[PATCH 2/6] seqno-fence: Hardware dma-buf implementation of fencing (v4)

[Rob Clark]

On Mon, Feb 17, 2014 at 10:56 AM, Maarten Lankhorst
<maarten.lankhorst at canonical.com> wrote:
> This type of fence can be used with hardware synchronization for simple
> hardware that can block execution until the condition
> (dma_buf[offset] - value) >= 0 has been met.
>
> A software fallback still has to be provided in case the fence is used
> with a device that doesn't support this mechanism. It is useful to expose
> this for graphics cards that have an op to support this.
>
> Some cards like i915 can export those, but don't have an option to wait,
> so they need the software fallback.
>
> I extended the original patch by Rob Clark.
>
> v1: Original
> v2: Renamed from bikeshed to seqno, moved into dma-fence.c since
>     not much was left of the file. Lots of documentation added.
> v3: Use fence_ops instead of custom callbacks. Moved to own file
>     to avoid circular dependency between dma-buf.h and fence.h
> v4: Add spinlock pointer to seqno_fence_init
>
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst at canonical.com>


Reviewed-by: Rob Clark <robdclark at gmail.com>


> ---
>  Documentation/DocBook/device-drivers.tmpl |    1
>  drivers/base/fence.c                      |   50 +++++++++++++
>  include/linux/seqno-fence.h               |  109 +++++++++++++++++++++++++++++
>  3 files changed, 160 insertions(+)
>  create mode 100644 include/linux/seqno-fence.h
>
> diff --git a/Documentation/DocBook/device-drivers.tmpl b/Documentation/DocBook/device-drivers.tmpl
> index 7a0c9ddb4818..8c85c20942c2 100644
> --- a/Documentation/DocBook/device-drivers.tmpl
> +++ b/Documentation/DocBook/device-drivers.tmpl
> @@ -131,6 +131,7 @@ X!Edrivers/base/interface.c
>  !Edrivers/base/dma-buf.c
>  !Edrivers/base/fence.c
>  !Iinclude/linux/fence.h
> +!Iinclude/linux/seqno-fence.h
>  !Edrivers/base/reservation.c
>  !Iinclude/linux/reservation.h
>  !Edrivers/base/dma-coherent.c
> diff --git a/drivers/base/fence.c b/drivers/base/fence.c
> index 12df2bf62034..cd0937127a89 100644
> --- a/drivers/base/fence.c
> +++ b/drivers/base/fence.c
> @@ -25,6 +25,7 @@
>  #include <linux/export.h>
>  #include <linux/atomic.h>
>  #include <linux/fence.h>
> +#include <linux/seqno-fence.h>
>
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/fence.h>
> @@ -413,3 +414,52 @@ __fence_init(struct fence *fence, const struct fence_ops *ops,
>         trace_fence_init(fence);
>  }
>  EXPORT_SYMBOL(__fence_init);
> +
> +static const char *seqno_fence_get_driver_name(struct fence *fence) {
> +       struct seqno_fence *seqno_fence = to_seqno_fence(fence);
> +       return seqno_fence->ops->get_driver_name(fence);
> +}
> +
> +static const char *seqno_fence_get_timeline_name(struct fence *fence) {
> +       struct seqno_fence *seqno_fence = to_seqno_fence(fence);
> +       return seqno_fence->ops->get_timeline_name(fence);
> +}
> +
> +static bool seqno_enable_signaling(struct fence *fence)
> +{
> +       struct seqno_fence *seqno_fence = to_seqno_fence(fence);
> +       return seqno_fence->ops->enable_signaling(fence);
> +}
> +
> +static bool seqno_signaled(struct fence *fence)
> +{
> +       struct seqno_fence *seqno_fence = to_seqno_fence(fence);
> +       return seqno_fence->ops->signaled && seqno_fence->ops->signaled(fence);
> +}
> +
> +static void seqno_release(struct fence *fence)
> +{
> +       struct seqno_fence *f = to_seqno_fence(fence);
> +
> +       dma_buf_put(f->sync_buf);
> +       if (f->ops->release)
> +               f->ops->release(fence);
> +       else
> +               kfree(f);
> +}
> +
> +static long seqno_wait(struct fence *fence, bool intr, signed long timeout)
> +{
> +       struct seqno_fence *f = to_seqno_fence(fence);
> +       return f->ops->wait(fence, intr, timeout);
> +}
> +
> +const struct fence_ops seqno_fence_ops = {
> +       .get_driver_name = seqno_fence_get_driver_name,
> +       .get_timeline_name = seqno_fence_get_timeline_name,
> +       .enable_signaling = seqno_enable_signaling,
> +       .signaled = seqno_signaled,
> +       .wait = seqno_wait,
> +       .release = seqno_release,
> +};
> +EXPORT_SYMBOL(seqno_fence_ops);
