Date:   Wed, 1 Mar 2023 10:50:20 +0700
From:   Bagas Sanjaya <bagasdotme@gmail.com>
To:     Rob Clark <robdclark@gmail.com>, dri-devel@lists.freedesktop.org
Cc:     freedreno@lists.freedesktop.org, Daniel Vetter <daniel@ffwll.ch>,
        Christian =?utf-8?B?S8O2bmln?= <ckoenig.leichtzumerken@gmail.com>,
        Michel =?utf-8?Q?D=C3=A4nzer?= <michel@daenzer.net>,
        Tvrtko Ursulin <tvrtko.ursulin@intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        Pekka Paalanen <ppaalanen@gmail.com>,
        Simon Ser <contact@emersion.fr>,
        Luben Tuikov <luben.tuikov@amd.com>,
        Rob Clark <robdclark@chromium.org>,
        Christian =?utf-8?B?S8O2bmln?= <christian.koenig@amd.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Gustavo Padovan <gustavo@padovan.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@lists.linaro.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v7 01/15] dma-buf/dma-fence: Add deadline awareness
Message-ID: <Y/7LfLxhIjDpD4D3@debian.me>
References: <20230227193535.2822389-1-robdclark@gmail.com>
 <20230227193535.2822389-2-robdclark@gmail.com>
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On Mon, Feb 27, 2023 at 11:35:07AM -0800, Rob Clark wrote:
> diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-=
api/dma-buf.rst
> index 622b8156d212..183e480d8cea 100644
> --- a/Documentation/driver-api/dma-buf.rst
> +++ b/Documentation/driver-api/dma-buf.rst
> @@ -164,6 +164,12 @@ DMA Fence Signalling Annotations
>  .. kernel-doc:: drivers/dma-buf/dma-fence.c
>     :doc: fence signalling annotation
> =20
> +DMA Fence Deadline Hints
> +~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +.. kernel-doc:: drivers/dma-buf/dma-fence.c
> +   :doc: deadline hints
> +
>  DMA Fences Functions Reference
>  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> =20
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index 0de0482cd36e..e103e821d993 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -912,6 +912,65 @@ dma_fence_wait_any_timeout(struct dma_fence **fences=
, uint32_t count,
>  }
>  EXPORT_SYMBOL(dma_fence_wait_any_timeout);
> =20
> +/**
> + * DOC: deadline hints
> + *
> + * In an ideal world, it would be possible to pipeline a workload suffic=
iently
> + * that a utilization based device frequency governor could arrive at a =
minimum
> + * frequency that meets the requirements of the use-case, in order to mi=
nimize
> + * power consumption.  But in the real world there are many workloads wh=
ich
> + * defy this ideal.  For example, but not limited to:
> + *
> + * * Workloads that ping-pong between device and CPU, with alternating p=
eriods
> + *   of CPU waiting for device, and device waiting on CPU.  This can res=
ult in
> + *   devfreq and cpufreq seeing idle time in their respective domains an=
d in
> + *   result reduce frequency.
> + *
> + * * Workloads that interact with a periodic time based deadline, such a=
s double
> + *   buffered GPU rendering vs vblank sync'd page flipping.  In this sce=
nario,
> + *   missing a vblank deadline results in an *increase* in idle time on =
the GPU
> + *   (since it has to wait an additional vblank period), sending a singl=
e to
> + *   the GPU's devfreq to reduce frequency, when in fact the opposite is=
 what is
> + *   needed.
> + *
> + * To this end, deadline hint(s) can be set on a &dma_fence via &dma_fen=
ce_set_deadline.
> + * The deadline hint provides a way for the waiting driver, or userspace=
, to
> + * convey an appropriate sense of urgency to the signaling driver.
> + *
> + * A deadline hint is given in absolute ktime (CLOCK_MONOTONIC for users=
pace
> + * facing APIs).  The time could either be some point in the future (suc=
h as
> + * the vblank based deadline for page-flipping, or the start of a compos=
itor's
> + * composition cycle), or the current time to indicate an immediate dead=
line
> + * hint (Ie. forward progress cannot be made until this fence is signale=
d).
> + *
> + * Multiple deadlines may be set on a given fence, even in parallel.  Se=
e the
> + * documentation for &dma_fence_ops.set_deadline.
> + *
> + * The deadline hint is just that, a hint.  The driver that created the =
fence
> + * may react by increasing frequency, making different scheduling choice=
s, etc.
> + * Or doing nothing at all.
> + */
> +
> +/**
> + * dma_fence_set_deadline - set desired fence-wait deadline hint
> + * @fence:    the fence that is to be waited on
> + * @deadline: the time by which the waiter hopes for the fence to be
> + *            signaled
> + *
> + * Give the fence signaler a hint about 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 but the fence is not
> + * yet signaled..
> + */
> +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..87c0d846dbb4 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -257,6 +257,24 @@ 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 is called without &dma_fence.lock held, it can be called
> +	 * multiple times and from any context.  Locking is up to the callee
> +	 * if it has some state to manage.  If multiple deadlines are set,
> +	 * the expectation is to track the soonest one.
> +	 *
> +	 * This callback is optional.
> +	 */
> +	void (*set_deadline)(struct dma_fence *fence, ktime_t deadline);
>  };
> =20
>  void dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops =
*ops,
> @@ -583,6 +601,8 @@ static inline signed long dma_fence_wait(struct dma_f=
ence *fence, bool intr)
>  	return ret < 0 ? ret : 0;
>  }
> =20
> +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);

The doc LGTM, thanks!

Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>

--=20
An old man doll... just what I always wanted! - Clara

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