New subject: [RFC] Synchronizing access to buffers shared with dma-buf between drivers/devices

4 Jun 2012

Some comments inline.. at this stage mostly superficial issues about
how the API works, etc..  not had a chance to dig too much into the
implementation yet (although some of my comments about the API would
change those anyways).
Anyways, thanks for getting the ball rolling on this, and I think I
can volunteer linaro to pick up and run w/ this if needed.
On Fri, May 25, 2012 at 7:08 PM, Tom Cooksey tom.cooksey@arm.com wrote:
...
Hi All,
I realise it's been a while since this was last discussed, however I'd like
to bring up kernel-side synchronization again. By kernel-side
synchronization, I mean allowing multiple drivers/devices wanting to access
the same buffer to do so without bouncing up to userspace to resolve
dependencies such as "the display controller can't start scanning out a
buffer until the GPU has finished rendering into it". As such, this is
really just an optimization which reduces latency between E.g. The GPU
finishing a rendering job and that buffer being scanned out. I appreciate
this particular example is already solved on desktop graphics cards as the
display controller and 3D core are both controlled by the same driver, so no
"generic" mechanism is needed. However on ARM SoCs, the 3D core (like an ARM
Mali) and display controller tend to be driven by separate drivers, so some
mechanism is needed to allow both drivers to synchronize their access to
buffers.
There are multiple ways synchronization can be achieved, fences/sync objects
is one common approach, however we're presenting a different approach.
Personally, I quite like fence sync objects, however we believe it requires
a lot of userspace interfaces to be changed to pass around sync object
handles. Our hope is that the kds approach will require less effort to make
use of as no existing userspace interfaces need to be changed. E.g. To use
explicit fences, the struct drm_mode_crtc_page_flip would need a new members
to pass in the handle(s) of sync object(s) which the flip depends on (I.e.
don't flip until these fences fire). The additional benefit of our approach
is that it prevents userspace specifying dependency loops which can cause a
deadlock (see kds.txt for an explanation of what I mean here).
I have waited until now to bring this up again because I am now able to
share the code I was trying (and failing I think) to explain previously. The
code has now been released under the GPLv2 from ARM Mali's developer portal,
however I've attempted to turn that into a patch to allow it to be discussed
on this list. Please find the patch inline below.
While KDS defines a very generic mechanism, I am proposing that this code or
at least the concepts be merged with the existing dma_buf code, so a the
struct kds_resource members get moved to struct dma_buf, kds_* functions get
renamed to dma_buf_* functions, etc. So I guess what I'm saying is please
don't review the actual code just yet, only the concepts the code describes,
where kds_resource == dma_duf.
Cheers,
Tom
Author: Tom Cooksey tom.cooksey@arm.com
Date:   Fri May 25 10:45:27 2012 +0100
Add new system to allow synchronizing access to resources
See Documentation/kds.txt for details, however the general
   idea is that this kds framework synchronizes multiple drivers
   ("clients") wanting to access the same resources, where a
   resource is typically a 2D image buffer being shared around
   using dma-buf.
Note: This patch is created by extracting the sources from the
   tarball on <http://www.malideveloper.com/open-source-mali-gpus-lin
   ux-kernel-device-drivers---dev-releases.php> and putting them in
   roughly the right places.

diff --git a/Documentation/kds.txt b/Documentation/kds.txt
fwiw, I think the documentation could be made a bit more generic, but
this and code style, etc shouldn't be too hard to fix
...
new file mode 100644
index 0000000..a96db21
--- /dev/null
+++ b/Documentation/kds.txt
@@ -0,0 +1,113 @@
+#
+# (C) COPYRIGHT 2012 ARM Limited. All rights reserved.
+#
+# This program is free software and is provided to you under the terms of
the GNU General Public License version 2
+# as published by the Free Software Foundation, and any use by you of this
program is subject to the terms of such GNU licence.
+#
+# A copy of the licence is included with the program, and can also be
obtained from Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
02110-1301, USA.
+#
+#




+==============================
+kds - Kernel Dependency System
+==============================



+Introduction
+------------
+kds provides a mechanism for clients to atomically lock down multiple
abstract resources.
+This can be done either synchronously or asynchronously.
+Abstract resources is used to allow a set of clients to use kds to control
access to any
+resource, an example is structured memory buffers.



