Re: [PATCH 02/11] dma-buf: add new dma_fence_chain container v2

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Am 03.12.18 um 14:44 schrieb Chunming Zhou:

在 2018/12/3 21:28, Christian König 写道:
Am 03.12.18 um 14:18 schrieb Chunming Zhou:
在 2018/12/3 19:00, Christian König 写道:
Am 03.12.18 um 06:25 schrieb zhoucm1:
On 2018年11月28日 22:50, Christian König wrote:
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.

Signed-off-by: Christian König <christian.koenig@xxxxxxx>
---
    drivers/dma-buf/Makefile          |   3 +-
    drivers/dma-buf/dma-fence-chain.c | 235
++++++++++++++++++++++++++++++++++++++
    include/linux/dma-fence-chain.h   |  79 +++++++++++++
    3 files changed, 316 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..de05101fc48d
--- /dev/null
+++ b/drivers/dma-buf/dma-fence-chain.c
@@ -0,0 +1,235 @@
+/*
+ * 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);
+        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) {
+        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);
+}
+
+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_chain_for_each(fence) {
+        struct dma_fence_chain *chain = to_dma_fence_chain(fence);
+        struct dma_fence *f = chain ? chain->fence : fence;
+
+        if (!dma_fence_add_callback(f, &head->cb,
dma_fence_chain_cb))
+            return true;
+    }
+    return false;
+}
+
+static bool dma_fence_chain_signaled(struct dma_fence *fence)
+{
+    dma_fence_chain_for_each(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 && seqno > prev->seqno &&
+        __dma_fence_is_later(seqno, prev->seqno)) {
As your patch#1 makes __dma_fence_is_later only be valid for 32bit,
we cannot use it for 64bit here, we should remove it from here, just
compare seqno directly.
That is intentional. We must make sure that the number both increments
as 64bit number as well as not wraps around as 32bit number.

In other words the largest difference between two sequence numbers
userspace is allowed to submit is 1<<31.
Why? no one can make sure that, application users would only think it is
an uint64 sequence nubmer, and they can signal any advanced point. I
already see umd guys writing timeline test use max_uint64-1 as a final
signal.
We shouldn't add this limitation here.
We need to be backward compatible to hardware which can only do 32bit
signaling with the dma-fence implementation.
I see that, you already explained that before.
but can't we just grep low 32bit of seqno only when 32bit hardware try
to use?

then we can make dma_fence_later use 64bit comparation.

The problem is that we don't know at all times when to use a 32bit compare and when to use a 64bit compare.

What we could do is test if any of the upper 32bits of a sequence number is not 0 and if that is the case do a 64bit compare. This way max_uint64_t would still be handled correctly.


Christian.


Otherwise dma_fence_later() could return an inconsistent result and
break at other places.

So if userspace wants to use more than 1<<31 difference between
sequence numbers we need to push back on this.
It's rare case, but I don't think we can convince them add this
limitation. So we cannot add this limitation here.

-David

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