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

[Christian König <ckoenig.leichtzumerken@xxxxxxxxx>]

Am 21.01.22 um 08:41 schrieb Thomas Hellström (Intel):
> 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?

Good question.

I think that all containers which also implement the dma_fence interface 
should be part of the DMA-buf subsystem and not driver specific. Drivers 
just tend to reinvent something incorrectly.

Where/How should we document that?

Regards,
Christian.

> Thanks,
> /Thomas
>
>
> > +
> >   #endif /* __LINUX_DMA_FENCE_H */