On Thu, Dec 13, 2018 at 5:47 PM Koenig, Christian
<Christian.Koenig@xxxxxxx> wrote:
>
> Am 13.12.18 um 17:01 schrieb Daniel Vetter:
> > On Thu, Dec 13, 2018 at 12:24:57PM +0000, Koenig, Christian wrote:
> >> Am 13.12.18 um 13:21 schrieb Chris Wilson:
> >>> Quoting Koenig, Christian (2018-12-13 12:11:10)
> >>>> Am 13.12.18 um 12:37 schrieb Chris Wilson:
> >>>>> Quoting Chunming Zhou (2018-12-11 10:34:45)
> >>>>>> From: Christian König <ckoenig.leichtzumerken@xxxxxxxxx>
> >>>>>>
> >>>>>> Implement finding the right timeline point in drm_syncobj_find_fence.
> >>>>>>
> >>>>>> v2: return -EINVAL when the point is not submitted yet.
> >>>>>> v3: fix reference counting bug, add flags handling as well
> >>>>>>
> >>>>>> Signed-off-by: Christian König <christian.koenig@xxxxxxx>
> >>>>>> ---
> >>>>>> drivers/gpu/drm/drm_syncobj.c | 43 ++++++++++++++++++++++++++++++++---
> >>>>>> 1 file changed, 40 insertions(+), 3 deletions(-)
> >>>>>>
> >>>>>> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> >>>>>> index 76ce13dafc4d..d964b348ecba 100644
> >>>>>> --- a/drivers/gpu/drm/drm_syncobj.c
> >>>>>> +++ b/drivers/gpu/drm/drm_syncobj.c
> >>>>>> @@ -231,16 +231,53 @@ int drm_syncobj_find_fence(struct drm_file *file_private,
> >>>>>> struct dma_fence **fence)
> >>>>>> {
> >>>>>> struct drm_syncobj *syncobj = drm_syncobj_find(file_private, handle);
> >>>>>> - int ret = 0;
> >>>>>> + struct syncobj_wait_entry wait;
> >>>>>> + int ret;
> >>>>>>
> >>>>>> if (!syncobj)
> >>>>>> return -ENOENT;
> >>>>>>
> >>>>>> *fence = drm_syncobj_fence_get(syncobj);
> >>>>>> - if (!*fence) {
> >>>>>> + drm_syncobj_put(syncobj);
> >>>>>> +
> >>>>>> + if (*fence) {
> >>>>>> + ret = dma_fence_chain_find_seqno(fence, point);
> >>>>>> + if (!ret)
> >>>>>> + return 0;
> >>>>>> + dma_fence_put(*fence);
> >>>>>> + } else {
> >>>>>> ret = -EINVAL;
> >>>>>> }
> >>>>>> - drm_syncobj_put(syncobj);
> >>>>>> +
> >>>>>> + if (!(flags & DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT))
> >>>>>> + return ret;
> >>>>>> +
> >>>>>> + memset(&wait, 0, sizeof(wait));
> >>>>>> + wait.task = current;
> >>>>>> + wait.point = point;
> >>>>>> + drm_syncobj_fence_add_wait(syncobj, &wait);
> >>>>>> +
> >>>>>> + do {
> >>>>>> + set_current_state(TASK_INTERRUPTIBLE);
> >>>>>> + if (wait.fence) {
> >>>>>> + ret = 0;
> >>>>>> + break;
> >>>>>> + }
> >>>>>> +
> >>>>>> + if (signal_pending(current)) {
> >>>>>> + ret = -ERESTARTSYS;
> >>>>>> + break;
> >>>>>> + }
> >>>>>> +
> >>>>>> + schedule();
> >>>>>> + } while (1);
> >>>>> I've previously used a dma_fence_proxy so that we could do nonblocking
> >>>>> waits on future submits. That would be preferrable (a requirement for
> >>>>> our stupid BKL-driven code).
> >>>> That is exactly what I would definitely NAK.
> >>>>
> >>>> I would rather say we should come up with a wait_multiple_events() macro
> >>>> and completely nuke the custom implementation of this in:
> >>>> 1. dma_fence_default_wait and dma_fence_wait_any_timeout
> >>>> 2. the radeon fence implementation
> >>>> 3. the nouveau fence implementation
> >>>> 4. the syncobj code
> >>>>
> >>>> Cause all of them do exactly the same. The dma_fence implementation
> >>>> unfortunately came up with a custom event handling mechanism instead of
> >>>> extending the core Linux wait_event() system.
> >>> I don't want a blocking wait at all.
> >> Ok I wasn't clear enough :) That is exactly what I would NAK!
> >>
> >> The wait must be blocking or otherwise you would allow wait-before-signal.
> > Well the current implementation is pulling a rather big trick on readers
> > in this regard: It looks like a dma_fence, it's implemented as one even,
> > heck you even open-code a dma_fence_wait here.
> >
> > Except the semantics are completely different.
> >
> > So aside from the discussion whether we really want to fully chain them I
> > think it just doesn't make sense to implement the "wait for fence submit"
> > as a dma_fence wait. And I'd outright remove that part from the uapi, and
> > force the wait. The current radv/anv plans I discussed with Jason was that
> > we'd have a separate submit thread, and hence unconditionally blocking
> > until the fence has materialized is the right thing to do. Even allowing
> > that option, either through a flag, or making these things look like
> > dma_fences (they are _not_) just tricks folks into misunderstanding what's
> > going on.
>
> Good, that sounds strongly like something I can agree on as well.
>
> > Code sharing just because the code looks similar is imo a really
> > bad idea, when the semantics are entirely different (that was also the
> > reason behind not reusing all the cpu event stuff for dma_fence, they're
> > not normal cpu events).
>
> Ok, the last sentence is what I don't understand.
>
> What exactly is the semantic difference between the dma_fence_wait and
> the wait_event interface?
>
> I mean the wait_event interface was introduced to prevent drivers from
> openly coding an event interface and getting it wrong all the time.
>
> So a good part of the bugs we have seen around waiting for dma-fences
> are exactly why wait_event was invented in the first place.
>
> The only big thing I can see missing in the wait_event interface is
> waiting for many events at the same time, but that should be a rather
> easy addition.
So this bikeshed was years ago, maybe I should type a patch to
document it, but as far as I remember the big difference is:
- wait_event and friends generally Just Work. It can go wrong of
course, but the usual pattern is that the waker-side does and
uncoditional wake_up_all, and hence all the waiter needs to do is add
themselves to the waiter list.
- dma_buf otoh is entirely different: We wanted to support all kinds
fo signalling modes, including having interrupts disabled by default
(not sure whether we actually achieve this still with all the cpu-side
scheduling the big drivers do). Which means the waker does not
unconditionally call wake_up_all, at least not timeline, and waiters
need to call dma_fence_enable_signalling before they can add
themselves to the waiter list and call schedule().
The other bit difference is how you check for the classic wakeup races
where the event happens between when you checked for it and when you
go to sleep. Because hw is involved, the rules are again a bit
different, and their different between drivers because hw is
incoherent/broken in all kinds of ways. So there's also really tricky
things going on between adding the waiter to the waiter list and
dma_fence_enable_signalling. For pure cpu events you can ignore this
and bake the few necessary barriers into the various macros, dma_fence
needs more.
Adding Maarten, maybe there was more. I definitely remember huge&very
long discussions about all this.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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