+kds supports that buffer is locked for exclusive access and sharing of
buffers.



+kds can be built as either a integrated feature of the kernel or as a
module.
+It supports being compiled as a module both in-tree and out-of-tree.




+Concepts
+--------
+A core concept in kds is abstract resources.
+A kds resource is just an abstraction for some client object, kds doesn't
care what it is.
+Typically EGL will consider UMP buffers as being a resource, thus each UMP
buffer has
+a kds resource for synchronization to the buffer.



+kds allows a client to create and destroy the abstract resource objects.
+A new resource object is made available asap (it is just a simple malloc
with some initializations),
+while destroy it requires some external synchronization.



+The other core concept in kds is consumer of resources.
+kds is requested to allow a client to consume a set of resources and the
client will be notified when it can consume the resources.



+Exclusive access allows only one client to consume a resource.
+Shared access permits multiple consumers to acceess a resource
concurrently.




+APIs
+----
+kds provides simple resource allocate and destroy functions.
+Clients use this to instantiate and control the lifetime of the resources
kds manages.



+kds provides two ways to wait for resources:
+- Asynchronous wait: the client specifies a function pointer to be called
when wait is over
+- Synchronous wait: Function blocks until access is gained.



+The synchronous API has a timeout for the wait.
+The call can early out if a signal is delivered.



+After a client is done consuming the resource kds must be notified to
release the resources and let some other client take ownership.
+This is done via resource set release call.



+A Windows comparison:
+kds implements WaitForMultipleObjectsEx(..., bWaitAll = TRUE, ...) but also
has an asynchronous version in addition.
+kds resources can be seen as being the same as NT object manager resources.



+Internals
+---------
+kds guarantees atomicity when a set of resources is operated on.
+This is implemented via a global resource lock which is taken by kds when
it updates resource objects.



+Internally a resource in kds is a linked list head with some flags.



+When a consumer requests access to a set of resources it is queued on each
of the resources.
+The link from the consumer to the resources can be triggered. Once all
links are triggered
+the registered callback is called or the blocking function returns.
+A link is considered triggered if it is the first on the list of consumers
of a resource,
+or if all the links ahead of it is marked as shared and itself is of the
type shared.



+When the client is done consuming the consumer object is removed from the
linked lists of
+the resources and a potential new consumer becomes the head of the
resources.
+As we add and remove consumers atomically across all resources we can
guarantee that
+we never introduces a A->B + B->A type of loops/deadlocks.




+kbase/base implementation
+-------------------------
+A HW job needs access to a set of shared resources.
+EGL tracks this and encodes the set along with the atom in the ringbuffer.
+EGL allocates a (k)base dep object to represent the dependency to the set
of resources and encodes that along with the list of resources.
+This dep object is use to create a dependency from a job chain(atom) to the
resources it needs to run.
+When kbase decodes the atom in the ringbuffer it finds the set of resources
and calls kds to request all the needed resources.
+As EGL needs to know when the kds request is delivered a new base event
object is needed: atom enqueued. This event is only delivered for atoms
which uses kds.
+The callback kbase registers trigger the dependency object described which
would trigger the existing JD system to release the job chain.
+When the atom is done kds resource set release is call to release the
resources.



+EGL will typically use exclusive access to the render target, while all
buffers used as input can be marked as shared.




+Buffer publish/vsync
+--------------------
+EGL will use a separate ioctl or DRM flip to request the flip.
+If the LCD driver is integrated with kds EGL can do these operations early.
+The LCD driver must then implement the ioctl or DRM flip to be asynchronous
with kds async call.
+The LCD driver binds a kds resource to each virtual buffer (2 buffers in
case of double-buffering).
+EGL will make a dependency to the target kds resource in the kbase atom.
+After EGL receives a atom enqueued event it can ask the LCD driver to pan
to the target kds resource.
+When the atom is completed it'll release the resource and the LCD driver
will get its callback.
+In the callback it'll load the target buffer into the DMA unit of the LCD
hardware.
+The LCD driver will be the consumer of both buffers for a short period.
+The LCD driver will call kds resource set release on the previous on-screen
buffer when the next vsync/dma read end is handled.




diff --git a/drivers/misc/kds.c b/drivers/misc/kds.c
new file mode 100644
index 0000000..8d7d55e
--- /dev/null
+++ b/drivers/misc/kds.c
@@ -0,0 +1,461 @@
+/*






(C) COPYRIGHT 2012 ARM Limited. All rights reserved.







