Re: [PATCH 02/11] dma-buf: add new dma_fence_chain container v4

[Lionel Landwerlin via dri-devel <dri-devel@lists.freedesktop.org>]

Hi Christian, David,

For timeline semaphore we need points to signaled in order.
I'm struggling to understand how this fence-chain implementation 
preserves ordering of the seqnos.

One of the scenario I can see an issue happening is when you have a 
timeline with points 1 & 2 and userspace submits for 2 different engines :
     - first with let's say a blitter style engine on point 2
     - then a 3d style engine on point 1

Another scenario would be signaling a timeline with points 1 & 2 with 
those points in reverse order in the submission array.

-Lionel

On 07/12/2018 09:55, Chunming Zhou wrote:
> From: Christian König <ckoenig.leichtzumerken@gmail.com>
>
> Lockless container implementation similar to a dma_fence_array, but with
> only two elements per node and automatic garbage collection.
>
> v2: properly document dma_fence_chain_for_each, add dma_fence_chain_find_seqno,
>      drop prev reference during garbage collection if it's not a chain fence.
> v3: use head and iterator for dma_fence_chain_for_each
> v4: fix reference count in dma_fence_chain_enable_signaling
>
> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
>   drivers/dma-buf/Makefile          |   3 +-
>   drivers/dma-buf/dma-fence-chain.c | 241 ++++++++++++++++++++++++++++++
>   include/linux/dma-fence-chain.h   |  81 ++++++++++
>   3 files changed, 324 insertions(+), 1 deletion(-)
>   create mode 100644 drivers/dma-buf/dma-fence-chain.c
>   create mode 100644 include/linux/dma-fence-chain.h
>
> diff --git a/drivers/dma-buf/Makefile b/drivers/dma-buf/Makefile
> index 0913a6ccab5a..1f006e083eb9 100644
> --- a/drivers/dma-buf/Makefile
> +++ b/drivers/dma-buf/Makefile
> @@ -1,4 +1,5 @@
> -obj-y := dma-buf.o dma-fence.o dma-fence-array.o reservation.o seqno-fence.o
> +obj-y := dma-buf.o dma-fence.o dma-fence-array.o dma-fence-chain.o \
> +	 reservation.o seqno-fence.o
>   obj-$(CONFIG_SYNC_FILE)		+= sync_file.o
>   obj-$(CONFIG_SW_SYNC)		+= sw_sync.o sync_debug.o
>   obj-$(CONFIG_UDMABUF)		+= udmabuf.o
> diff --git a/drivers/dma-buf/dma-fence-chain.c b/drivers/dma-buf/dma-fence-chain.c
> new file mode 100644
> index 000000000000..0c5e3c902fa0
> --- /dev/null
> +++ b/drivers/dma-buf/dma-fence-chain.c
> @@ -0,0 +1,241 @@
> +/*
> + * fence-chain: chain fences together in a timeline
> + *
> + * Copyright (C) 2018 Advanced Micro Devices, Inc.
> + * Authors:
> + *	Christian König <christian.koenig@amd.com>
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms of the GNU General Public License version 2 as published by
> + * the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful, but WITHOUT
> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
> + * more details.
> + */
> +
> +#include <linux/dma-fence-chain.h>
> +
> +static bool dma_fence_chain_enable_signaling(struct dma_fence *fence);
> +
> +/**
> + * dma_fence_chain_get_prev - use RCU to get a reference to the previous fence
> + * @chain: chain node to get the previous node from
> + *
> + * Use dma_fence_get_rcu_safe to get a reference to the previous fence of the
> + * chain node.
> + */
> +static struct dma_fence *dma_fence_chain_get_prev(struct dma_fence_chain *chain)
> +{
> +	struct dma_fence *prev;
> +
> +	rcu_read_lock();
> +	prev = dma_fence_get_rcu_safe(&chain->prev);
> +	rcu_read_unlock();
> +	return prev;
> +}
> +
> +/**
> + * dma_fence_chain_walk - chain walking function
> + * @fence: current chain node
> + *
> + * Walk the chain to the next node. Returns the next fence or NULL if we are at
> + * the end of the chain. Garbage collects chain nodes which are already
> + * signaled.
> + */
> +struct dma_fence *dma_fence_chain_walk(struct dma_fence *fence)
> +{
> +	struct dma_fence_chain *chain, *prev_chain;
> +	struct dma_fence *prev, *replacement, *tmp;
> +
> +	chain = to_dma_fence_chain(fence);
> +	if (!chain) {
> +		dma_fence_put(fence);
> +		return NULL;
> +	}
> +
