[Linaro-mm-sig] [RFC] Synchronizing access to buffers shared with dma-buf between drivers/devices
John Reitan
john.reitan at arm.com
Wed Jun 6 06:33:13 PDT 2012
Hi All,
I'm the original designer of the KDS system that Tom posted
while I was on paternity leave. Find my responses inline...
> -----Original Message-----
> From: linaro-mm-sig-bounces at lists.linaro.org [mailto:linaro-mm-sig-
> bounces at lists.linaro.org] On Behalf Of Rob Clark
> Sent: Monday, June 04, 2012 10:31 PM
> To: Tom Cooksey
> Cc: linaro-mm-sig at lists.linaro.org; Pauli; dri-
> devel at lists.freedesktop.org
> Subject: Re: [Linaro-mm-sig] [RFC] Synchronizing access to buffers
> shared with dma-buf between drivers/devices
>
> 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).
It was the API we really wanted input on.
The implementation still leaves a bit to be desired.
> 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 at 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 at 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..
Better to use proper ref-counting, agree.
Then the term is just a simple deref.
Then we might want to add a deref_and_wait version to
know that the callback has completed?
> > +
> > +/* 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)?
Well, you must have one extra arg with your solution:
Two pointers plus an int for the current version
Two pointers plus two ints for your suggestion.
Other than that there is no real reason to not change the API really...
>
> > + 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?
Guess so :)
This was done as callbacks needed two different things to operate on,
and instead of mallocing a struct to track the two objects we used the
existing alloc holding the original ptr.
But continuing with your next comment:
> 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.
That sounds like a better approach.
Will make a cleaner API, will look into it.
> /me wonders what sort of fun would otherwise happen if the cb ever
> indirectly called kds_resource_set_release()?
It should work.
The callback is performed without any kds side locks held.
When a callback returns the resource set object should not be
accessed again, so it's safe for it to have been freed by
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 ;-)
With the kthread change the lock problems will be reduced.
But the real point of this framework is to remove lock order problems
when you need to lock down two or more more resources, but you're
not in control of all the code operating on the buffers, which is
typical for many SoC projects.
But yes, this system must be vetted for all potential deadlocks
that it could introduce.
Cheers,
John
>
>
> BR,
> -R
>
>
> > +
> > +#endif /* _KDS_H_ */
> > +
> >
> >
> >
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/dri-devel
>
> _______________________________________________
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