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

[veeras@xxxxxxxxxxxxxx]

On 2020-11-19 03:58, Daniel Vetter wrote:
> On Thu, Nov 12, 2020 at 7:27 PM Veera Sundaram Sankaran
> <veeras@xxxxxxxxxxxxxx> 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.
> >
> > Signed-off-by: Veera Sundaram Sankaran <veeras@xxxxxxxxxxxxxx>
>
> So with they "why?" question fully resolved, I think this is a bit too
> much a hack. I think much better if we pass the timestamp explicitly,
> in a new dma_fence_signal_timestamp variant. That means a bit more
> work, but I think it will handle this special case cleaner.
>
> Also means we need to wire the timestamp through the entire call stack
> on the drm side too. So we need a drm_send_event_locked_timestamp
> variant too for send_vblank_event.
> -Daniel

@Sumit Semwal, @Gustavo Padovan, @Steven Rostedt
Can you please help in getting review for the v2 patches.
I have addressed the earlier comments from Daniel Vetter.
https://patchwork.kernel.org/project/dri-devel/list/?series=388881&archive=both
Thanks,
Veera

> > ---
> >  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@xxxxxxxxx>
> > @@ -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@xxxxxxxxx>
> > @@ -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;
> > --
> > 2.7.4
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