linux-kernel - Re: [RFC PATCH] dma-fence: Remove 64-bit flag

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Message-ID: <aPK04r1E7IbAZ9QY@lstrano-desk.jf.intel.com>
Date: Fri, 17 Oct 2025 14:28:02 -0700
From: Matthew Brost <matthew.brost@...el.com>
To: Philipp Stanner <phasta@...nel.org>
CC: Sumit Semwal <sumit.semwal@...aro.org>, Gustavo Padovan
	<gustavo@...ovan.org>, Christian König
	<christian.koenig@....com>, <tursulin@...ulin.net>,
	<linux-media@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
	<linaro-mm-sig@...ts.linaro.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] dma-fence: Remove 64-bit flag

On Fri, Oct 17, 2025 at 11:31:47AM +0200, Philipp Stanner wrote:
> It seems that DMA_FENCE_FLAG_SEQNO64_BIT has no real effects anymore,
> since seqno is a u64 everywhere.
> 
> Remove the unneeded flag.
> 
> Signed-off-by: Philipp Stanner <phasta@...nel.org>
> ---
> Seems to me that this flag doesn't really do anything anymore?
> 
> I *suspect* that it could be that some drivers pass a u32 to
> dma_fence_init()? I guess they could be ported, couldn't they.
> 

Xe uses 32-bit hardware fence sequence numbers—see [1] and [2]. We could
switch to 64-bit hardware fence sequence numbers, but that would require
changes on the driver side. If you sent this to our CI, I’m fairly
certain we’d see a bunch of failures. I suspect this would also break
several other drivers.

As I mentioned, all Xe-supported platforms could be updated since their
rings support 64-bit store instructions. However, I suspect that very
old i915 platforms don’t support such instructions in the ring. I agree
this is a legacy issue, and we should probably use 64-bit sequence
numbers in Xe. But again, platforms and drivers that are decades old
might break as a result.

Matt

[1] https://elixir.bootlin.com/linux/v6.17.1/source/drivers/gpu/drm/xe/xe_hw_fence.c#L264
[2] https://elixir.bootlin.com/linux/v6.17.1/source/drivers/gpu/drm/xe/xe_hw_fence_types.h#L51

> P.
> ---
>  drivers/dma-buf/dma-fence.c |  3 +--
>  include/linux/dma-fence.h   | 10 +---------
>  2 files changed, 2 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index 3f78c56b58dc..24794c027813 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -1078,8 +1078,7 @@ void
>  dma_fence_init64(struct dma_fence *fence, const struct dma_fence_ops *ops,
>  		 spinlock_t *lock, u64 context, u64 seqno)
>  {
> -	__dma_fence_init(fence, ops, lock, context, seqno,
> -			 BIT(DMA_FENCE_FLAG_SEQNO64_BIT));
> +	__dma_fence_init(fence, ops, lock, context, seqno, 0);
>  }
>  EXPORT_SYMBOL(dma_fence_init64);
>  
> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> index 64639e104110..4eca2db28625 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -98,7 +98,6 @@ struct dma_fence {
>  };
>  
>  enum dma_fence_flag_bits {
> -	DMA_FENCE_FLAG_SEQNO64_BIT,
>  	DMA_FENCE_FLAG_SIGNALED_BIT,
>  	DMA_FENCE_FLAG_TIMESTAMP_BIT,
>  	DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
> @@ -470,14 +469,7 @@ dma_fence_is_signaled(struct dma_fence *fence)
>   */
>  static inline bool __dma_fence_is_later(struct dma_fence *fence, u64 f1, u64 f2)
>  {
> -	/* This is for backward compatibility with drivers which can only handle
> -	 * 32bit sequence numbers. Use a 64bit compare when the driver says to
> -	 * do so.
> -	 */
> -	if (test_bit(DMA_FENCE_FLAG_SEQNO64_BIT, &fence->flags))
> -		return f1 > f2;
> -
> -	return (int)(lower_32_bits(f1) - lower_32_bits(f2)) > 0;
> +	return f1 > f2;
>  }
>  
>  /**
> -- 
> 2.49.0
>