This program is free software and is provided to you under the terms of



the GNU General Public License version 2


as published by the Free Software Foundation, and any use by you of this



program is subject to the terms of such GNU licence.






A copy of the licence is included with the program, and can also be



obtained from Free Software


Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA



02110-1301, USA.





*/




+#include <linux/slab.h>
+#include <linux/list.h>
+#include <linux/mutex.h>
+#include <linux/wait.h>
+#include <linux/sched.h>
+#include <linux/err.h>
+#include <linux/module.h>
+#include <linux/workqueue.h>
+#include <linux/kds.h>




+#define KDS_LINK_TRIGGERED (1u << 0)
+#define KDS_LINK_EXCLUSIVE (1u << 1)



+#define KDS_IGNORED NULL
+#define KDS_INVALID (void*)-2
+#define KDS_RESOURCE (void*)-1



+struct kds_resource_set
+{

  unsigned long         num_resources;


  unsigned long         pending;


  unsigned long         locked_resources;


  struct kds_callback * cb;


  void *                callback_parameter;


  void *                callback_extra_parameter;


  struct list_head      callback_link;


  struct work_struct    callback_work;


  struct kds_link       resources[0];



+};



+static DEFINE_MUTEX(kds_lock);



+int kds_callback_init(struct kds_callback * cb, int direct, kds_callback_fn
user_cb)
+{

  int ret = 0;



  cb->direct = direct;


  cb->user_cb = user_cb;



  if (!direct)


  {


          cb->wq = alloc_workqueue("kds", WQ_UNBOUND | WQ_HIGHPRI,



WQ_UNBOUND_MAX_ACTIVE);

          if (!cb->wq)


                  ret = -ENOMEM;


  }


  else


  {


          cb->wq = NULL;


  }



  return ret;



+}
+EXPORT_SYMBOL(kds_callback_init);



+void kds_callback_term(struct kds_callback * cb)
+{

  if (!cb->direct)


  {


          BUG_ON(!cb->wq);


          destroy_workqueue(cb->wq);


  }


  else


  {


          BUG_ON(cb->wq);


  }



+}



+EXPORT_SYMBOL(kds_callback_term);



+static void kds_do_user_callback(struct kds_resource_set * rset)
+{

  rset->cb->user_cb(rset->callback_parameter,



rset->callback_extra_parameter);
+}



+static void kds_queued_callback(struct work_struct * work)
+{

  struct kds_resource_set * rset;


  rset = container_of( work, struct kds_resource_set, callback_work);



  kds_do_user_callback(rset);



+}



+static void kds_callback_perform(struct kds_resource_set * rset)
+{

  if (rset->cb->direct)


          kds_do_user_callback(rset);


  else


  {


          int result;


          result = queue_work(rset->cb->wq, &rset->callback_work);


          /* if we got a 0 return it means we've triggered the same



rset twice! */

          BUG_ON(!result);


  }



+}



+void kds_resource_init(struct kds_resource * res)
+{

  BUG_ON(!res);


  INIT_LIST_HEAD(&res->waiters.link);


  res->waiters.parent = KDS_RESOURCE;



+}
+EXPORT_SYMBOL(kds_resource_init);



+void kds_resource_term(struct kds_resource * res)
+{

  BUG_ON(!res);


  BUG_ON(!list_empty(&res->waiters.link));


  res->waiters.parent = KDS_INVALID;



+}
+EXPORT_SYMBOL(kds_resource_term);



+int kds_async_waitall(

          struct kds_resource_set ** pprset,


          unsigned long              flags,


          struct kds_callback *      cb,


          void *                     callback_parameter,


          void *                     callback_extra_parameter,


          int                        number_resources,


          unsigned long *            exclusive_access_bitmap,


          struct kds_resource **     resource_list)



