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

[Lionel Landwerlin via amd-gfx <amd-gfx-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org>]

On 15/02/2019 14:32, Koenig, Christian wrote:
> Am 15.02.19 um 15:23 schrieb Lionel Landwerlin:
>> 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
> Yeah, and where exactly is the problem?
>
> Seqno 1 will signal when the 3d style engine finishes work.
>
> And seqno 2 will signal when both seqno 1 is signaled and the blitter
> style engine has finished its work.

That's not really how I understood the spec, but I might be wrong.

What makes me thing 1 should be signaled as soon as 2 is signaled
(regardless of whether the fence attached on point 1 is been signaled),
is that the spec defines wait & signal operations in term of the value
of the timeline.


-Lionel

>
>> Another scenario would be signaling a timeline with points 1 & 2 with
>> those points in reverse order in the submission array.
> That is actually illegal in the spec, but actually handled gracefully as
> well.
>
> E.g. when you add seqno 1 to the syncobj container it will only signal
> when 2 is signaled as well.




>
> Regards,
> Christian.
>
>> -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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