> diff --git a/include/linux/seqno-fence.h b/include/linux/seqno-fence.h
> new file mode 100644
> index 000000000000..952f7909128c
> --- /dev/null
> +++ b/include/linux/seqno-fence.h
> @@ -0,0 +1,109 @@
> +/*
> + * seqno-fence, using a dma-buf to synchronize fencing
> + *
> + * Copyright (C) 2012 Texas Instruments
> + * Copyright (C) 2012 Canonical Ltd
> + * Authors:
> + * Rob Clark <robdclark at gmail.com>
> + *   Maarten Lankhorst <maarten.lankhorst at canonical.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.
> + *
> + * You should have received a copy of the GNU General Public License along with
> + * this program.  If not, see <http://www.gnu.org/licenses/>.
> + */
> +
> +#ifndef __LINUX_SEQNO_FENCE_H
> +#define __LINUX_SEQNO_FENCE_H
> +
> +#include <linux/fence.h>
> +#include <linux/dma-buf.h>
> +
> +struct seqno_fence {
> +       struct fence base;
> +
> +       const struct fence_ops *ops;
> +       struct dma_buf *sync_buf;
> +       uint32_t seqno_ofs;
> +};
> +
> +extern const struct fence_ops seqno_fence_ops;
> +
> +/**
> + * to_seqno_fence - cast a fence to a seqno_fence
> + * @fence: fence to cast to a seqno_fence
> + *
> + * Returns NULL if the fence is not a seqno_fence,
> + * or the seqno_fence otherwise.
> + */
> +static inline struct seqno_fence *
> +to_seqno_fence(struct fence *fence)
> +{
> +       if (fence->ops != &seqno_fence_ops)
> +               return NULL;
> +       return container_of(fence, struct seqno_fence, base);
> +}
> +
> +/**
> + * seqno_fence_init - initialize a seqno fence
> + * @fence: seqno_fence to initialize
> + * @lock: pointer to spinlock to use for fence
> + * @sync_buf: buffer containing the memory location to signal on
> + * @context: the execution context this fence is a part of
> + * @seqno_ofs: the offset within @sync_buf
> + * @seqno: the sequence # to signal on
> + * @ops: the fence_ops for operations on this seqno fence
> + *
> + * This function initializes a struct seqno_fence with passed parameters,
> + * and takes a reference on sync_buf which is released on fence destruction.
> + *
> + * A seqno_fence is a dma_fence which can complete in software when
> + * enable_signaling is called, but it also completes when
> + * (s32)((sync_buf)[seqno_ofs] - seqno) >= 0 is true
> + *
> + * The seqno_fence will take a refcount on the sync_buf until it's
> + * destroyed, but actual lifetime of sync_buf may be longer if one of the
> + * callers take a reference to it.
> + *
> + * Certain hardware have instructions to insert this type of wait condition
> + * in the command stream, so no intervention from software would be needed.
> + * This type of fence can be destroyed before completed, however a reference
> + * on the sync_buf dma-buf can be taken. It is encouraged to re-use the same
> + * dma-buf for sync_buf, since mapping or unmapping the sync_buf to the
> + * device's vm can be expensive.
> + *
> + * It is recommended for creators of seqno_fence to call fence_signal
> + * before destruction. This will prevent possible issues from wraparound at
> + * time of issue vs time of check, since users can check fence_is_signaled
> + * before submitting instructions for the hardware to wait on the fence.
> + * However, when ops.enable_signaling is not called, it doesn't have to be
> + * done as soon as possible, just before there's any real danger of seqno
> + * wraparound.
> + */
> +static inline void
> +seqno_fence_init(struct seqno_fence *fence, spinlock_t *lock,
> +                struct dma_buf *sync_buf,  uint32_t context, uint32_t seqno_ofs,
> +                uint32_t seqno, const struct fence_ops *ops)
> +{
> +       BUG_ON(!fence || !sync_buf || !ops);
> +       BUG_ON(!ops->wait || !ops->enable_signaling || !ops->get_driver_name || !ops->get_timeline_name);
> +
> +       /*
> +        * ops is used in __fence_init for get_driver_name, so needs to be
> +        * initialized first
> +        */
> +       fence->ops = ops;
> +       __fence_init(&fence->base, &seqno_fence_ops, lock, context, seqno);
> +       get_dma_buf(sync_buf);
> +       fence->sync_buf = sync_buf;
> +       fence->seqno_ofs = seqno_ofs;
> +}
> +
> +#endif /* __LINUX_SEQNO_FENCE_H */
>