Re: [Linaro-mm-sig] [PATCH 3/4] dma-seqno-fence: Hardware dma-buf implementation of fencing (v2)

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Aug 10, 2012 at 04:57:58PM +0200, Maarten Lankhorst wrote:
> This type of fence can be used with hardware synchronization for simple
> hardware that can block execution until the condition
> (dma_buf[offset] - value) >= 0 has been met.
> 
> A software fallback still has to be provided in case the fence is used
> with a device that doesn't support this mechanism. It is useful to expose
> this for graphics cards that have an op to support this.
> 
> Some cards like i915 can export those, but don't have an option to wait,
> so they need the software fallback.
> 
> I extended the original patch by Rob Clark.
> 
> v1: Original
> v2: Renamed from bikeshed to seqno, moved into dma-fence.c since
>     not much was left of the file. Lots of documentation added.
> 
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>

Patch looks good, two bikesheds inline. Either way
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> ---
>  drivers/base/dma-fence.c  |   21 +++++++++++++++
>  include/linux/dma-fence.h |   61 +++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 82 insertions(+)
> 
> diff --git a/drivers/base/dma-fence.c b/drivers/base/dma-fence.c
> index 93448e4..4092a58 100644
> --- a/drivers/base/dma-fence.c
> +++ b/drivers/base/dma-fence.c
> @@ -266,3 +266,24 @@ struct dma_fence *dma_fence_create(void *priv)
>  	return fence;
>  }
>  EXPORT_SYMBOL_GPL(dma_fence_create);
> +
> +static int seqno_enable_signaling(struct dma_fence *fence)
> +{
> +	struct dma_seqno_fence *seqno_fence = to_seqno_fence(fence);
> +	return seqno_fence->enable_signaling(seqno_fence);
> +}
> +
> +static void seqno_release(struct dma_fence *fence)
> +{
> +	struct dma_seqno_fence *f = to_seqno_fence(fence);
> +
> +	if (f->release)
> +		f->release(f);
> +	dma_buf_put(f->sync_buf);
> +}
> +
> +const struct dma_fence_ops dma_seqno_fence_ops = {
> +	.enable_signaling = seqno_enable_signaling,
> +	.release = seqno_release
> +};
> +EXPORT_SYMBOL_GPL(dma_seqno_fence_ops);
> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> index e0ceddd..3ef0da0 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -91,6 +91,19 @@ struct dma_fence_ops {
>  	void (*release)(struct dma_fence *fence);
>  };
>  
> +struct dma_seqno_fence {
> +	struct dma_fence base;
> +
> +	struct dma_buf *sync_buf;
> +	uint32_t seqno_ofs;
> +	uint32_t seqno;
> +
> +	int (*enable_signaling)(struct dma_seqno_fence *fence);
> +	void (*release)(struct dma_seqno_fence *fence);

I think using dma_fence_ops here is the better color. We lose type-safety
at compile-time, but still keep type-safety at runtime (thanks to
to_dma_seqno_fence). In addition people seem to like to constify function
pointers, we'd save a pointer and if we extend the sw dma_fence interface.

> +};
> +
> +extern const struct dma_fence_ops dma_seqno_fence_ops;
> +
>  struct dma_fence *dma_fence_create(void *priv);
>  
>  /**
> @@ -121,4 +134,52 @@ int dma_fence_wait(struct dma_fence *fence, bool intr, unsigned long timeout);
>  int dma_fence_add_callback(struct dma_fence *fence, struct dma_fence_cb *cb,
>  			   dma_fence_func_t func, void *priv);
>  
> +/**
> + * to_seqno_fence - cast a dma_fence to a dma_seqno_fence
> + * @fence: dma_fence to cast to a dma_seqno_fence
> + *
> + * Returns NULL if the dma_fence is not a dma_seqno_fence,
> + * or the dma_seqno_fence otherwise.
> + */
> +static inline struct dma_seqno_fence *
> +to_seqno_fence(struct dma_fence *fence)
> +{
> +	if (fence->ops != &dma_seqno_fence_ops)
> +		return NULL;
> +	return container_of(fence, struct dma_seqno_fence, base);
> +}

I think adding an is_dma_seqno_fence would be nice ...

> +
> +/**
> + * dma_seqno_fence_init - initialize a seqno fence
> + * @fence: dma_seqno_fence to initialize
> + * @sync_buf: buffer containing the memory location to signal on
> + * @seqno_ofs: the offset within @sync_buf
> + * @seqno: the sequence # to signal on
> + * @priv: value of priv member
> + * @enable_signaling: callback which is called when some other device is
> + *    waiting for sw notification of fence
> + * @release: callback called during destruction before object is freed.
> + *
> + * This function initializes a struct dma_seqno_fence with passed parameters,
> + * and takes a reference on sync_buf which is released on fence destruction.
> + */
> +static inline void
> +dma_seqno_fence_init(struct dma_seqno_fence *fence,
> +			struct dma_buf *sync_buf,
> +			uint32_t seqno_ofs, uint32_t seqno, void *priv,
> +			int (*enable_signaling)(struct dma_seqno_fence *),
> +			void (*release)(struct dma_seqno_fence *))
> +{
> +	BUG_ON(!fence || !sync_buf || !enable_signaling);
> +
> +	__dma_fence_init(&fence->base, &dma_seqno_fence_ops, priv);
> +
> +	get_dma_buf(sync_buf);
> +	fence->sync_buf = sync_buf;
> +	fence->seqno_ofs = seqno_ofs;
> +	fence->seqno = seqno;
> +	fence->enable_signaling = enable_signaling;
> +	fence->release = release;
> +}
> +
>  #endif /* __DMA_FENCE_H__ */
> 
> 
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-- 
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48