+{

  struct kds_resource_set * rset = NULL;


  int i;


  int triggered;


  int err = -EFAULT;



  BUG_ON(!pprset);


  BUG_ON(!resource_list);


  BUG_ON(!cb);



  mutex_lock(&kds_lock);



  if ((flags & KDS_FLAG_LOCKED_ACTION) == KDS_FLAG_LOCKED_FAIL)


  {


          for (i = 0; i < number_resources; i++)


          {


                  if (resource_list[i]->lock_count)


                  {


                          err = -EBUSY;


                          goto errout;


                  }


          }


  }



  rset = kmalloc(sizeof(*rset) + number_resources * sizeof(struct



kds_link), GFP_KERNEL);

  if (!rset)


  {


          err = -ENOMEM;


          goto errout;


  }



  rset->num_resources = number_resources;


  rset->pending = number_resources;


  rset->locked_resources = 0;


  rset->cb = cb;


  rset->callback_parameter = callback_parameter;


  rset->callback_extra_parameter = callback_extra_parameter;


  INIT_LIST_HEAD(&rset->callback_link);


  INIT_WORK(&rset->callback_work, kds_queued_callback);



  for (i = 0; i < number_resources; i++)


  {


          unsigned long link_state = 0;



          INIT_LIST_HEAD(&rset->resources[i].link);


          rset->resources[i].parent = rset;



          if (test_bit(i, exclusive_access_bitmap))


          {


                  link_state |= KDS_LINK_EXCLUSIVE;


          }



          /* no-one else waiting? */


          if (list_empty(&resource_list[i]->waiters.link))


          {


                  link_state |= KDS_LINK_TRIGGERED;


                  rset->pending--;


          }


          /* Adding a non-exclusive and the current tail is a



triggered non-exclusive? */

          else if (((link_state & KDS_LINK_EXCLUSIVE) == 0) &&


                  (((list_entry(resource_list[i]->waiters.link.prev,



struct kds_link, link)->state & (KDS_LINK_EXCLUSIVE | KDS_LINK_TRIGGERED))
== KDS_LINK_TRIGGERED)))

          {


                  link_state |= KDS_LINK_TRIGGERED;


                  rset->pending--;


          }


          /* locked & ignore locked? */


          else if ((resource_list[i]->lock_count) && ((flags &



KDS_FLAG_LOCKED_ACTION) == KDS_FLAG_LOCKED_IGNORE) )

          {


                  link_state |= KDS_LINK_TRIGGERED;


                  rset->pending--;


                  rset->resources[i].parent = KDS_IGNORED; /* to



disable decrementing the pending count when we get the ignored resource */

          }


          rset->resources[i].state = link_state;


          list_add_tail(&rset->resources[i].link,



&resource_list[i]->waiters.link);

  }



  triggered = (rset->pending == 0);



  mutex_unlock(&kds_lock);



  /* set the pointer before the callback is called so it sees it */


  *pprset = rset;



  if (triggered)


  {


          /* all resources obtained, trigger callback */


          kds_callback_perform(rset);


  }



  return 0;




+errout:

  mutex_unlock(&kds_lock);


  return err;



+}
+EXPORT_SYMBOL(kds_async_waitall);



+static void wake_up_sync_call(void * callback_parameter, void *
callback_extra_parameter)
+{

  wait_queue_head_t * wait = (wait_queue_head_t*)callback_parameter;


  wake_up(wait);



+}



+static struct kds_callback sync_cb =
+{

  wake_up_sync_call,


  1,


  NULL,



+};



+struct kds_resource_set * kds_waitall(

          int                    number_resources,


          unsigned long *        exclusive_access_bitmap,


          struct kds_resource ** resource_list,


          unsigned long          jiffies_timeout)



+{

  struct kds_resource_set * rset;


  int i;


  int triggered = 0;


  DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wake);



  rset = kmalloc(sizeof(*rset) + number_resources * sizeof(struct



kds_link), GFP_KERNEL);

  if (!rset)


          return rset;



  rset->num_resources = number_resources;


  rset->pending = number_resources;


  rset->locked_resources = 1;


  INIT_LIST_HEAD(&rset->callback_link);


  INIT_WORK(&rset->callback_work, kds_queued_callback);



  mutex_lock(&kds_lock);



  for (i = 0; i < number_resources; i++)