> +	while ((prev = dma_fence_chain_get_prev(chain))) {
> +
> +		prev_chain = to_dma_fence_chain(prev);
> +		if (prev_chain) {
> +			if (!dma_fence_is_signaled(prev_chain->fence))
> +				break;
> +
> +			replacement = dma_fence_chain_get_prev(prev_chain);
> +		} else {
> +			if (!dma_fence_is_signaled(prev))
> +				break;
> +
> +			replacement = NULL;
> +		}
> +
> +		tmp = cmpxchg(&chain->prev, prev, replacement);
> +		if (tmp == prev)
> +			dma_fence_put(tmp);
> +		else
> +			dma_fence_put(replacement);
> +		dma_fence_put(prev);
> +	}
> +
> +	dma_fence_put(fence);
> +	return prev;
> +}
> +EXPORT_SYMBOL(dma_fence_chain_walk);
> +
> +/**
> + * dma_fence_chain_find_seqno - find fence chain node by seqno
> + * @pfence: pointer to the chain node where to start
> + * @seqno: the sequence number to search for
> + *
> + * Advance the fence pointer to the chain node which will signal this sequence
> + * number. If no sequence number is provided then this is a no-op.
> + *
> + * Returns EINVAL if the fence is not a chain node or the sequence number has
> + * not yet advanced far enough.
> + */
> +int dma_fence_chain_find_seqno(struct dma_fence **pfence, uint64_t seqno)
> +{
> +	struct dma_fence_chain *chain;
> +
> +	if (!seqno)
> +		return 0;
> +
> +	chain = to_dma_fence_chain(*pfence);
> +	if (!chain || chain->base.seqno < seqno)
> +		return -EINVAL;
> +
> +	dma_fence_chain_for_each(*pfence, &chain->base) {
> +		if ((*pfence)->context != chain->base.context ||
> +		    to_dma_fence_chain(*pfence)->prev_seqno < seqno)
> +			break;
> +	}
> +	dma_fence_put(&chain->base);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(dma_fence_chain_find_seqno);
> +
> +static const char *dma_fence_chain_get_driver_name(struct dma_fence *fence)
> +{
> +        return "dma_fence_chain";
> +}
> +
> +static const char *dma_fence_chain_get_timeline_name(struct dma_fence *fence)
> +{
> +        return "unbound";
> +}
> +
> +static void dma_fence_chain_irq_work(struct irq_work *work)
> +{
> +	struct dma_fence_chain *chain;
> +
> +	chain = container_of(work, typeof(*chain), work);
> +
> +	/* Try to rearm the callback */
> +	if (!dma_fence_chain_enable_signaling(&chain->base))
> +		/* Ok, we are done. No more unsignaled fences left */
> +		dma_fence_signal(&chain->base);
> +	dma_fence_put(&chain->base);
> +}
> +
> +static void dma_fence_chain_cb(struct dma_fence *f, struct dma_fence_cb *cb)
> +{
> +	struct dma_fence_chain *chain;
> +
> +	chain = container_of(cb, typeof(*chain), cb);
> +	irq_work_queue(&chain->work);
> +	dma_fence_put(f);
> +}
> +
> +static bool dma_fence_chain_enable_signaling(struct dma_fence *fence)
> +{
> +	struct dma_fence_chain *head = to_dma_fence_chain(fence);
> +
> +	dma_fence_get(&head->base);
> +	dma_fence_chain_for_each(fence, &head->base) {
> +		struct dma_fence_chain *chain = to_dma_fence_chain(fence);
> +		struct dma_fence *f = chain ? chain->fence : fence;
> +
> +		dma_fence_get(f);
> +		if (!dma_fence_add_callback(f, &head->cb, dma_fence_chain_cb)) {
> +			dma_fence_put(fence);
> +			return true;
> +		}
> +		dma_fence_put(f);
> +	}
> +	dma_fence_put(&head->base);
> +	return false;
> +}
> +
> +static bool dma_fence_chain_signaled(struct dma_fence *fence)
> +{
> +	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;
> +
> +		if (!dma_fence_is_signaled(f)) {
> +			dma_fence_put(fence);
> +			return false;
> +		}
> +	}
> +
> +	return true;
> +}
> +
> +static void dma_fence_chain_release(struct dma_fence *fence)
> +{
> +	struct dma_fence_chain *chain = to_dma_fence_chain(fence);
> +
> +	dma_fence_put(chain->prev);
> +	dma_fence_put(chain->fence);
> +	dma_fence_free(fence);
> +}
> +
> +const struct dma_fence_ops dma_fence_chain_ops = {
> +	.get_driver_name = dma_fence_chain_get_driver_name,
> +	.get_timeline_name = dma_fence_chain_get_timeline_name,
> +	.enable_signaling = dma_fence_chain_enable_signaling,
> +	.signaled = dma_fence_chain_signaled,
> +	.release = dma_fence_chain_release,
> +};
> +EXPORT_SYMBOL(dma_fence_chain_ops);
> +
> +/**
> + * dma_fence_chain_init - initialize a fence chain
