Re: [PATCH drm-next v6 02/13] drm: manager to keep track of GPUs VA mappings

Boris Brezillon
2023-06-30T08:09:36Z

On Fri, 30 Jun 2023 10:02:52 +0200
Boris Brezillon <boris.brezillon@collabora.com> wrote:

> Hi Danilo,
>
> On Fri, 30 Jun 2023 00:25:18 +0200
> Danilo Krummrich <dakr@redhat.com> wrote:
>
> > + * int driver_gpuva_remap(struct drm_gpuva_op *op, void *__ctx)
> > + * {
> > + * struct driver_context *ctx = __ctx;
> > + *
> > + * drm_gpuva_remap(ctx->prev_va, ctx->next_va, &op->remap);
> > + *
> > + * drm_gpuva_unlink(op->remap.unmap->va);
> > + * kfree(op->remap.unmap->va);
> > + *
> > + * if (op->remap.prev) {
> > + * drm_gpuva_link(ctx->prev_va);
>
> I ended up switching to dma_resv-based locking for the GEMs and I
> wonder what the locking is supposed to look like in the async-mapping
> case, where we insert/remove the VA nodes in the drm_sched::run_job()
> path.
>
> What I have right now is something like:
>
> dma_resv_lock(vm->resv);
>
> // split done in drm_gpuva_sm_map(), each iteration
> // of the loop is a call to the driver ->[re,un]map()
> // hook
> for_each_sub_op() {
>
> // Private BOs have their resv field pointing to the
> // VM resv and we take the VM resv lock before calling
> // drm_gpuva_sm_map()
> if (vm->resv != gem->resv)
> dma_resv_lock(gem->resv);
>
> drm_gpuva_[un]link(va);
> gem_[un]pin(gem);
>
> if (vm->resv != gem->resv)
> dma_resv_unlock(gem->resv);
> }
>
> dma_resv_unlock(vm->resv);
>
> In practice, I don't expect things to deadlock, because the VM resv is
> not supposed to be taken outside the VM context and the locking order
> is always the same (VM lock first, and then each shared BO
> taken/released independently), but I'm not super thrilled by this
> nested lock, and I'm wondering if we shouldn't have a pass collecting
> locks in a drm_exec context first, and then have
> the operations executed. IOW, something like that:
>
> drm_exec_init(exec, DRM_EXEC_IGNORE_DUPLICATES)
> drm_exec_until_all_locked(exec) {
> // Dummy GEM is the dummy GEM object I use to make the VM
> // participate in the locking without having to teach
> // drm_exec how to deal with raw dma_resv objects.
> ret = drm_exec_lock_obj(exec, vm->dummy_gem);
> drm_exec_retry_on_contention(exec);
> if (ret)
> return ret;
>
> // Could take the form of drm_gpuva_sm_[un]map_acquire_locks()
> // helpers
> for_each_sub_op() {
> ret = drm_exec_lock_obj(exec, gem);
> if (ret)
> return ret;
> }
> }
>
> // each iteration of the loop is a call to the driver
> // ->[re,un]map() hook
> for_each_sub_op() {
> ...
> gem_[un]pin_locked(gem);

Just wanted to clarify that the pages have been pinned at VM_BIND job
creation time, so this gem_pin_locked() call is effectively just a
pin_count++, not the whole page allocation, which we don't want to
happen in a dma-signaling path.

> drm_gpuva_[un]link(va);
> ...
> }
>
> drm_exec_fini(exec);