On Sat, Aug 11, 2012 at 06:00:46PM +0200, Maarten Lankhorst wrote:
Op 11-08-12 17:14, Rob Clark schreef:
On Fri, Aug 10, 2012 at 3:29 PM, Daniel Vetter daniel@ffwll.ch wrote:
+/**
- dma_fence_signal - signal completion of a fence
- @fence: the fence to signal
- All registered callbacks will be called directly (synchronously) and
- all blocked waters will be awoken. This should be always be called on
- software only fences, or alternatively be called after
- dma_fence_ops::enable_signaling is called.
I think we need to be cleare here when dma_fence_signal can be called:
- for a sw-only fence (i.e. created with dma_fence_create) dma_fence_signal _must_ be called under all circumstances.
- for any other fences, dma_fence_signal may be called, but it _must_ be called once the ->enable_signalling func has been called and return 0 (i.e. success).
- it may be called only _once_.
As we discussed on irc it might be beneficial to be able to have it called twice, the second time would be a noop, however.
Agreed.
[snip]
/* At this point, if enable_signaling returns any error* a wakeup has to be performanced regardless.* -ENOENT signals fence was already signaled. Any other error* inidicates a catastrophic hardware error.** If any hardware error occurs, nothing can be done against* it, so it's treated like the fence was already signaled.* No synchronization can be performed, so we have to assume* the fence was already signaled.*/ret = fence->ops->enable_signaling(fence);if (ret) {signaled = true;dma_fence_signal(fence);I think we should call dma_fence_signal only for -ENOENT and pass all other errors back as-is. E.g. on -ENOMEM or so we might want to retry ...
Also discussed on irc, boolean might be a better solution until we start dealing with hardware on fire. :) This would however likely be dealt in the same way as signaling, however.
Agreed.
[snip]
if (!ret) {cb->base.flags = 0;cb->base.func = __dma_fence_wake_func;cb->base.private = priv;cb->fence = fence;cb->func = func;__add_wait_queue(&fence->event_queue, &cb->base);}spin_unlock_irqrestore(&fence->event_queue.lock, flags);return ret;+} +EXPORT_SYMBOL_GPL(dma_fence_add_callback);
I think for api completenes we should also have a dma_fence_remove_callback function.
We did originally but Maarten found it was difficult to deal with properly when the gpu's hang. I think his alternative was just to require the hung driver to signal the fence. I had kicked around the idea of a dma_fence_cancel() alternative to signal that could pass an error thru to the waiting driver.. although not sure if the other driver could really do anything differently at that point.
No, there is a very real reason I removed dma_fence_remove_callback. It is absolutely non-trivial to cancel it once added, since you have to deal with all kinds of race conditions.. See i915_gem_reset_requests in my git tree: http://cgit.freedesktop.org/~mlankhorst/linux/commit/?id=673c4b2550bc63ec134...
I don't see the point in that code ... Why can't we drop the kref _outside_ of the critical section protected by event_queue_lock? Then you pretty much have an open-coded version of dma_fence_callback_cancel in there.
This is the only way to do it completely deadlock/memory corruption free since you essentially have a locking inversion to avoid. I had it wrong the first 2 times too, even when I knew about a lot of the locking complications. If you want to do it, in most cases it will likely be easier to just eat the signal and ignore it instead of canceling.
+/**
- dma_fence_wait - wait for a fence to be signaled
- @fence: [in] The fence to wait on
- @intr: [in] if true, do an interruptible wait
- @timeout: [in] absolute time for timeout, in jiffies.
I don't quite like this, I think we should keep the styl of all other wait_*_timeout functions and pass the arg as timeout in jiffies (and also the same return semantics). Otherwise well have funny code that needs to handle return values differently depending upon whether it waits upon a dma_fence or a native object (where it would us the wait_*_timeout functions directly).
We did start out this way, but there was an ugly jiffies roll-over problem that was difficult to deal with properly. Using an absolute time avoided the problem.
Yeah, this makes it easier to wait on multiple fences, instead of resetting the timeout over and over and over again, or manually recalculating.
I don't see how updating the jiffies_left timeout is that onerous, and in any case we can easily wrap that up into a little helper function, passing in an array of dma_fence pointers.
Creating interfaces that differ from established kernel api patterns otoh isn't good imo. I.e. I want dma_fence_wait_bla to be a drop-in replacement for the corresponding wait_event_bla function/macro, which the same semantics for the timeout and return values.
Differing in such things only leads to confusion when reading patches imo.
Also, I think we should add the non-_timeout variants, too, just for completeness.
Would it be ok if timeout == 0 is special, then?
See above ;-)
[snip]
+struct dma_fence_ops {
int (*enable_signaling)(struct dma_fence *fence);I think we should mandate that enable_signalling can be called from atomic context, but not irq context (since I don't see a use-case for calling this from irq context).
What would not having this called from irq context get you? I do agree that you would be crazy to do so, but not sure why we should restrict it.
If we allow ->enable_signalling to be called from irq context, all spinlocks the driver grabs need to be irq safe. If we disallow this I guess some drivers could easily get by with plain spinlocks.
And since we both agree that it would be crazy to call ->enable_signalling from irq context, I think we should bake this constrain into the interface.
Cheers, Daniel