Fence, timeline and android sync points

Daniel Vetter daniel at ffwll.ch
Wed Aug 13 01:28:22 PDT 2014

On Tue, Aug 12, 2014 at 06:13:41PM -0400, Jerome Glisse wrote:
> Hi,
> So i want over the whole fence and sync point stuff as it's becoming a pressing
> issue. I think we first need to agree on what is the problem we want to solve
> and what would be the requirements to solve it.
> Problem :
>   Explicit synchronization btw different hardware block over a buffer object.
> Requirements :
>   Share common infrastructure.
>   Allow optimal hardware command stream scheduling accross hardware block.
>   Allow android sync point to be implemented on top of it.
>   Handle/acknowledge exception (like good old gpu lockup).
>   Minimize driver changes.
> Glossary :
>   hardware timeline: timeline bound to a specific hardware block.
>   pipeline timeline: timeline bound to a userspace rendering pipeline, each
>                      point on that timeline can be a composite of several
>                      different hardware pipeline point.
>   pipeline: abstract object representing userspace application graphic pipeline
>             of each of the application graphic operations.
>   fence: specific point in a timeline where synchronization needs to happen.
> So now, current include/linux/fence.h implementation is i believe missing the
> objective by confusing hardware and pipeline timeline and by bolting fence to
> buffer object while what is really needed is true and proper timeline for both
> hardware and pipeline. But before going further down that road let me look at
> things and explain how i see them.

fences can be used free-standing and no one forces you to integrate them
with buffers. We actually plan to go this way with the intel svm stuff.
Ofc for dma-buf the plan is to synchronize using such fences, but that's
somewhat orthogonal I think. At least you only talk about fences and
timeline and not dma-buf here.
> Current ttm fence have one and a sole purpose, allow synchronization for buffer
> object move even thought some driver like radeon slightly abuse it and use them
> for things like lockup detection.
> The new fence want to expose an api that would allow some implementation of a
> timeline. For that it introduces callback and some hard requirement on what the
> driver have to expose :
>   enable_signaling
>   [signaled]
>   wait
> Each of those have to do work inside the driver to which the fence belongs and
> each of those can be call more or less from unexpected (with restriction like
> outside irq) context. So we end up with thing like :
>  Process 1              Process 2                   Process 3
>  I_A_schedule(fence0)
>                         CI_A_F_B_signaled(fence0)
>                                                     I_A_signal(fence0)
>                                                     CI_B_F_A_callback(fence0)
>                         CI_A_F_B_wait(fence0)
> Lexique:
> I_x  in driver x (I_A == in driver A)
> CI_x_F_y call in driver X from driver Y (CI_A_F_B call in driver A from driver B)
> So this is an happy mess everyone call everyone and this bound to get messy.
> Yes i know there is all kind of requirement on what happen once a fence is
> signaled. But those requirement only looks like they are trying to atone any
> mess that can happen from the whole callback dance.
> While i was too seduced by the whole callback idea long time ago, i think it is
> a highly dangerous path to take where the combinatorial of what could happen
> are bound to explode with the increase in the number of players.
> So now back to how to solve the problem we are trying to address. First i want
> to make an observation, almost all GPU that exist today have a command ring
> on to which userspace command buffer are executed and inside the command ring
> you can do something like :
>   if (condition) execute_command_buffer else skip_command_buffer
> where condition is a simple expression (memory_address cop value)) with cop one
> of the generic comparison (==, <, >, <=, >=). I think it is a safe assumption
> that any gpu that slightly matter can do that. Those who can not should fix
> there command ring processor.
> With that in mind, i think proper solution is implementing timeline and having
> fence be a timeline object with a way simpler api. For each hardware timeline
> driver provide a system memory address at which the lastest signaled fence
> sequence number can be read. Each fence object is uniquely associated with
> both a hardware and a pipeline timeline. Each pipeline timeline have a wait
> queue.
> When scheduling something that require synchronization on a hardware timeline
> a fence is created and associated with the pipeline timeline and hardware
> timeline. Other hardware block that need to wait on a fence can use there
> command ring conditional execution to directly check the fence sequence from
> the other hw block so you do optimistic scheduling. If optimistic scheduling
> fails (which would be reported by hw block specific solution and hidden) then
> things can fallback to software cpu wait inside what could be considered the
> kernel thread of the pipeline timeline.
> From api point of view there is no inter-driver call. All the driver needs to
> do is wakeup the pipeline timeline wait_queue when things are signaled or
> when things go sideway (gpu lockup).
> So how to implement that with current driver ? Well easy. Currently we assume
> implicit synchronization so all we need is an implicit pipeline timeline per
> userspace process (note this do not prevent inter process synchronization).
> Everytime a command buffer is submitted it is added to the implicit timeline
> with the simple fence object :
> struct fence {
>   struct list_head   list_hwtimeline;
>   struct list_head   list_pipetimeline;
>   struct hw_timeline *hw_timeline;
>   uint64_t           seq_num;
>   work_t             timedout_work;
>   void               *csdata;
> };
> So with set of helper function call by each of the driver command execution
> ioctl you have the implicit timeline that is properly populated and each
> dirver command execution get the dependency from the implicit timeline.
> Of course to take full advantages of all flexibilities this could offer we
> would need to allow userspace to create pipeline timeline and to schedule
> against the pipeline timeline of there choice. We could create file for
> each of the pipeline timeline and have file operation to wait/query
> progress.
> Note that the gpu lockup are considered exceptional event, the implicit
> timeline will probably want to continue on other job on other hardware
> block but the explicit one probably will want to decide wether to continue
> or abort or retry without the fault hw block.
> I realize i am late to the party and that i should have taken a serious
> look at all this long time ago. I apologize for that and if you consider
> this is to late then just ignore me modulo the big warning the crazyness
> that callback will introduce an how bad things bound to happen. I am not
> saying that bad things can not happen with what i propose just that
> because everything happen inside the process context that is the one
> asking/requiring synchronization there will be not interprocess kernel
> callback (a callback that was registered by one process and that is call
> inside another process time slice because fence signaling is happening
> inside this other process time slice).

