Re: [PATCH 01/11] drm/amdgpu: Comply with implicit fencing rules

[Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>]

On Fri, May 21, 2021 at 11:10 AM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
> Docs for struct dma_resv are fairly clear:
>
> "A reservation object can have attached one exclusive fence (normally
> associated with write operations) or N shared fences (read
> operations)."
>
> https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html#reservation-objects
>
> Furthermore a review across all of upstream.
>
> First of render drivers and how they set implicit fences:
>
> - nouveau follows this contract, see in validate_fini_no_ticket()
>
>                         nouveau_bo_fence(nvbo, fence, !!b->write_domains);
>
>   and that last boolean controls whether the exclusive or shared fence
>   slot is used.
>
> - radeon follows this contract by setting
>
>                 p->relocs[i].tv.num_shared = !r->write_domain;
>
>   in radeon_cs_parser_relocs(), which ensures that the call to
>   ttm_eu_fence_buffer_objects() in radeon_cs_parser_fini() will do the
>   right thing.
>
> - vmwgfx seems to follow this contract with the shotgun approach of
>   always setting ttm_val_buf->num_shared = 0, which means
>   ttm_eu_fence_buffer_objects() will only use the exclusive slot.
>
> - etnaviv follows this contract, as can be trivially seen by looking
>   at submit_attach_object_fences()
>
> - i915 is a bit a convoluted maze with multiple paths leading to
>   i915_vma_move_to_active(). Which sets the exclusive flag if
>   EXEC_OBJECT_WRITE is set. This can either come as a buffer flag for
>   softpin mode, or through the write_domain when using relocations. It
>   follows this contract.
>
> - lima follows this contract, see lima_gem_submit() which sets the
>   exclusive fence when the LIMA_SUBMIT_BO_WRITE flag is set for that
>   bo
>
> - msm follows this contract, see msm_gpu_submit() which sets the
>   exclusive flag when the MSM_SUBMIT_BO_WRITE is set for that buffer
>
> - panfrost follows this contract with the shotgun approach of just
>   always setting the exclusive fence, see
>   panfrost_attach_object_fences(). Benefits of a single engine I guess
>
> - v3d follows this contract with the same shotgun approach in
>   v3d_attach_fences_and_unlock_reservation(), but it has at least an
>   XXX comment that maybe this should be improved
>
> - v4c uses the same shotgun approach of always setting an exclusive
>   fence, see vc4_update_bo_seqnos()
>
> - vgem also follows this contract, see vgem_fence_attach_ioctl() and
>   the VGEM_FENCE_WRITE. This is used in some igts to validate prime
>   sharing with i915.ko without the need of a 2nd gpu
>
> - vritio follows this contract again with the shotgun approach of
>   always setting an exclusive fence, see virtio_gpu_array_add_fence()
>
> This covers the setting of the exclusive fences when writing.
>
> Synchronizing against the exclusive fence is a lot more tricky, and I
> only spot checked a few:
>
> - i915 does it, with the optional EXEC_OBJECT_ASYNC to skip all
>   implicit dependencies (which is used by vulkan)
>
> - etnaviv does this. Implicit dependencies are collected in
>   submit_fence_sync(), again with an opt-out flag
>   ETNA_SUBMIT_NO_IMPLICIT. These are then picked up in
>   etnaviv_sched_dependency which is the
>   drm_sched_backend_ops->dependency callback.
>
> - v4c seems to not do much here, maybe gets away with it by not having
>   a scheduler and only a single engine. Since all newer broadcom chips than
>   the OG vc4 use v3d for rendering, which follows this contract, the
>   impact of this issue is fairly small.
>
> - v3d does this using the drm_gem_fence_array_add_implicit() helper,
>   which then it's drm_sched_backend_ops->dependency callback
>   v3d_job_dependency() picks up.
>
> - panfrost is nice here and tracks the implicit fences in
>   panfrost_job->implicit_fences, which again the
>   drm_sched_backend_ops->dependency callback panfrost_job_dependency()
>   picks up. It is mildly questionable though since it only picks up
>   exclusive fences in panfrost_acquire_object_fences(), but not buggy
>   in practice because it also always sets the exclusive fence. It
>   should pick up both sets of fences, just in case there's ever going
>   to be a 2nd gpu in a SoC with a mali gpu. Or maybe a mali SoC with a
>   pcie port and a real gpu, which might actually happen eventually. A
>   bug, but easy to fix. Should probably use the
>   drm_gem_fence_array_add_implicit() helper.
>
> - lima is nice an easy, uses drm_gem_fence_array_add_implicit() and
>   the same schema as v3d.
>
> - msm is mildly entertaining. It also supports MSM_SUBMIT_NO_IMPLICIT,
>   but because it doesn't use the drm/scheduler it handles fences from
>   the wrong context with a synchronous dma_fence_wait. See
>   submit_fence_sync() leading to msm_gem_sync_object(). Investing into
>   a scheduler might be a good idea.
>
> - all the remaining drivers are ttm based, where I hope they do
>   appropriately obey implicit fences already. I didn't do the full
>   audit there because a) not follow the contract would confuse ttm
>   quite well and b) reading non-standard scheduler and submit code
>   which isn't based on drm/scheduler is a pain.
>
> Onwards to the display side.
>
> - Any driver using the drm_gem_plane_helper_prepare_fb() helper will
>   correctly. Overwhelmingly most drivers get this right, except a few
>   totally dont. I'll follow up with a patch to make this the default
