Re: [PATCH v4 01/14] dma-buf/dma-fence: Add deadline awareness
From: Tvrtko Ursulin
Date: Wed Feb 22 2023 - 12:05:27 EST
On 22/02/2023 15:28, Christian König wrote:
Am 22.02.23 um 11:23 schrieb Tvrtko Ursulin:
On 18/02/2023 21:15, Rob Clark wrote:
From: Rob Clark <robdclark@xxxxxxxxxxxx>
Add a way to hint to the fence signaler of an upcoming deadline, such as
vblank, which the fence waiter would prefer not to miss. This is to aid
the fence signaler in making power management decisions, like boosting
frequency as the deadline approaches and awareness of missing deadlines
so that can be factored in to the frequency scaling.
v2: Drop dma_fence::deadline and related logic to filter duplicate
deadlines, to avoid increasing dma_fence size. The fence-context
implementation will need similar logic to track deadlines of all
the fences on the same timeline. [ckoenig]
v3: Clarify locking wrt. set_deadline callback
Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
Reviewed-by: Christian König <christian.koenig@xxxxxxx>
---
drivers/dma-buf/dma-fence.c | 20 ++++++++++++++++++++
include/linux/dma-fence.h | 20 ++++++++++++++++++++
2 files changed, 40 insertions(+)
diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 0de0482cd36e..763b32627684 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -912,6 +912,26 @@ dma_fence_wait_any_timeout(struct dma_fence
**fences, uint32_t count,
}
EXPORT_SYMBOL(dma_fence_wait_any_timeout);
+
+/**
+ * dma_fence_set_deadline - set desired fence-wait deadline
+ * @fence: the fence that is to be waited on
+ * @deadline: the time by which the waiter hopes for the fence to be
+ * signaled
+ *
+ * Inform the fence signaler of an upcoming deadline, such as
vblank, by
+ * which point the waiter would prefer the fence to be signaled by.
This
+ * is intended to give feedback to the fence signaler to aid in power
+ * management decisions, such as boosting GPU frequency if a periodic
+ * vblank deadline is approaching.
+ */
+void dma_fence_set_deadline(struct dma_fence *fence, ktime_t deadline)
+{
+ if (fence->ops->set_deadline && !dma_fence_is_signaled(fence))
+ fence->ops->set_deadline(fence, deadline);
+}
+EXPORT_SYMBOL(dma_fence_set_deadline);
+
/**
* dma_fence_describe - Dump fence describtion into seq_file
* @fence: the 6fence to describe
diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
index 775cdc0b4f24..d77f6591c453 100644
--- a/include/linux/dma-fence.h
+++ b/include/linux/dma-fence.h
@@ -99,6 +99,7 @@ enum dma_fence_flag_bits {
DMA_FENCE_FLAG_SIGNALED_BIT,
DMA_FENCE_FLAG_TIMESTAMP_BIT,
DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
+ DMA_FENCE_FLAG_HAS_DEADLINE_BIT,
Would this bit be better left out from core implementation, given how
the approach is the component which implements dma-fence has to track
the actual deadline and all?
Also taking a step back - are we all okay with starting to expand the
relatively simple core synchronisation primitive with side channel
data like this? What would be the criteria for what side channel data
would be acceptable? Taking note the thing lives outside drivers/gpu/.
I had similar concerns and it took me a moment as well to understand the
background why this is necessary. I essentially don't see much other
approach we could do.
Yes, this is GPU/CRTC specific, but we somehow need a common interface
for communicating it between drivers and that's the dma_fence object as
far as I can see.
Yeah I also don't see any other easy options. Just wanted to raise this
as something which probably needs some wider acks.
Also what about the "low level" part of my question about the reason, or
benefits, of defining the deadline bit in the common layer?
Regards,
Tvrtko