  {


          unsigned long link_state = 0;



          if (likely(resource_list[i]->lock_count < ULONG_MAX))


                  resource_list[i]->lock_count++;


          else


                  break;



          if (test_bit(i, exclusive_access_bitmap))


          {


                  link_state |= KDS_LINK_EXCLUSIVE;


          }



          if (list_empty(&resource_list[i]->waiters.link))


          {


                  link_state |= KDS_LINK_TRIGGERED;


                  rset->pending--;


          }


          /* Adding a non-exclusive and the current tail is a



triggered non-exclusive? */

          else if (((link_state & KDS_LINK_EXCLUSIVE) == 0) &&


                  (((list_entry(resource_list[i]->waiters.link.prev,



struct kds_link, link)->state & (KDS_LINK_EXCLUSIVE | KDS_LINK_TRIGGERED))
== KDS_LINK_TRIGGERED)))

          {


                  link_state |= KDS_LINK_TRIGGERED;


                  rset->pending--;


          }



          INIT_LIST_HEAD(&rset->resources[i].link);


          rset->resources[i].parent = rset;


          rset->resources[i].state = link_state;


          list_add_tail(&rset->resources[i].link,



&resource_list[i]->waiters.link);

  }



  if (i < number_resources)


  {


          /* an overflow was detected, roll back */


          while (i--)


          {


                  list_del(&rset->resources[i].link);


                  resource_list[i]->lock_count--;


          }


          mutex_unlock(&kds_lock);


          kfree(rset);


          return ERR_PTR(-EFAULT);


  }



  if (rset->pending == 0)


          triggered = 1;


  else


  {


          rset->cb = &sync_cb;


          rset->callback_parameter = &wake;


          rset->callback_extra_parameter = NULL;


  }



  mutex_unlock(&kds_lock);



  if (!triggered)


  {


          long wait_res;


          if ( KDS_WAIT_BLOCKING == jiffies_timeout )


          {


                  wait_res = wait_event_interruptible(wake,



rset->pending == 0);

          }


          else


          {


                  wait_res = wait_event_interruptible_timeout(wake,



rset->pending == 0, jiffies_timeout);

          }


          if ((wait_res == -ERESTARTSYS) || (wait_res == 0))


          {


                  /* use \a kds_resource_set_release to roll back */


                  kds_resource_set_release(&rset);


                  return ERR_PTR(wait_res);


          }


  }


  return rset;



+}
+EXPORT_SYMBOL(kds_waitall);



+void kds_resource_set_release(struct kds_resource_set ** pprset)
+{

  struct list_head triggered = LIST_HEAD_INIT(triggered);


  struct kds_resource_set * rset;


  struct kds_resource_set * it;


  int i;



  BUG_ON(!pprset);



  mutex_lock(&kds_lock);



  rset = *pprset;


  if (!rset)


  {


          /* caught a race between a cancelation


           * and a completion, nothing to do */


          mutex_unlock(&kds_lock);


          return;


  }



  /* clear user pointer so we'll be the only


   * thread handling the release */


  *pprset = NULL;



  for (i = 0; i < rset->num_resources; i++)


  {


          struct kds_resource * resource;


          struct kds_link * it = NULL;



          /* fetch the previous entry on the linked list */


          it = list_entry(rset->resources[i].link.prev, struct



kds_link, link);

          /* unlink ourself */


          list_del(&rset->resources[i].link);



          /* any waiters? */


          if (list_empty(&it->link))


                  continue;



          /* were we the head of the list? (head if prev is a



resource) */

          if (it->parent != KDS_RESOURCE)


                  continue;



          /* we were the head, find the kds_resource */


          resource = container_of(it, struct kds_resource, waiters);



          if (rset->locked_resources)


          {


                  resource->lock_count--;


          }



          /* we know there is someone waiting from the any-waiters



test above */


          /* find the head of the waiting list */


          it = list_first_entry(&resource->waiters.link, struct



kds_link, link);


          /* new exclusive owner? */


          if (it->state & KDS_LINK_EXCLUSIVE)


          {


                  /* link now triggered */


                  it->state |= KDS_LINK_TRIGGERED;


