[PATCH RESEND 1/2] dma-fence: allow signaling drivers to set fence timestamp

[Daniel Vetter]

On Wed, Nov 18, 2020 at 9:28 PM <veeras at codeaurora.org> wrote:
>
> On 2020-11-12 10:27, Veera Sundaram Sankaran wrote:
> > Some drivers have hardware capability to get the precise timestamp of
> > certain events based on which the fences are triggered. This allows it
> > to set accurate timestamp factoring out any software and IRQ latencies.
> > Move the timestamp parameter out of union in dma_fence struct to allow
> > signaling drivers to set it. If the parameter is not set, ktime_get is
> > used to set the current time to fence timestamp during
> > dma_fence_signal.
> >
>
> @Sumit Semwal / @Gustavo Padovan,
> Can you please help in reviewing this change as it falls in dma-fence
> files.

Please answer my question on the 2nd patch. This is new uabi
behaviour, we have a bunch of requirements for this kind of work:

https://dri.freedesktop.org/docs/drm/gpu/drm-uapi.html#open-source-userspace-requirements

Thanks, Daniel

> Thanks,
> Veera
>
> > Signed-off-by: Veera Sundaram Sankaran <veeras at codeaurora.org>
> > ---
> >  drivers/dma-buf/dma-fence.c | 18 ++++++++++--------
> >  include/linux/dma-fence.h   | 15 +++------------
> >  2 files changed, 13 insertions(+), 20 deletions(-)
> >
> > diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> > index 43624b4..7cef49a 100644
> > --- a/drivers/dma-buf/dma-fence.c
> > +++ b/drivers/dma-buf/dma-fence.c
> > @@ -4,6 +4,7 @@
> >   *
> >   * Copyright (C) 2012 Canonical Ltd
> >   * Copyright (C) 2012 Texas Instruments
> > + * Copyright (c) 2020 The Linux Foundation. All rights reserved.
> >   *
> >   * Authors:
> >   * Rob Clark <robdclark at gmail.com>
> > @@ -329,7 +330,6 @@ void __dma_fence_might_wait(void)
> >  int dma_fence_signal_locked(struct dma_fence *fence)
> >  {
> >       struct dma_fence_cb *cur, *tmp;
> > -     struct list_head cb_list;
> >
> >       lockdep_assert_held(fence->lock);
> >
> > @@ -337,16 +337,18 @@ int dma_fence_signal_locked(struct dma_fence
> > *fence)
> >                                     &fence->flags)))
> >               return -EINVAL;
> >
> > -     /* Stash the cb_list before replacing it with the timestamp */
> > -     list_replace(&fence->cb_list, &cb_list);
> > -
> > -     fence->timestamp = ktime_get();
> > +     /* set current time, if not set by signaling driver */
> > +     if (!fence->timestamp)
> > +             fence->timestamp = ktime_get();
> >       set_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags);
> >       trace_dma_fence_signaled(fence);
> >
> > -     list_for_each_entry_safe(cur, tmp, &cb_list, node) {
> > -             INIT_LIST_HEAD(&cur->node);
> > -             cur->func(fence, cur);
> > +     if (!list_empty(&fence->cb_list)) {
> > +             list_for_each_entry_safe(cur, tmp, &fence->cb_list, node) {
> > +                     INIT_LIST_HEAD(&cur->node);
> > +                     cur->func(fence, cur);
> > +             }
> > +             INIT_LIST_HEAD(&fence->cb_list);
> >       }
> >
> >       return 0;
> > diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> > index 09e23ad..a9eebaf 100644
> > --- a/include/linux/dma-fence.h
> > +++ b/include/linux/dma-fence.h
> > @@ -4,6 +4,7 @@
> >   *
> >   * Copyright (C) 2012 Canonical Ltd
> >   * Copyright (C) 2012 Texas Instruments
> > + * Copyright (c) 2020 The Linux Foundation. All rights reserved.
> >   *
> >   * Authors:
> >   * Rob Clark <robdclark at gmail.com>
> > @@ -70,26 +71,16 @@ struct dma_fence {
> >        * release the fence it is unused. No one should be adding to the
> >        * cb_list that they don't themselves hold a reference for.
> >        *
> > -      * The lifetime of the timestamp is similarly tied to both the
> > -      * rcu freelist and the cb_list. The timestamp is only set upon
> > -      * signaling while simultaneously notifying the cb_list. Ergo, we
> > -      * only use either the cb_list of timestamp. Upon destruction,
> > -      * neither are accessible, and so we can use the rcu. This means
> > -      * that the cb_list is *only* valid until the signal bit is set,
> > -      * and to read either you *must* hold a reference to the fence,
> > -      * and not just the rcu_read_lock.
> > -      *
> >        * Listed in chronological order.
> >        */
> >       union {
> >               struct list_head cb_list;
> > -             /* @cb_list replaced by @timestamp on dma_fence_signal() */
> > -             ktime_t timestamp;
> > -             /* @timestamp replaced by @rcu on dma_fence_release() */
> > +             /* @cb_list replaced by @rcu on dma_fence_release() */
> >               struct rcu_head rcu;
> >       };
> >       u64 context;
> >       u64 seqno;
> > +     ktime_t timestamp;
> >       unsigned long flags;
> >       struct kref refcount;
> >       int error;



-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch