Re: [PATCH 08/53] drm/i915: Update alloc_request to return the allocated request

[Tomas Elf <tomas.elf@xxxxxxxxx>]

On 05/03/2015 15:46, John Harrison wrote:
> On 05/03/2015 15:27, Tomas Elf wrote:
> > On 19/02/2015 17:17, John.C.Harrison@xxxxxxxxx wrote:
> > > From: John Harrison <John.C.Harrison@xxxxxxxxx>
> > >
> > > The alloc_request() function does not actually return the newly
> > > allocated
> > > request. Instead, it must be pulled from
> > > ring->outstanding_lazy_request. This
> > > patch fixes this so that code can create a request and start using it
> > > knowing
> > > exactly which request it actually owns.
> > >
> > > For: VIZ-5115
> > > Signed-off-by: John Harrison <John.C.Harrison@xxxxxxxxx>
> > > ---
> > >   drivers/gpu/drm/i915/i915_drv.h            |    3 ++-
> > >   drivers/gpu/drm/i915/i915_gem_execbuffer.c |    3 ++-
> > >   drivers/gpu/drm/i915/intel_lrc.c           |   13 +++++++++----
> > >   drivers/gpu/drm/i915/intel_lrc.h           |    3 ++-
> > >   drivers/gpu/drm/i915/intel_ringbuffer.c    |   14 ++++++++++----
> > >   drivers/gpu/drm/i915/intel_ringbuffer.h    |    3 ++-
> > >   6 files changed, 27 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/i915/i915_drv.h
> > > b/drivers/gpu/drm/i915/i915_drv.h
> > > index 87a4a2e..90223f208 100644
> > > --- a/drivers/gpu/drm/i915/i915_drv.h
> > > +++ b/drivers/gpu/drm/i915/i915_drv.h
> > > @@ -1909,7 +1909,8 @@ struct drm_i915_private {
> > >       /* Abstract the submission mechanism (legacy ringbuffer or
> > > execlists) away */
> > >       struct {
> > >           int (*alloc_request)(struct intel_engine_cs *ring,
> > > -                     struct intel_context *ctx);
> > > +                     struct intel_context *ctx,
> > > +                     struct drm_i915_gem_request **req_out);
> > >           int (*do_execbuf)(struct i915_execbuffer_params *params,
> > >                     struct drm_i915_gem_execbuffer2 *args,
> > >                     struct list_head *vmas);
> > > diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> > > b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> > > index 61471e9..37dcc6f 100644
> > > --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> > > +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> > > @@ -1353,6 +1353,7 @@ i915_gem_do_execbuffer(struct drm_device *dev,
> > > void *data,
> > >       struct i915_address_space *vm;
> > >       struct i915_execbuffer_params params_master; /* XXX: will be
> > > removed later */
> > >       struct i915_execbuffer_params *params = &params_master;
> > > +    struct drm_i915_gem_request *request;
> >
> > Please initialize request to NULL. If you accidentally dereference it
> > before it is allocated (seeing as the allocation is several pages down
> > from here) you get a null pointer exception, which is a clear
> > indication that you did something stupid. Otherwise it's not clear
> > what will happen (sure, page fault, but null pointer exception is a
> > better failure indication).
>
> That should generate a 'use before assignment' compiler warning.

That assumes that the developer in question isn't too busy hacking to 
check for warnings (in all honesty that developer probably would've been 
me ;)). Sure, you should always check for warnings but if we can save 
this developer some time by giving them a clear run-time indication 
aside from the compile-time warning then that would not be a bad thing. 
I've been there myself a few times and I know times in the past where 
this would've saved me the time it takes to rebuild and redeploy the 
kernel once.

