Am 12.12.19 um 15:50 schrieb Grodzovsky, Andrey:
[AMD Official Use Only - Internal Distribution Only]
______________________________________
From: Christian König <ckoenig.leichtzumerken@xxxxxxxxx>
Sent: 12 December 2019 03:31
To: Alex Deucher; Grodzovsky, Andrey
Cc: Deucher, Alexander; Ma, Le; Quan, Evan; amd-gfx list; Zhang, Hawking
Subject: Re: [PATCH 2/5] drm: Add Reusable task barrier.
Am 12.12.19 um 09:24 schrieb Christian König:
Am 11.12.19 um 21:19 schrieb Alex Deucher:
On Wed, Dec 11, 2019 at 3:07 PM Andrey Grodzovsky
<andrey.grodzovsky@xxxxxxx> wrote:
It is used to synchronize N threads at a rendevouz point before
execution
of critical code that has to be started by all the threads at
approximatly
the same time.
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@xxxxxxx>
You should resend to dri-devel since this task barrier is being added
to common code.
Additional to that this whole thing has the potential to raise lockdep
warnings and if I'm not completely mistaken doesn't even work correctly.
Can you give me a potential lockdep scenario ?
Lockdep usually complains if a lock is released from another thread than
where it was locked from.
In the code you let each thread do a down() and then the last one does
multiple up() calls.
But I think that is only illegal for mutexes, but legal for semaphores.
Christian.
Andrey
See Linux kernel semaphores don't allow negative values (the count
field in struct semaphore is unsigned).
Ok, forget what I've wrote. That indeed seems to be supported, some
other drivers are already using semaphores the same way.
Regards,
Christian.
Regards,
Christian.
Alex
---
include/drm/task_barrier.h | 106
+++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 106 insertions(+)
create mode 100644 include/drm/task_barrier.h
diff --git a/include/drm/task_barrier.h b/include/drm/task_barrier.h
new file mode 100644
index 0000000..81fb0f7
--- /dev/null
+++ b/include/drm/task_barrier.h
@@ -0,0 +1,106 @@
+/*
+ * Copyright 2019 Advanced Micro Devices, Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person
obtaining a
+ * copy of this software and associated documentation files (the
"Software"),
+ * to deal in the Software without restriction, including without
limitation
+ * the rights to use, copy, modify, merge, publish, distribute,
sublicense,
+ * and/or sell copies of the Software, and to permit persons to
whom the
+ * Software is furnished to do so, subject to the following
conditions:
+ *
+ * The above copyright notice and this permission notice shall be
included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO
EVENT SHALL
+ * THE COPYRIGHT HOLDER(S) OR AUTHOR(S) BE LIABLE FOR ANY CLAIM,
DAMAGES OR
+ * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
OTHERWISE,
+ * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
USE OR
+ * OTHER DEALINGS IN THE SOFTWARE.
+ *
+ */
+#include <linux/semaphore.h>
+#include <linux/atomic.h>
+
+/*
+ * Reusable 2 PHASE task barrier (randevouz point) implementation
for N tasks.
+ * Based on the Little book of sempahores -
https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgreenteapress.com%2Fwp%2Fsemaphores%2F&data=02%7C01%7Candrey.grodzovsky%40amd.com%7C96f8c28ae4bd43f2922208d77eddb60f%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637117363029062995&sdata=6xXZXyDIKmQM8ET5hPIZ%2FbJrOMQqK4oYw8dGYS1rxcY%3D&reserved=0
+ */
+
+
+
+#ifndef DRM_TASK_BARRIER_H_
+#define DRM_TASK_BARRIER_H_
+
+/*
+ * Represents an instance of a task barrier.
+ */
+struct task_barrier {
+ unsigned int n;
+ atomic_t count;
+ struct semaphore enter_turnstile;
+ struct semaphore exit_turnstile;
+};
+
+static inline void task_barrier_signal_turnstile(struct semaphore
*turnstile,
+ unsigned int n)
+{
+ int i;
+
+ for (i = 0 ; i < n; i++)
+ up(turnstile);
+}
+
+static inline void task_barrier_init(struct task_barrier *tb)
+{
+ tb->n = 0;
+ atomic_set(&tb->count, 0);
+ sema_init(&tb->enter_turnstile, 0);
+ sema_init(&tb->exit_turnstile, 0);
+}
+
+static inline void task_barrier_add_task(struct task_barrier *tb)
+{
+ tb->n++;
+}
+
+static inline void task_barrier_rem_task(struct task_barrier *tb)
+{
+ tb->n--;
+}
+
+/*
+ * Lines up all the threads BEFORE the critical point.
+ *
+ * When all thread passed this code the entry barrier is back to
locked state.
+ */
+static inline void task_barrier_enter(struct task_barrier *tb)
+{
+ if (atomic_inc_return(&tb->count) == tb->n)
+ task_barrier_signal_turnstile(&tb->enter_turnstile, tb->n);
+
+ down(&tb->enter_turnstile);
+}
+
+/*
+ * Lines up all the threads AFTER the critical point.
+ *
+ * This function is used to avoid any one thread running ahead of
the reset if
+ * the barrier is used in a loop (repeatedly) .
+ */
+static inline void task_barrier_exit(struct task_barrier *tb)
+{
+ if (atomic_dec_return(&tb->count) == 0)
+ task_barrier_signal_turnstile(&tb->exit_turnstile, tb->n);
+
+ down(&tb->exit_turnstile);
+}
+
+static inline void task_barrier_full(struct task_barrier *tb)
+{
+ task_barrier_enter(tb);
+ task_barrier_exit(tb);
+}
+
+#endif
--
2.7.4
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