Re: [Linaro-mm-sig] [PATCH 1/9] dma-buf: consolidate dma_fence subclass checking

[Thomas Hellström (Intel) <thomas_os@xxxxxxxxxxxx>]

On 1/20/22 14:27, Christian König wrote:
> Consolidate the wrapper functions to check for dma_fence
> subclasses in the dma_fence header.
>
> This makes it easier to document and also check the different
> requirements for fence containers in the subclasses.
>
> Signed-off-by: Christian König <christian.koenig@xxxxxxx>
> ---
>   include/linux/dma-fence-array.h | 15 +------------
>   include/linux/dma-fence-chain.h |  3 +--
>   include/linux/dma-fence.h       | 38 +++++++++++++++++++++++++++++++++
>   3 files changed, 40 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/dma-fence-array.h b/include/linux/dma-fence-array.h
> index 303dd712220f..fec374f69e12 100644
> --- a/include/linux/dma-fence-array.h
> +++ b/include/linux/dma-fence-array.h
> @@ -45,19 +45,6 @@ struct dma_fence_array {
>   	struct irq_work work;
>   };
>   
> -extern const struct dma_fence_ops dma_fence_array_ops;
> -
> -/**
> - * dma_fence_is_array - check if a fence is from the array subsclass
> - * @fence: fence to test
> - *
> - * Return true if it is a dma_fence_array and false otherwise.
> - */
> -static inline bool dma_fence_is_array(struct dma_fence *fence)
> -{
> -	return fence->ops == &dma_fence_array_ops;
> -}
> -
>   /**
>    * to_dma_fence_array - cast a fence to a dma_fence_array
>    * @fence: fence to cast to a dma_fence_array
> @@ -68,7 +55,7 @@ static inline bool dma_fence_is_array(struct dma_fence *fence)
>   static inline struct dma_fence_array *
>   to_dma_fence_array(struct dma_fence *fence)
>   {
> -	if (fence->ops != &dma_fence_array_ops)
> +	if (!fence || !dma_fence_is_array(fence))
>   		return NULL;
>   
>   	return container_of(fence, struct dma_fence_array, base);
> diff --git a/include/linux/dma-fence-chain.h b/include/linux/dma-fence-chain.h
> index 54fe3443fd2c..ee906b659694 100644
> --- a/include/linux/dma-fence-chain.h
> +++ b/include/linux/dma-fence-chain.h
> @@ -49,7 +49,6 @@ struct dma_fence_chain {
>   	spinlock_t lock;
>   };
>   
> -extern const struct dma_fence_ops dma_fence_chain_ops;
>   
>   /**
>    * to_dma_fence_chain - cast a fence to a dma_fence_chain
> @@ -61,7 +60,7 @@ extern const struct dma_fence_ops dma_fence_chain_ops;
>   static inline struct dma_fence_chain *
>   to_dma_fence_chain(struct dma_fence *fence)
>   {
> -	if (!fence || fence->ops != &dma_fence_chain_ops)
> +	if (!fence || !dma_fence_is_chain(fence))
>   		return NULL;
>   
>   	return container_of(fence, struct dma_fence_chain, base);
> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> index 1ea691753bd3..775cdc0b4f24 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -587,4 +587,42 @@ struct dma_fence *dma_fence_get_stub(void);
>   struct dma_fence *dma_fence_allocate_private_stub(void);
>   u64 dma_fence_context_alloc(unsigned num);
>   
> +extern const struct dma_fence_ops dma_fence_array_ops;
> +extern const struct dma_fence_ops dma_fence_chain_ops;
> +
> +/**
> + * dma_fence_is_array - check if a fence is from the array subclass
> + * @fence: the fence to test
> + *
> + * Return true if it is a dma_fence_array and false otherwise.
> + */
> +static inline bool dma_fence_is_array(struct dma_fence *fence)
> +{
> +	return fence->ops == &dma_fence_array_ops;
> +}
> +
> +/**
> + * dma_fence_is_chain - check if a fence is from the chain subclass
> + * @fence: the fence to test
> + *
> + * Return true if it is a dma_fence_chain and false otherwise.
> + */
> +static inline bool dma_fence_is_chain(struct dma_fence *fence)
> +{
> +	return fence->ops == &dma_fence_chain_ops;
> +}
> +
> +/**
> + * dma_fence_is_container - check if a fence is a container for other fences
> + * @fence: the fence to test
> + *
> + * Return true if this fence is a container for other fences, false otherwise.
> + * This is important since we can't build up large fence structure or otherwise
> + * we run into recursion during operation on those fences.
> + */
> +static inline bool dma_fence_is_container(struct dma_fence *fence)
> +{
> +	return dma_fence_is_array(fence) || dma_fence_is_chain(fence);
> +}

What's the strategy here moving forward if we add more dma_resv 
containers, or if a driver adds a container that similarly has risc of 
recursion? Should we perhaps add an ops bool for this, or require that 
all dma_resv containers that has this limitation be part of the dma-buf 
subsystem rather than driver-specific?

Thanks,
/Thomas


> +
>   #endif /* __LINUX_DMA_FENCE_H */