                  /* a parent to update? */


                  if (it->parent != KDS_IGNORED)


                  {


                          if (0 == --it->parent->pending)


                          {


                                  /* new owner now triggered, track



for callback later */

                                  list_add(&it->parent->callback_link,



&triggered);

                          }


                  }


          }


          /* exclusive releasing ? */


          else if (rset->resources[i].state & KDS_LINK_EXCLUSIVE)


          {


                  /* trigger non-exclusive until end-of-list or first



exclusive */

                  list_for_each_entry(it, &resource->waiters.link,



link)

                  {


                          /* exclusive found, stop triggering */


                          if (it->state & KDS_LINK_EXCLUSIVE)


                                  break;



                          it->state |= KDS_LINK_TRIGGERED;


                          /* a parent to update? */


                          if (it->parent != KDS_IGNORED)


                          {


                                  if (0 == --it->parent->pending)


                                  {


                                          /* new owner now triggered,



track for callback later */



list_add(&it->parent->callback_link, &triggered);

                                  }


                          }


                  }


          }



  }



  mutex_unlock(&kds_lock);



  while (!list_empty(&triggered))


  {


          it = list_first_entry(&triggered, struct kds_resource_set,



callback_link);

          list_del(&it->callback_link);


          kds_callback_perform(it);


  }



  cancel_work_sync(&rset->callback_work);



  /* free the resource set */


  kfree(rset);



+}
+EXPORT_SYMBOL(kds_resource_set_release);



+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("ARM Ltd.");
+MODULE_VERSION("1.0");
diff --git a/include/linux/kds.h b/include/linux/kds.h
new file mode 100644
index 0000000..65e5706
--- /dev/null
+++ b/include/linux/kds.h
@@ -0,0 +1,154 @@
+/*






(C) COPYRIGHT 2012 ARM Limited. All rights reserved.







This program is free software and is provided to you under the terms of



the GNU General Public License version 2


as published by the Free Software Foundation, and any use by you of this



program is subject to the terms of such GNU licence.






A copy of the licence is included with the program, and can also be



obtained from Free Software


Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA



02110-1301, USA.





*/




+#ifndef _KDS_H_
+#define _KDS_H_



+#include <linux/list.h>
+#include <linux/workqueue.h>



+#define KDS_WAIT_BLOCKING (ULONG_MAX)



+/* what to do when waitall must wait for a synchronous locked resource: */
+#define KDS_FLAG_LOCKED_FAIL    (0u <<  0) /* fail waitall */
+#define KDS_FLAG_LOCKED_IGNORE  (1u <<  0) /* don't wait, but block other
that waits */
+#define KDS_FLAG_LOCKED_WAIT    (2u <<  0) /* wait (normal */
+#define KDS_FLAG_LOCKED_ACTION  (3u <<  0) /* mask to extract the action to
do on locked resources */



+struct kds_resource_set;



+typedef void (*kds_callback_fn) (void * callback_parameter, void *
callback_extra_parameter);



+struct kds_callback
+{

  kds_callback_fn  user_cb; /* real cb */


  int direct;               /* do direct or queued call? */


  struct workqueue_struct * wq;



+};



+struct kds_link
+{

  struct kds_resource_set * parent;


  struct list_head          link;


  unsigned long             state;



+};



+struct kds_resource
+{

  struct kds_link waiters;


  unsigned long lock_count;



+};



+/* callback API */



+/* Initialize a callback object.






Typically created per context or per hw resource.







Callbacks can be performed directly if no nested locking can



happen in the client.







Nested locking can occur when a lock is held during the



kds_async_waitall or


kds_resource_set_release call. If the callback needs to take the same



lock


nested locking will happen.







If nested locking could happen non-direct callbacks can be requested.



Callbacks will then be called asynchronous to the triggering call.


*/

+int kds_callback_init(struct kds_callback * cb, int direct, kds_callback_fn
user_cb);



+/* Terminate the use of a callback object.






If the callback object was set up as non-direct



any pending callbacks will be flushed first.



Note that to avoid a deadlock the lock callbacks needs



can't be held when a callback object is terminated.


