Re: [PATCH v2 0/5] dma-fence: Deadline awareness

From: Christian König
Date: Tue Aug 17 2021 - 05:01:51 EST

Am 17.08.21 um 00:29 schrieb Rob Clark:
dma_fence_array looks simple enough, just propagate the deadline to
all children.

I guess dma_fence_chain is similar (ie. fence is signalled when all
children are signalled), the difference being simply that children are
added dynamically?

No, new chain nodes are always added at the top.

So when you have a dma_fence_chain as a starting point the linked nodes after it will stay the same (except for garbage collection).

The tricky part is you can't use recursion, cause that would easily exceed the kernels stack depth. So you need something similar to dma_fence_chain_signaled().

Something like this should do it:

static bool dma_fence_chain_set_deadline(struct dma_fence *fence, ktime_t deadline)
        dma_fence_chain_for_each(fence, fence) {
                struct dma_fence_chain *chain = to_dma_fence_chain(fence);
                struct dma_fence *f = chain ? chain->fence : fence;

                dma_fence_set_deadline(f, deadline);



On Mon, Aug 16, 2021 at 3:17 AM Christian König
<christian.koenig@xxxxxxx> wrote:
The general approach seems to make sense now I think.

One minor thing which I'm missing is adding support for this to the
dma_fence_array and dma_fence_chain containers.


Am 07.08.21 um 20:37 schrieb Rob Clark:
From: Rob Clark <robdclark@xxxxxxxxxxxx>

Based on discussion from a previous series[1] to add a "boost" mechanism
when, for example, vblank deadlines are missed. Instead of a boost
callback, this approach adds a way to set a deadline on the fence, by
which the waiter would like to see the fence signalled.

I've not yet had a chance to re-work the drm/msm part of this, but
wanted to send this out as an RFC in case I don't have a chance to
finish the drm/msm part this week.

Original description:

In some cases, like double-buffered rendering, missing vblanks can
trick the GPU into running at a lower frequence, when really we
want to be running at a higher frequency to not miss the vblanks
in the first place.

This is partially inspired by a trick i915 does, but implemented
via dma-fence for a couple of reasons:

1) To continue to be able to use the atomic helpers
2) To support cases where display and gpu are different drivers


v2: Move filtering out of later deadlines to fence implementation
to avoid increasing the size of dma_fence

Rob Clark (5):
dma-fence: Add deadline awareness
drm/vblank: Add helper to get next vblank time
drm/atomic-helper: Set fence deadline for vblank
drm/scheduler: Add fence deadline support
drm/msm: Add deadline based boost support

drivers/dma-buf/dma-fence.c | 20 +++++++
drivers/gpu/drm/drm_atomic_helper.c | 36 ++++++++++++
drivers/gpu/drm/drm_vblank.c | 31 ++++++++++
drivers/gpu/drm/msm/msm_fence.c | 76 +++++++++++++++++++++++++
drivers/gpu/drm/msm/msm_fence.h | 20 +++++++
drivers/gpu/drm/msm/msm_gpu.h | 1 +
drivers/gpu/drm/msm/msm_gpu_devfreq.c | 20 +++++++
drivers/gpu/drm/scheduler/sched_fence.c | 25 ++++++++
drivers/gpu/drm/scheduler/sched_main.c | 3 +
include/drm/drm_vblank.h | 1 +
include/drm/gpu_scheduler.h | 6 ++
include/linux/dma-fence.h | 16 ++++++
12 files changed, 255 insertions(+)