[RFC 1/4] dma-fence: Add deadline awareness

[Christian König]

Am 27.07.21 um 16:25 schrieb Rob Clark:
> On Tue, Jul 27, 2021 at 12:11 AM Christian König
> <ckoenig.leichtzumerken at gmail.com> wrote:
>> Am 27.07.21 um 01:38 schrieb Rob Clark:
>>> From: Rob Clark <robdclark at chromium.org>
>>>
>>> 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.
>>>
>>> Signed-off-by: Rob Clark <robdclark at chromium.org>
>>> ---
>>>    drivers/dma-buf/dma-fence.c | 39 +++++++++++++++++++++++++++++++++++++
>>>    include/linux/dma-fence.h   | 17 ++++++++++++++++
>>>    2 files changed, 56 insertions(+)
>>>
>>> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
>>> index ce0f5eff575d..2e0d25ab457e 100644
>>> --- a/drivers/dma-buf/dma-fence.c
>>> +++ b/drivers/dma-buf/dma-fence.c
>>> @@ -910,6 +910,45 @@ 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)
>>> +{
>>> +     unsigned long flags;
>>> +
>>> +     if (dma_fence_is_signaled(fence))
>>> +             return;
>>> +
>>> +     spin_lock_irqsave(fence->lock, flags);
>>> +
>>> +     /* If we already have an earlier deadline, keep it: */
>>> +     if (test_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT, &fence->flags) &&
>>> +         ktime_before(fence->deadline, deadline)) {
>>> +             spin_unlock_irqrestore(fence->lock, flags);
>>> +             return;
>>> +     }
>>> +
>>> +     fence->deadline = deadline;
>>> +     set_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT, &fence->flags);
>>> +
>>> +     spin_unlock_irqrestore(fence->lock, flags);
>>> +
>>> +     if (fence->ops->set_deadline)
>>> +             fence->ops->set_deadline(fence, deadline);
>>> +}
>>> +EXPORT_SYMBOL(dma_fence_set_deadline);
>>> +
>>>    /**
>>>     * dma_fence_init - Initialize a custom fence.
>>>     * @fence: the fence to initialize
>>> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
>>> index 6ffb4b2c6371..4e6cfe4e6fbc 100644
>>> --- a/include/linux/dma-fence.h
>>> +++ b/include/linux/dma-fence.h
>>> @@ -88,6 +88,7 @@ struct dma_fence {
>>>                /* @timestamp replaced by @rcu on dma_fence_release() */
>>>                struct rcu_head rcu;
>>>        };
>>> +     ktime_t deadline;
>> Mhm, adding the flag sounds ok to me but I'm a bit hesitating adding the
>> deadline as extra field here.
>>
>> We tuned the dma_fence structure intentionally so that it is only 64 bytes.
> Hmm, then I guess you wouldn't be a fan of also adding an hrtimer?
>
> We could push the ktime_t (and timer) down into the derived fence
> class, but I think there is going to need to be some extra storage
> *somewhere*.. maybe the fence signaler could get away with just
> storing the nearest upcoming deadline per fence-context instead?

I would just push that into the driver instead.

You most likely don't want the deadline per fence anyway in complex 
scenarios, but rather per frame. And a frame is usually composed from 
multiple fences.

Regards,
Christian.

>
> BR,
> -R
>
>> Regards,
>> Christian.
>>
>>>        u64 context;
>>>        u64 seqno;
>>>        unsigned long flags;
>>> @@ -99,6 +100,7 @@ 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,
>>>        DMA_FENCE_FLAG_USER_BITS, /* must always be last member */
>>>    };
>>>
>>> @@ -261,6 +263,19 @@ struct dma_fence_ops {
>>>         */
>>>        void (*timeline_value_str)(struct dma_fence *fence,
>>>                                   char *str, int size);
>>> +
>>> +     /**
>>> +      * @set_deadline:
>>> +      *
>>> +      * Callback to allow a fence waiter to 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.
>>> +      *
>>> +      * This callback is optional.
>>> +      */
>>> +     void (*set_deadline)(struct dma_fence *fence, ktime_t deadline);
>>>    };
>>>
>>>    void dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
>>> @@ -586,6 +601,8 @@ static inline signed long dma_fence_wait(struct dma_fence *fence, bool intr)
>>>        return ret < 0 ? ret : 0;
>>>    }
>>>
>>> +void dma_fence_set_deadline(struct dma_fence *fence, ktime_t deadline);
>>> +
>>>    struct dma_fence *dma_fence_get_stub(void);
>>>    struct dma_fence *dma_fence_allocate_private_stub(void);
>>>    u64 dma_fence_context_alloc(unsigned num);