> + * @chain: the chain node to initialize
> + * @prev: the previous fence
> + * @fence: the current fence
> + *
> + * Initialize a new chain node and either start a new chain or add the node to
> + * the existing chain of the previous fence.
> + */
> +void dma_fence_chain_init(struct dma_fence_chain *chain,
> +			  struct dma_fence *prev,
> +			  struct dma_fence *fence,
> +			  uint64_t seqno)
> +{
> +	struct dma_fence_chain *prev_chain = to_dma_fence_chain(prev);
> +	uint64_t context;
> +
> +	spin_lock_init(&chain->lock);
> +	chain->prev = prev;
> +	chain->fence = fence;
> +	chain->prev_seqno = 0;
> +	init_irq_work(&chain->work, dma_fence_chain_irq_work);
> +
> +	/* Try to reuse the context of the previous chain node. */
> +	if (prev_chain && __dma_fence_is_later(seqno, prev->seqno)) {
> +		context = prev->context;
> +		chain->prev_seqno = prev->seqno;
> +	} else {
> +		context = dma_fence_context_alloc(1);
> +		/* Make sure that we always have a valid sequence number. */
> +		if (prev_chain)
> +			seqno = max(prev->seqno, seqno);
> +	}
> +
> +	dma_fence_init(&chain->base, &dma_fence_chain_ops,
> +		       &chain->lock, context, seqno);
> +}
> +EXPORT_SYMBOL(dma_fence_chain_init);
> diff --git a/include/linux/dma-fence-chain.h b/include/linux/dma-fence-chain.h
> new file mode 100644
> index 000000000000..a5c2e8c6915c
> --- /dev/null
> +++ b/include/linux/dma-fence-chain.h
> @@ -0,0 +1,81 @@
> +/*
> + * fence-chain: chain fences together in a timeline
> + *
> + * Copyright (C) 2018 Advanced Micro Devices, Inc.
> + * Authors:
> + *	Christian König <christian.koenig@amd.com>
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms of the GNU General Public License version 2 as published by
> + * the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful, but WITHOUT
> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
> + * more details.
> + */
> +
> +#ifndef __LINUX_DMA_FENCE_CHAIN_H
> +#define __LINUX_DMA_FENCE_CHAIN_H
> +
> +#include <linux/dma-fence.h>
> +#include <linux/irq_work.h>
> +
> +/**
> + * struct dma_fence_chain - fence to represent an node of a fence chain
> + * @base: fence base class
> + * @lock: spinlock for fence handling
> + * @prev: previous fence of the chain
> + * @prev_seqno: original previous seqno before garbage collection
> + * @fence: encapsulated fence
> + * @cb: callback structure for signaling
> + * @work: irq work item for signaling
> + */
> +struct dma_fence_chain {
> +	struct dma_fence base;
> +	spinlock_t lock;
> +	struct dma_fence *prev;
> +	u64 prev_seqno;
> +	struct dma_fence *fence;
> +	struct dma_fence_cb cb;
> +	struct irq_work work;
> +};
> +
> +extern const struct dma_fence_ops dma_fence_chain_ops;
> +
> +/**
> + * to_dma_fence_chain - cast a fence to a dma_fence_chain
> + * @fence: fence to cast to a dma_fence_array
> + *
> + * Returns NULL if the fence is not a dma_fence_chain,
> + * or the dma_fence_chain otherwise.
> + */
> +static inline struct dma_fence_chain *
> +to_dma_fence_chain(struct dma_fence *fence)
> +{
> +	if (!fence || fence->ops != &dma_fence_chain_ops)
> +		return NULL;
> +
> +	return container_of(fence, struct dma_fence_chain, base);
> +}
> +
> +/**
> + * dma_fence_chain_for_each - iterate over all fences in chain
> + * @iter: current fence
> + * @head: starting point
> + *
> + * Iterate over all fences in the chain. We keep a reference to the current
> + * fence while inside the loop which must be dropped when breaking out.
> + */
> +#define dma_fence_chain_for_each(iter, head)	\
> +	for (iter = dma_fence_get(head); iter; \
> +	     iter = dma_fence_chain_walk(head))
> +
> +struct dma_fence *dma_fence_chain_walk(struct dma_fence *fence);
> +int dma_fence_chain_find_seqno(struct dma_fence **pfence, uint64_t seqno);
> +void dma_fence_chain_init(struct dma_fence_chain *chain,
> +			  struct dma_fence *prev,
> +			  struct dma_fence *fence,
> +			  uint64_t seqno);
> +
> +#endif /* __LINUX_DMA_FENCE_CHAIN_H */


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