> >   drivers/gpu/drm/i915/i915_sw_fence.h          | 21 +++++++++----------
> >   drivers/gpu/drm/i915/i915_sw_fence_work.c     |  2 +-
> >   .../gpu/drm/i915/selftests/i915_sw_fence.c    |  2 +-
> >   drivers/gpu/drm/i915/selftests/lib_sw_fence.c |  4 ++--
> >   8 files changed, 23 insertions(+), 26 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_display.c 
> > b/drivers/gpu/drm/i915/display/intel_display.c
> > index a7ca38613f89..6d5bb55ffc82 100644
> > --- a/drivers/gpu/drm/i915/display/intel_display.c
> > +++ b/drivers/gpu/drm/i915/display/intel_display.c
> > @@ -10323,7 +10323,7 @@ static void intel_atomic_commit_work(struct 
> > work_struct *work)
> >       intel_atomic_commit_tail(state);
> >   }
> > -static int __i915_sw_fence_call
> > +static int
> >   intel_atomic_commit_ready(struct i915_sw_fence *fence,
> >                 enum i915_sw_fence_notify notify)
> >   {
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c 
> > b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > index c2ab0e22db0a..df5fec5c3da8 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > @@ -800,7 +800,7 @@ static void free_engines_rcu(struct rcu_head *rcu)
> >       free_engines(engines);
> >   }
> > -static int __i915_sw_fence_call
> > +static int
> >   engines_notify(struct i915_sw_fence *fence, enum 
> > i915_sw_fence_notify state)
> >   {
> >       struct i915_gem_engines *engines =
> > diff --git a/drivers/gpu/drm/i915/i915_request.c 
> > b/drivers/gpu/drm/i915/i915_request.c
> > index ce446716d092..945d3025a0b6 100644
> > --- a/drivers/gpu/drm/i915/i915_request.c
> > +++ b/drivers/gpu/drm/i915/i915_request.c
> > @@ -719,7 +719,7 @@ void i915_request_cancel(struct i915_request *rq, 
> > int error)
> >       intel_context_cancel_request(rq->context, rq);
> >   }
> > -static int __i915_sw_fence_call
> > +static int
> >   submit_notify(struct i915_sw_fence *fence, enum i915_sw_fence_notify 
> > state)
> >   {
> >       struct i915_request *request =
> > @@ -755,7 +755,7 @@ submit_notify(struct i915_sw_fence *fence, enum 
> > i915_sw_fence_notify state)
> >       return NOTIFY_DONE;
> >   }
> > -static int __i915_sw_fence_call
> > +static int
> >   semaphore_notify(struct i915_sw_fence *fence, enum 
> > i915_sw_fence_notify state)
> >   {
> >       struct i915_request *rq = container_of(fence, typeof(*rq), 
> > semaphore);
> > diff --git a/drivers/gpu/drm/i915/i915_sw_fence.c 
> > b/drivers/gpu/drm/i915/i915_sw_fence.c
> > index c589a681da77..1c080dd1f718 100644
> > --- a/drivers/gpu/drm/i915/i915_sw_fence.c
> > +++ b/drivers/gpu/drm/i915/i915_sw_fence.c
> > @@ -34,7 +34,7 @@ enum {
> >   static void *i915_sw_fence_debug_hint(void *addr)
> >   {
> > -    return (void *)(((struct i915_sw_fence *)addr)->flags & 
> > I915_SW_FENCE_MASK);
> > +    return (void *)(((struct i915_sw_fence *)addr)->fn);
> >   }
> >   #ifdef CONFIG_DRM_I915_SW_FENCE_DEBUG_OBJECTS
> > @@ -126,10 +126,7 @@ static inline void debug_fence_assert(struct 
> > i915_sw_fence *fence)
> >   static int __i915_sw_fence_notify(struct i915_sw_fence *fence,
> >                     enum i915_sw_fence_notify state)
> >   {
> > -    i915_sw_fence_notify_t fn;
> > -
> > -    fn = (i915_sw_fence_notify_t)(fence->flags & I915_SW_FENCE_MASK);
> > -    return fn(fence, state);
> > +    return fence->fn(fence, state);
> >   }
> >   #ifdef CONFIG_DRM_I915_SW_FENCE_DEBUG_OBJECTS
> > @@ -242,10 +239,11 @@ void __i915_sw_fence_init(struct i915_sw_fence 
> > *fence,
> >                 const char *name,
> >                 struct lock_class_key *key)
> >   {
> > -    BUG_ON(!fn || (unsigned long)fn & ~I915_SW_FENCE_MASK);
> > +    BUG_ON(!fn);
> >       __init_waitqueue_head(&fence->wait, name, key);
> > -    fence->flags = (unsigned long)fn;
> > +    fence->fn = fn;
> > +    fence->flags = 0;
> >       i915_sw_fence_reinit(fence);
> >   }
> > diff --git a/drivers/gpu/drm/i915/i915_sw_fence.h 
> > b/drivers/gpu/drm/i915/i915_sw_fence.h
> > index 30a863353ee6..70ba1789aa89 100644
> > --- a/drivers/gpu/drm/i915/i915_sw_fence.h
> > +++ b/drivers/gpu/drm/i915/i915_sw_fence.h
> > @@ -17,26 +17,25 @@
> >   struct completion;
> >   struct dma_resv;
> > +struct i915_sw_fence;
> > +
> > +enum i915_sw_fence_notify {
> > +    FENCE_COMPLETE,
> > +    FENCE_FREE
> > +};
> > +
> > +typedef int (*i915_sw_fence_notify_t)(struct i915_sw_fence *,
> > +                      enum i915_sw_fence_notify state);
> >   struct i915_sw_fence {
> >       wait_queue_head_t wait;
> > +    i915_sw_fence_notify_t fn;
> >       unsigned long flags;
>
> Looks good to me. I'd just make the flags narrower now that they can be, 
> and put them down..
>
> >       atomic_t pending;
>
> .. here as unsigned int and so we save 4 bytes, maybe.