So I read through it all and presuming I understand it correctly your
proposal and what we currently have is about the same. The big difference
is that you make a timeline a first-class object and move the callback
queue from the fence to the timeline, which requires callers to check the
fence/seqno/whatever themselves instead of pushing that responsibility to

If you actually mandate that the fence is just a seqno or similar which
can be read lockless then I could register my own special callback into
that waitqueue (other stuff than waking up threads is allowed) and from
hard-irq context check the seqno and readd my own callback if that's not
yet happened (that needs to go through some other context for hilarity).

So from that pov (presuming I didn't miss anything) your proposal is
identical to what we have, minor some different color choices (like where
to place the callback queue).

I guess that leaves the topic of how do you wait upon other fences, and
the current stuff allows you to do that all from process context like you
propose. And wrt the hardirq context callbacks I'm not worried at all
since lockdep will yell at any driver write who gets this wrong, so
debugging this stuff won't be tricky. And you can always reject any such
patches if they concern your own driver.

So the only substantial proposal I can distill here is to make timelines
first-class citizens, and we can always do that later on. We actually
discussed this, but didn't really see a point in having them around.

And yeah you're fairly late to the party, this stuff has been floating
around and been discussed since years ;-)

Cheers, Daniel

> Pseudo code for explicitness :
> drm_cs_ioctl_wrapper(struct drm_device *dev, void *data, struct file *filp)
> {
>    struct fence *dependency[16], *fence;
>    int m;
>    m = timeline_schedule(filp->implicit_pipeline, dev->hw_pipeline,
>                          dependency, 16, &fence);
>    if (m < 0)
>      return m;
>    if (m >= 16) {
>        // alloc m and recall;
>    }
>    dev->cs_ioctl(dev, data, filp, dev->implicit_pipeline, dependency, fence);
> }
> int timeline_schedule(ptimeline, hwtimeline, timeout,
>                        dependency, mdep, **fence)
> {
>    // allocate fence set hw_timeline and init work
>    // build up list of dependency by looking at list of pending fence in
>    // timeline
> }
> // If device driver schedule job hopping for all dependency to be signaled then
> // it must also call this function with csdata being a copy of what needs to be
> // executed once all dependency are signaled
> void timeline_missed_schedule(timeline, fence, void *csdata)
> {
>    INITWORK(fence->work, timeline_missed_schedule_worker)
>    fence->csdata = csdata;
>    schedule_delayed_work(fence->work, default_timeout)
> }
> void timeline_missed_schedule_worker(work)
> {
>    driver = driver_from_fence_hwtimeline(fence)
>    // Make sure that each of the hwtimeline dependency will fire irq by
>    // calling a driver function.
>    timeline_wait_for_fence_dependency(fence);
>    driver->execute_cs(driver, fence);
> }
> // This function is call by driver code that signal fence (could be call from
> // interrupt context). It is responsabilities of device driver to call that
> // function.
> void timeline_signal(hwtimeline)
> {
>   for_each_fence(fence, hwtimeline->fences, list_hwtimeline) {
>     wakeup(fence->pipetimeline->wait_queue);
>   }
> }
> Cheers,
> Jérôme

Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

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