Thanks,
Tomas


> > Thanks,
> > Tomas
> >
> > >       const u32 ctx_id = i915_execbuffer2_get_context_id(*args);
> > >       u32 dispatch_flags;
> > >       int ret;
> > > @@ -1531,7 +1532,7 @@ i915_gem_do_execbuffer(struct drm_device *dev,
> > > void *data,
> > >           params->batch_obj_vm_offset =
> > > i915_gem_obj_offset(batch_obj, vm);
> > >
> > >       /* Allocate a request for this batch buffer nice and early. */
> > > -    ret = dev_priv->gt.alloc_request(ring, ctx);
> > > +    ret = dev_priv->gt.alloc_request(ring, ctx, &request);
> > >       if (ret)
> > >           goto err;
> > >
> > > diff --git a/drivers/gpu/drm/i915/intel_lrc.c
> > > b/drivers/gpu/drm/i915/intel_lrc.c
> > > index 8628abf..c3c783f 100644
> > > --- a/drivers/gpu/drm/i915/intel_lrc.c
> > > +++ b/drivers/gpu/drm/i915/intel_lrc.c
> > > @@ -857,13 +857,17 @@ void intel_lr_context_unpin(struct
> > > intel_engine_cs *ring,
> > >   }
> > >
> > >   int intel_logical_ring_alloc_request(struct intel_engine_cs *ring,
> > > -                     struct intel_context *ctx)
> > > +                     struct intel_context *ctx,
> > > +                     struct drm_i915_gem_request **req_out)
> > >   {
> > >       struct drm_i915_gem_request *request;
> > >       struct drm_i915_private *dev_private = ring->dev->dev_private;
> > >       int ret;
> > >
> > > -    if (ring->outstanding_lazy_request)
> > > +    if (!req_out)
> > > +        return -EINVAL;
> > > +
> > > +    if ((*req_out = ring->outstanding_lazy_request) != NULL)
> > >           return 0;
> > >
> > >       request = kzalloc(sizeof(*request), GFP_KERNEL);
> > > @@ -898,7 +902,7 @@ int intel_logical_ring_alloc_request(struct
> > > intel_engine_cs *ring,
> > >       i915_gem_context_reference(request->ctx);
> > >       request->ringbuf = ctx->engine[ring->id].ringbuf;
> > >
> > > -    ring->outstanding_lazy_request = request;
> > > +    *req_out = ring->outstanding_lazy_request = request;
> > >       return 0;
> > >   }
> > >
> > > @@ -1051,6 +1055,7 @@ static int logical_ring_prepare(struct
> > > intel_ringbuffer *ringbuf,
> > >   int intel_logical_ring_begin(struct intel_ringbuffer *ringbuf,
> > >                    struct intel_context *ctx, int num_dwords)
> > >   {
> > > +    struct drm_i915_gem_request *req;
> > >       struct intel_engine_cs *ring = ringbuf->ring;
> > >       struct drm_device *dev = ring->dev;
> > >       struct drm_i915_private *dev_priv = dev->dev_private;
> > > @@ -1066,7 +1071,7 @@ int intel_logical_ring_begin(struct
> > > intel_ringbuffer *ringbuf,
> > >           return ret;
> > >
> > >       /* Preallocate the olr before touching the ring */
> > > -    ret = intel_logical_ring_alloc_request(ring, ctx);
> > > +    ret = intel_logical_ring_alloc_request(ring, ctx, &req);
> > >       if (ret)
> > >           return ret;
> > >
> > > diff --git a/drivers/gpu/drm/i915/intel_lrc.h
> > > b/drivers/gpu/drm/i915/intel_lrc.h
> > > index 3cc38bd..77de8ac 100644
> > > --- a/drivers/gpu/drm/i915/intel_lrc.h
> > > +++ b/drivers/gpu/drm/i915/intel_lrc.h
> > > @@ -37,7 +37,8 @@
> > >
> > >   /* Logical Rings */
> > >   int __must_check intel_logical_ring_alloc_request(struct
> > > intel_engine_cs *ring,
> > > -                          struct intel_context *ctx);
> > > +                          struct intel_context *ctx,
> > > +                          struct drm_i915_gem_request **req_out);
> > >   void intel_logical_ring_stop(struct intel_engine_cs *ring);
> > >   void intel_logical_ring_cleanup(struct intel_engine_cs *ring);
> > >   int intel_logical_rings_init(struct drm_device *dev);
> > > diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c
> > > b/drivers/gpu/drm/i915/intel_ringbuffer.c
> > > index 635707a..1a9f884 100644
> > > --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> > > +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> > > @@ -2164,13 +2164,18 @@ int intel_ring_idle(struct intel_engine_cs
> > > *ring)
> > >   }
> > >
> > >   int
> > > -intel_ring_alloc_request(struct intel_engine_cs *ring, struct
> > > intel_context *ctx)
> > > +intel_ring_alloc_request(struct intel_engine_cs *ring,
> > > +             struct intel_context *ctx,
> > > +             struct drm_i915_gem_request **req_out)
> > >   {
> > >       int ret;
> > >       struct drm_i915_gem_request *request;
> > >       struct drm_i915_private *dev_private = ring->dev->dev_private;
> > >
> > > -    if (ring->outstanding_lazy_request)
> > > +    if (!req_out)
> > > +        return -EINVAL;
> > > +
> > > +    if ((*req_out = ring->outstanding_lazy_request) != NULL)
> > >           return 0;
> > >
> > >       request = kzalloc(sizeof(*request), GFP_KERNEL);
> > > @@ -2188,7 +2193,7 @@ intel_ring_alloc_request(struct intel_engine_cs
> > > *ring, struct intel_context *ctx
> > >           return ret;
> > >       }
> > >
> > > -    ring->outstanding_lazy_request = request;
> > > +    *req_out = ring->outstanding_lazy_request = request;
> > >       return 0;
> > >   }
> > >
> > > @@ -2216,6 +2221,7 @@ static int __intel_ring_prepare(struct
> > > intel_engine_cs *ring,
> > >   int intel_ring_begin(struct intel_engine_cs *ring,
> > >                int num_dwords)
> > >   {
> > > +    struct drm_i915_gem_request *req;
> > >       struct drm_i915_private *dev_priv = ring->dev->dev_private;
> > >       int ret;
> > >
> > > @@ -2229,7 +2235,7 @@ int intel_ring_begin(struct intel_engine_cs *ring,
> > >           return ret;
> > >
> > >       /* Preallocate the olr before touching the ring */
> > > -    ret = intel_ring_alloc_request(ring, NULL);
> > > +    ret = intel_ring_alloc_request(ring, NULL, &req);
> > >       if (ret)
> > >           return ret;
> > >
> > > diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h
> > > b/drivers/gpu/drm/i915/intel_ringbuffer.h
> > > index 2fd960a..4f8a14a 100644
> > > --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
> > > +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
> > > @@ -393,7 +393,8 @@ void intel_cleanup_ring_buffer(struct
> > > intel_engine_cs *ring);
> > >   int __must_check intel_ring_begin(struct intel_engine_cs *ring, int
> > > n);
> > >   int __must_check intel_ring_cacheline_align(struct intel_engine_cs
> > > *ring);
> > >   int __must_check intel_ring_alloc_request(struct intel_engine_cs
> > > *ring,
> > > -                      struct intel_context *ctx);
> > > +                      struct intel_context *ctx,
> > > +                      struct drm_i915_gem_request **req_out);
> > >   static inline void intel_ring_emit(struct intel_engine_cs *ring,
> > >                      u32 data)
> > >   {

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