Message-ID: <530248B1.2090405@vodafone.de>
Date:	Mon, 17 Feb 2014 18:36:49 +0100
From:	Christian König <deathsimple@...afone.de>
To:	Rob Clark <robdclark@...il.com>
CC:	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org,
	"linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
	Colin Cross <ccross@...gle.com>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>
Subject: Re: [PATCH 2/6] seqno-fence: Hardware dma-buf implementation of fencing
 (v4)

Am 17.02.2014 18:27, schrieb Rob Clark:
> On Mon, Feb 17, 2014 at 11:56 AM, Christian König
> <deathsimple@...afone.de> wrote:
>> Am 17.02.2014 16:56, schrieb Maarten Lankhorst:
>>
>>> 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.
>>
>> Can't we make that just "dma_buf[offset] != 0" instead? As far as I know
>> this way it would match the definition M$ uses in their WDDM specification
>> and so make it much more likely that hardware supports it.
> well 'buf[offset] >= value' at least means the same slot can be used
> for multiple operations (with increasing values of 'value').. not sure
> if that is something people care about.
>
>> =value seems to be possible with adreno and radeon.  I'm not really sure about others (although I presume it as least supported for nv desktop stuff).  For hw that cannot do >=value, we can either have a different fence implementation which uses the !=0 approach.  Or change seqno-fence implementation later if needed.  But if someone has hw that can do !=0 but not >=value, speak up now ;-)

Here! Radeon can only do >=value on the DMA and 3D engine, but not with 
UVD or VCE. And for the 3D engine it means draining the pipe, which 
isn't really a good idea.

Christian.

>
>> Apart from that I still don't like the idea of leaking a drivers IRQ context
>> outside of the driver, but without a proper GPU scheduler there probably
>> isn't much alternative.
> I guess it will be not uncommon scenario for gpu device to just need
> to kick display device to write a few registers for a page flip..
> probably best not to schedule a worker just for this (unless the
> signalled device otherwise needs to).  I think it is better in this
> case to give the signalee some rope to hang themselves, and make it
> the responsibility of the callback to kick things off to a worker if
> needed.
>
> BR,
> -R
>
>> Christian.
>>
>>> 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@...onical.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@...il.com>
>>> + *   Maarten Lankhorst <maarten.lankhorst@...onical.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 */
>>>
>>> _______________________________________________
>>> dri-devel mailing list
>>> dri-devel@...ts.freedesktop.org
>>> http://lists.freedesktop.org/mailman/listinfo/dri-devel
>>
>> _______________________________________________
>> dri-devel mailing list
>> dri-devel@...ts.freedesktop.org
>> http://lists.freedesktop.org/mailman/listinfo/dri-devel

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