On Mon, Mar 22, 2021 at 11:59 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> On Fri, Mar 19, 2021 at 05:38:56PM -0500, Jason Ekstrand wrote:
> > I'd love to delete the SINGLE_TIMELINE API because it leaks an
> > implementation detail of contexts through to the API and is something
> > that userspace can do itself, trivially. Unfortunately, it's used by
> > the media driver so we can't do that. We can, however, do the next-best
> > thing which is to embed a syncobj in the context and do exactly what
> > we'd expect from userspace internally.
> >
> > This has a couple of advantages. One is that we're no longer leaking a
> > detail of the current execlist scheduler which will be problematic when
> > we try to add GuC scheduling. Second is that, together with deleting
> > the CLONE_CONTEXT API, we should now have a 1:1 mapping between
> > intel_context and intel_timeline which should make some of our locking
> > mess a bit easier.
> >
> > Signed-off-by: Jason Ekstrand <jason@xxxxxxxxxxxxxx>
> > Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> > Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
> > ---
> > drivers/gpu/drm/i915/gem/i915_gem_context.c | 47 ++++---------------
> > .../gpu/drm/i915/gem/i915_gem_context_types.h | 8 +++-
> > .../gpu/drm/i915/gem/i915_gem_execbuffer.c | 15 ++++++
> > 3 files changed, 32 insertions(+), 38 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > index f88bac19333ec..e094f4a1ca4cd 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > @@ -67,6 +67,8 @@
> > #include <linux/log2.h>
> > #include <linux/nospec.h>
> >
> > +#include <drm/drm_syncobj.h>
> > +
> > #include "gt/gen6_ppgtt.h"
> > #include "gt/intel_context.h"
> > #include "gt/intel_engine_heartbeat.h"
> > @@ -224,10 +226,6 @@ static void intel_context_set_gem(struct intel_context *ce,
> > ce->vm = vm;
> > }
> >
> > - GEM_BUG_ON(ce->timeline);
> > - if (ctx->timeline)
> > - ce->timeline = intel_timeline_get(ctx->timeline);
> > -
> > if (ctx->sched.priority >= I915_PRIORITY_NORMAL &&
> > intel_engine_has_timeslices(ce->engine))
> > __set_bit(CONTEXT_USE_SEMAPHORES, &ce->flags);
> > @@ -344,8 +342,8 @@ void i915_gem_context_release(struct kref *ref)
> > mutex_destroy(&ctx->engines_mutex);
> > mutex_destroy(&ctx->lut_mutex);
> >
> > - if (ctx->timeline)
> > - intel_timeline_put(ctx->timeline);
> > + if (ctx->syncobj)
> > + drm_syncobj_put(ctx->syncobj);
> >
> > put_pid(ctx->pid);
> > mutex_destroy(&ctx->mutex);
> > @@ -790,33 +788,11 @@ static void __assign_ppgtt(struct i915_gem_context *ctx,
> > i915_vm_close(vm);
> > }
> >
> > -static void __set_timeline(struct intel_timeline **dst,
> > - struct intel_timeline *src)
> > -{
> > - struct intel_timeline *old = *dst;
> > -
> > - *dst = src ? intel_timeline_get(src) : NULL;
> > -
> > - if (old)
> > - intel_timeline_put(old);
> > -}
> > -
> > -static void __apply_timeline(struct intel_context *ce, void *timeline)
> > -{
> > - __set_timeline(&ce->timeline, timeline);
> > -}
> > -
> > -static void __assign_timeline(struct i915_gem_context *ctx,
> > - struct intel_timeline *timeline)
> > -{
> > - __set_timeline(&ctx->timeline, timeline);
> > - context_apply_all(ctx, __apply_timeline, timeline);
> > -}
> > -
> > static struct i915_gem_context *
> > i915_gem_create_context(struct drm_i915_private *i915, unsigned int flags)
> > {
> > struct i915_gem_context *ctx;
> > + int ret;
> >
> > if (flags & I915_CONTEXT_CREATE_FLAGS_SINGLE_TIMELINE &&
> > !HAS_EXECLISTS(i915))
> > @@ -845,16 +821,13 @@ i915_gem_create_context(struct drm_i915_private *i915, unsigned int flags)
> > }
> >
> > if (flags & I915_CONTEXT_CREATE_FLAGS_SINGLE_TIMELINE) {
> > - struct intel_timeline *timeline;
> > -
> > - timeline = intel_timeline_create(&i915->gt);
> > - if (IS_ERR(timeline)) {
> > + ret = drm_syncobj_create(&ctx->syncobj,
> > + DRM_SYNCOBJ_CREATE_SIGNALED,
> > + NULL);
> > + if (ret) {
> > context_close(ctx);
> > - return ERR_CAST(timeline);
> > + return ERR_PTR(ret);
> > }
> > -
> > - __assign_timeline(ctx, timeline);
> > - intel_timeline_put(timeline);
> > }
> >
> > trace_i915_context_create(ctx);
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> > index 676592e27e7d2..8a5fdd163b79d 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> > @@ -83,7 +83,13 @@ struct i915_gem_context {
> > struct i915_gem_engines __rcu *engines;
> > struct mutex engines_mutex; /* guards writes to engines */
> >
> > - struct intel_timeline *timeline;
> > + /**
> > + * @syncobj: Shared timeline syncobj
> > + *
> > + * When the SHARED_TIMELINE flag is set on context creation, this
> > + * provides automatic implicit synchronization across all engines.
>
> I think we should explain a bit more what this actually does, i.e. emulate
> it using a syncobj, but unlike the real timeline it does not make any
> total order guarantees. Since userspace doesn't expect that.
New comment:
/**
* @syncobj: Shared timeline syncobj
*
* When the SHARED_TIMELINE flag is set on context creation, we
* emulate a single timeline across all engines using this syncobj.
* For every execbuffer2 call, this syncobj is used as both an in-
* and out-fence. Unlike the real intel_timeline, this doesn't
* provide perfect atomic in-order guarantees if the client races
* with itself by calling execbuffer2 twice concurrently. However,
* if userspace races with itself, that's not likely to yield well-
* defined results anyway so we choose to not care.
*/
> > + */
> > + struct drm_syncobj *syncobj;
> >
> > /**
> > * @vm: unique address space (GTT)
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > index 96403130a373d..2c56796f6a71b 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > @@ -3295,6 +3295,15 @@ i915_gem_do_execbuffer(struct drm_device *dev,
> > goto err_vma;
> > }
> >
> > + if (eb.gem_context->syncobj) {
>
> I think would be good to wrap these in unlikely, least also because it's
> nice documentation.
Done.
> -Daniel
>
>
> > + struct dma_fence *fence;
> > +
> > + fence = drm_syncobj_fence_get(eb.gem_context->syncobj);
> > + err = i915_request_await_dma_fence(eb.request, fence);
> > + if (err)
> > + goto err_ext;
> > + }
> > +
> > if (in_fence) {
> > if (args->flags & I915_EXEC_FENCE_SUBMIT)
> > err = i915_request_await_execution(eb.request,
> > @@ -3351,6 +3360,12 @@ i915_gem_do_execbuffer(struct drm_device *dev,
> > fput(out_fence->file);
> > }
> > }
> > +
> > + if (eb.gem_context->syncobj) {
> > + drm_syncobj_replace_fence(eb.gem_context->syncobj,
> > + &eb.request->fence);
> > + }
> > +
> > i915_request_put(eb.request);
> >
> > err_vma:
> > --
> > 2.29.2
> >
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx@xxxxxxxxxxxxxxxxxxxxx
> > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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