Re: [PATCH 1/7] dma-buf: some dma_fence_chain improvements

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Jun 10, 2021 at 11:17:54AM +0200, Christian König wrote:
> The callback and the irq work are never used at the same
> time. Putting them into an union saves us 24 bytes and
> makes the structure only 120 bytes in size.

Yeah pushing below 128 bytes makes sense.

> 
> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
>  drivers/dma-buf/dma-fence-chain.c | 2 +-
>  include/linux/dma-fence-chain.h   | 8 +++++---
>  2 files changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-fence-chain.c b/drivers/dma-buf/dma-fence-chain.c
> index 7d129e68ac70..1b4cb3e5cec9 100644
> --- a/drivers/dma-buf/dma-fence-chain.c
> +++ b/drivers/dma-buf/dma-fence-chain.c
> @@ -137,6 +137,7 @@ static void dma_fence_chain_cb(struct dma_fence *f, struct dma_fence_cb *cb)
>  	struct dma_fence_chain *chain;
>  
>  	chain = container_of(cb, typeof(*chain), cb);
> +	init_irq_work(&chain->work, dma_fence_chain_irq_work);
>  	irq_work_queue(&chain->work);
>  	dma_fence_put(f);
>  }
> @@ -239,7 +240,6 @@ void dma_fence_chain_init(struct dma_fence_chain *chain,
>  	rcu_assign_pointer(chain->prev, prev);
>  	chain->fence = fence;
>  	chain->prev_seqno = 0;
> -	init_irq_work(&chain->work, dma_fence_chain_irq_work);
>  
>  	/* Try to reuse the context of the previous chain node. */
>  	if (prev_chain && __dma_fence_is_later(seqno, prev->seqno, prev->ops)) {
> diff --git a/include/linux/dma-fence-chain.h b/include/linux/dma-fence-chain.h
> index 10462a029da2..9d6a062be640 100644
> --- a/include/linux/dma-fence-chain.h
> +++ b/include/linux/dma-fence-chain.h
> @@ -25,12 +25,14 @@
>   */
>  struct dma_fence_chain {
>  	struct dma_fence base;
> -	spinlock_t lock;
>  	struct dma_fence __rcu *prev;
>  	u64 prev_seqno;
>  	struct dma_fence *fence;
> -	struct dma_fence_cb cb;
> -	struct irq_work work;

Can you pls pull the kerneldoc inline here for these too and extend the
comments that @work is only used from the callback, at which point we
don't need @cb anymore? For union lifetime tricks we really should
document this in the datastructure docs.

With that:

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

I also think it'd be good to specify this clearly in the kerneldoc for
dma_fence_add_callback() with something like:

"Note that the registered @cb structure is no longer in use by the
signalling code by the time @func is called, and can therefore be used
again. This is useful when @func launches a work item because it allows us
to put both the struct dma_fence_cb and the work struct (e.g. struct
work_struct) into a union to save space."

Feel free to includ this in this patch here or do a separate one.

Cheers, Daniel

> +	union {
> +		struct dma_fence_cb cb;
> +		struct irq_work work;
> +	};
> +	spinlock_t lock;
>  };
>  
>  extern const struct dma_fence_ops dma_fence_chain_ops;
> -- 
> 2.25.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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