*/

+void kds_callback_term(struct kds_callback * cb);
hmm, not hugely a fan of this.. having callbacks that might need to
aquire locks be potentially called synchronously in special cases.
It seems like it can get simpler if pending callbacks could hold a
reference to the underlying object until the callback completes.
Although not quite sure offhand how that can work without coupling kds
to dmabuf or GEM..
...



+/* resource object API */



+/* initialize a resource handle for a shared resource */
+void kds_resource_init(struct kds_resource * resource);



+/*


Will assert if the resource is being used or waited on.



The caller should NOT try to terminate a resource that could still have



clients.


After the function returns the resource is no longer known by kds.


*/

+void kds_resource_term(struct kds_resource * resource);



+/* Asynchronous wait for a set of resources.


Callback will be called when all resources are available.



If all the resources was available the callback will be called before



kds_async_waitall returns.


So one must not hold any locks the callback code-flow can take when



calling kds_async_waitall.


Caller considered to own/use the resources until \a kds_rset_release is



called.


flags is one or more of the KDS_FLAG_* set.



exclusive_access_bitmap is a bitmap where a high bit means exclusive



access while a low bit means shared access.


Use the Linux __set_bit API, where the index of the buffer to control is



used as the bit index.






Standard Linux error return value.


*/

+int kds_async_waitall(

          struct kds_resource_set ** pprset,


          unsigned long              flags,


          struct kds_callback *      cb,


          void *                     callback_parameter,


          void *                     callback_extra_parameter,


          int                        number_resources,


          unsigned long *            exclusive_access_bitmap,



hmm, is there advantage of passing the requested resources this way,
vs. just having two arrays (one for exclusive access, one for shared
access)?
...

          struct kds_resource **     resource_list);



callback_parameter + callback_extra_parameter seems a bit, well, odd.
I'm guessing that is some implementation detail about mali driver
peaking through?
But maybe instead of inventing something new, we can just use 'struct
kthread_work' instead of 'struct kds_callback' plus the two 'void *'s?
 If the user needs some extra args they can embed 'struct
kthread_work' in their own struct and use container_of() magic in the
cb.
Plus this is a natural fit if you want to dispatch callbacks instead
on a kthread_worker, which seems like it would simplify a few things
when it comes to deadlock avoidance.. ie., not resource deadlock
avoidance, but dispatching callbacks when some lock is held.
/me wonders what sort of fun would otherwise happen if the cb ever
indirectly called kds_resource_set_release()?
...
+/* Synchronous wait for a set of resources.


Function will return when one of these have happened:




all resources have been obtained






timeout lapsed while waiting






a signal was received while waiting









Caller considered to own/use the resources when the function returns.



Caller must release the resources using \a kds_rset_release.







Calling this function while holding already locked resources or other



locking primitives is dangerous.


One must if this is needed decide on a lock order of the resources



and/or the other locking primitives


and always take the resources/locking primitives in the specific order.







Use the ERR_PTR framework to decode the return value.



NULL = time out



If IS_ERR then PTR_ERR gives:



ERESTARTSYS = signal received, retry call after signal



all other values = internal error, lock failed



Other values  = successful wait, now the owner, must call



kds_resource_set_release

*/

+struct kds_resource_set * kds_waitall(

          int                    number_resources,


          unsigned long *        exclusive_access_bitmap,


          struct kds_resource ** resource_list,


          unsigned long          jifies_timeout);




+/* Release resources after use.


Caller must handle that other async callbacks will trigger,



so must avoid holding any locks a callback will take.







The function takes a pointer to your poiner to handle a race



between a cancelation and a completion.







If the caller can't guarantee that a race can't occur then



the passed in pointer must be the same in both call paths



to allow kds to manage the potential race.


*/

+void kds_resource_set_release(struct kds_resource_set ** pprset);
maybe using a worker as mentioned above for dealing w/ async cb's
simplify's things?  Maybe I'm a bit paranoid about locking around all
of this, but basically the synchronization framework ends up having to
deal w/ all the potential deadlock and recursive lock issues that we
tried to hide from w/ dmabuf ;-)
BR,
-R
...



+#endif /* _KDS_H_ */




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Re: [Linaro-mm-sig] [RFC] Synchronizing access to buffers shared with dma-buf between drivers/devices