No this won't work due test_and_set_bit needs a long, oh well.

> In fact, unless CONFIG_DRM_I915_SW_FENCE_CHECK_DAG is on, which it won't 
> be on release builds, I don't think anything uses flags any more. So you 
> could even omit the flags in that case. Might be cumbersome so can leave 
> for later, but would sure be nice not to waste space if we can avoid it.
>
> Also please double check if i915_sw_fence_reinit() will be fine and 
> won't bug on since fence->flags is always zero now I think.

But these two are worth checking out.

Regards,

Tvrtko

> Regards,
>
> Tvrtko
>
> >       int error;
> >   };
> >   #define I915_SW_FENCE_CHECKED_BIT    0 /* used internally for DAG 
> > checking */
> > -#define I915_SW_FENCE_PRIVATE_BIT    1 /* available for use by owner */
> > -#define I915_SW_FENCE_MASK        (~3)
> > -
> > -enum i915_sw_fence_notify {
> > -    FENCE_COMPLETE,
> > -    FENCE_FREE
> > -};
> > -
> > -typedef int (*i915_sw_fence_notify_t)(struct i915_sw_fence *,
> > -                      enum i915_sw_fence_notify state);
> > -#define __i915_sw_fence_call __aligned(4)
> >   void __i915_sw_fence_init(struct i915_sw_fence *fence,
> >                 i915_sw_fence_notify_t fn,
> > diff --git a/drivers/gpu/drm/i915/i915_sw_fence_work.c 
> > b/drivers/gpu/drm/i915/i915_sw_fence_work.c
> > index 5b33ef23d54c..d2e56b387993 100644
> > --- a/drivers/gpu/drm/i915/i915_sw_fence_work.c
> > +++ b/drivers/gpu/drm/i915/i915_sw_fence_work.c
> > @@ -23,7 +23,7 @@ static void fence_work(struct work_struct *work)
> >       dma_fence_put(&f->dma);
> >   }
> > -static int __i915_sw_fence_call
> > +static int
> >   fence_notify(struct i915_sw_fence *fence, enum i915_sw_fence_notify 
> > state)
> >   {
> >       struct dma_fence_work *f = container_of(fence, typeof(*f), chain);
> > diff --git a/drivers/gpu/drm/i915/selftests/i915_sw_fence.c 
> > b/drivers/gpu/drm/i915/selftests/i915_sw_fence.c
> > index cbf45d85cbff..daa985e5a19b 100644
> > --- a/drivers/gpu/drm/i915/selftests/i915_sw_fence.c
> > +++ b/drivers/gpu/drm/i915/selftests/i915_sw_fence.c
> > @@ -28,7 +28,7 @@
> >   #include "../i915_selftest.h"
> > -static int __i915_sw_fence_call
> > +static int
> >   fence_notify(struct i915_sw_fence *fence, enum i915_sw_fence_notify 
> > state)
> >   {
> >       switch (state) {
> > diff --git a/drivers/gpu/drm/i915/selftests/lib_sw_fence.c 
> > b/drivers/gpu/drm/i915/selftests/lib_sw_fence.c
> > index 080b90b63d16..eb59a41bdb79 100644
> > --- a/drivers/gpu/drm/i915/selftests/lib_sw_fence.c
> > +++ b/drivers/gpu/drm/i915/selftests/lib_sw_fence.c
> > @@ -26,7 +26,7 @@
> >   /* Small library of different fence types useful for writing tests */
> > -static int __i915_sw_fence_call
> > +static int
> >   nop_fence_notify(struct i915_sw_fence *fence, enum 
> > i915_sw_fence_notify state)
> >   {
> >       return NOTIFY_DONE;
> > @@ -89,7 +89,7 @@ struct heap_fence {
> >       };
> >   };
> > -static int __i915_sw_fence_call
> > +static int
> >   heap_fence_notify(struct i915_sw_fence *fence, enum 
> > i915_sw_fence_notify state)
> >   {
> >       struct heap_fence *h = container_of(fence, typeof(*h), fence);