Quoting Chunming Zhou (2018-12-11 10:34:40)
> From: Christian König <ckoenig.leichtzumerken@xxxxxxxxx>
>
> 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@xxxxxxx>
> ---
> 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 @@
> +/*
SPDX-License-Identifier: GPL-2.0
> + * fence-chain: chain fences together in a timeline
> + *
> + * Copyright (C) 2018 Advanced Micro Devices, Inc.
> + * Authors:
> + * Christian König <christian.koenig@xxxxxxx>
> + *
> + * 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);
if (cmpxchg(&chain->prev, prev, replacement) == prev)
dma_fence_put(prev)
else
dma_fence_put(replacement);
would be a little easier for me to read.
> + dma_fence_put(prev);
> + }
> +
> + dma_fence_put(fence);
Putting the caller's fence caught me by surprise.
I'd be tempted to do
> +
> +/**
> + * 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))
dma_fence_chain_walk(head) <-- should be iter?
#define dma_fence_chain_for_each(iter, head) \
for (iter = dma_fence_get(head); iter; \
({ dma_fence_put(iter); iter = dma_fence_chain_walk(iter)))
just so that dma_fence_chain_walk() didn't look unbalanced and the
put/get more clearly inside the for_each.
> + 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)
That inout parameter took me by surprise. "find" I expected to return
the fence.
Not used in this patch, so I need to read on to see if its use
simplifies the code or is equally surprising ;)
-Chris
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