>   and avoid a bunch of bugs.
>
> - I didn't audit the ttm drivers, but given that dma_resv started
>   there I hope they get this right.
>
> In conclusion this IS the contract, both as documented and
> overwhelmingly implemented, specically as implemented by all render
> drivers except amdgpu.
>
> Amdgpu tried to fix this already in
>
> commit 049aca4363d8af87cab8d53de5401602db3b9999
> Author: Christian König <christian.koenig@amd.com>
> Date:   Wed Sep 19 16:54:35 2018 +0200
>
>     drm/amdgpu: fix using shared fence for exported BOs v2
>
> but this fix falls short on a number of areas:
>
> - It's racy, by the time the buffer is shared it might be too late. To
>   make sure there's definitely never a problem we need to set the
>   fences correctly for any buffer that's potentially exportable.
>
> - It's breaking uapi, dma-buf fds support poll() and differentitiate
>   between, which was introduced in
>
>         commit 9b495a5887994a6d74d5c261d012083a92b94738
>         Author: Maarten Lankhorst <maarten.lankhorst@canonical.com>
>         Date:   Tue Jul 1 12:57:43 2014 +0200
>
>             dma-buf: add poll support, v3
>
> - Christian König wants to nack new uapi building further on this
>   dma_resv contract because it breaks amdgpu, quoting
>
>   "Yeah, and that is exactly the reason why I will NAK this uAPI change.
>
>   "This doesn't works for amdgpu at all for the reasons outlined above."
>
>   https://lore.kernel.org/dri-devel/f2eb6751-2f82-9b23-f57e-548de5b729de@gmail.com/
>
>   Rejecting new development because your own driver is broken and
>   violates established cross driver contracts and uapi is really not
>   how upstream works.
>
> Now this patch will have a severe performance impact on anything that
> runs on multiple engines. So we can't just merge it outright, but need
> a bit a plan:
>
> - amdgpu needs a proper uapi for handling implicit fencing. The funny
>   thing is that to do it correctly, implicit fencing must be treated
>   as a very strange IPC mechanism for transporting fences, where both
>   setting the fence and dependency intercepts must be handled
>   explicitly. Current best practices is a per-bo flag to indicate
>   writes, and a per-bo flag to to skip implicit fencing in the CS
>   ioctl as a new chunk.
>
> - Since amdgpu has been shipping with broken behaviour we need an
>   opt-out flag from the butchered implicit fencing model to enable the
>   proper explicit implicit fencing model.
>
> - for kernel memory fences due to bo moves at least the i915 idea is
>   to use ttm_bo->moving. amdgpu probably needs the same.
>
> - since the current p2p dma-buf interface assumes the kernel memory
>   fence is in the exclusive dma_resv fence slot we need to add a new
>   fence slot for kernel fences, which must never be ignored. Since
>   currently only amdgpu supports this there's no real problem here
>   yet, until amdgpu gains a NO_IMPLICIT CS flag.
>
> - New userspace needs to ship in enough desktop distros so that users
>   wont notice the perf impact. I think we can ignore LTS distros who
>   upgrade their kernels but not their mesa3d snapshot.
>
> - Then when this is all in place we can merge this patch here.
>
> What is not a solution to this problem here is trying to make the
> dma_resv rules in the kernel more clever. The fundamental issue here
> is that the amdgpu CS uapi is the least expressive one across all
> drivers (only equalled by panfrost, which has an actual excuse) by not
> allowing any userspace control over how implicit sync is conducted.
>
> Until this is fixed it's completely pointless to make the kernel more
> clever to improve amdgpu, because all we're doing is papering over
> this uapi design issue. amdgpu needs to attain the status quo
> established by other drivers first, once that's achieved we can tackle
> the remaining issues in a consistent way across drivers.
>
> Cc: mesa-dev@lists.freedesktop.org
> Cc: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
> Cc: Dave Airlie <airlied@gmail.com>
> Cc: Rob Clark <robdclark@chromium.org>
> Cc: Kristian H. Kristensen <hoegsberg@google.com>
> Cc: Michel Dänzer <michel@daenzer.net>
> Cc: Daniel Stone <daniels@collabora.com>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: "Christian König" <christian.koenig@amd.com>
> Cc: Alex Deucher <alexander.deucher@amd.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Deepak R Varma <mh12gx2825@gmail.com>
> Cc: Chen Li <chenli@uniontech.com>
> Cc: Kevin Wang <kevin1.wang@amd.com>
> Cc: Dennis Li <Dennis.Li@amd.com>
> Cc: Luben Tuikov <luben.tuikov@amd.com>
> Cc: linaro-mm-sig@lists.linaro.org
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
> index 88a24a0b5691..cc8426e1e8a8 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
> @@ -617,8 +617,8 @@ static int amdgpu_cs_parser_bos(struct amdgpu_cs_parser *p,
>         amdgpu_bo_list_for_each_entry(e, p->bo_list) {
>                 struct amdgpu_bo *bo = ttm_to_amdgpu_bo(e->tv.bo);
>
> -               /* Make sure we use the exclusive slot for shared BOs */
> -               if (bo->prime_shared_count)
> +               /* Make sure we use the exclusive slot for all potentially shared BOs */
> +               if (!(bo->flags & AMDGPU_GEM_CREATE_VM_ALWAYS_VALID))
>                         e->tv.num_shared = 0;

I think it also makes sense to skip this with
AMDGPU_GEM_CREATE_EXPLICIT_SYNC? It can be shared but I don't think
anyone expects implicit sync to happen with those.
>                 e->bo_va = amdgpu_vm_bo_find(vm, bo);
>         }
> --
> 2.31.0
>