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

[Christian König]

Am 17.02.2014 19:24, schrieb Rob Clark:
> On Mon, Feb 17, 2014 at 12:36 PM, Christian König
> <deathsimple@xxxxxxxxxxx> wrote:
> > Am 17.02.2014 18:27, schrieb Rob Clark:
> >
> > > On Mon, Feb 17, 2014 at 11:56 AM, Christian König
> > > <deathsimple@xxxxxxxxxxx> 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.
> hmm, ok.. forgot you have a few extra rings compared to me.  Is UVD
> re-ordering from decode-order to display-order for you in hw?  If not,
> I guess you need sw intervention anyways when a frame is done for
> frame re-ordering, so maybe hw->hw sync doesn't really matter as much
> as compared to gpu/3d->display.  For dma<->3d interactions, seems like
> you would care more about hw<->hw sync, but I guess you aren't likely
> to use GPU A to do a resolve blit for GPU B..

No UVD isn't reordering, but since frame reordering is predictable you 
usually end up with pipelining everything to the hardware. E.g. you send 
the decode commands in decode order to the UVD block and if you have 
overlay active one of the frames are going to be the first to display 
and then you want to wait for it on the display side.

> For 3D ring, I assume you probably want a CP_WAIT_FOR_IDLE before a
> CP_MEM_WRITE to update fence value in memory (for the one signalling
> the fence).  But why would you need that before a CP_WAIT_REG_MEM (for
> the one waiting for the fence)?  I don't exactly have documentation
> for adreno version of CP_WAIT_REG_{MEM,EQ,GTE}..  but PFP and ME
> appear to be same instruction set as r600, so I'm pretty sure they
> should have similar capabilities.. CP_WAIT_REG_MEM appears to be same
> but with 32bit gpu addresses vs 64b.

You shouldn't use any of the CP commands for engine synchronization 
(neither for wait nor for signal). The PFP and ME are just the top of a 
quite deep pipeline and when you use any of the CP_WAIT functions you 
block them for something and that's draining the pipeline.

With the semaphore and fence commands the values are just attached as 
prerequisite to the draw command, e.g. the CP setups the draw 
environment and issues the command, but the actual execution of it is 
delayed until the "!= 0" condition hits. And in the meantime the CP 
already prepares the next draw operation.

But at least for compute queues wait semaphore aren't the perfect 
solution either. What you need then is a GPU scheduler that uses a 
kernel task for setting up the command submission for you when all 
prerequisites are meet.

Christian.

> BR,
> -R
>
> > 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@xxxxxxxxxxxxx>
> > > > > ---
> > > > >     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@xxxxxxxxx>
> > > > > + *   Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxx>
> > > > > + *
> > > > > + * 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@xxxxxxxxxxxxxxxxxxxxx
> > > > > http://lists.freedesktop.org/mailman/listinfo/dri-devel
> > > >
> > > > _______________________________________________
> > > > dri-devel mailing list
> > > > dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > > > http://lists.freedesktop.org/mailman/listinfo/dri-devel

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