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

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Message-ID: <aPYabivOBSJ1UChg@lstrano-desk.jf.intel.com>
Date: Mon, 20 Oct 2025 04:18:06 -0700
From: Matthew Brost <matthew.brost@...el.com>
To: <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 Mon, Oct 20, 2025 at 10:16:23AM +0200, Philipp Stanner wrote:
> On Fri, 2025-10-17 at 14:28 -0700, Matthew Brost wrote:
> > 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.
> 
> What exactly breaks? Help me out here; if you pass a u32 for a u64,

Seqno wraps.

> doesn't the C standard guarantee that the higher, unused 32 bits will
> be 0?

	return (int)(lower_32_bits(f1) - lower_32_bits(f2)) > 0;

Look at the above logic.

f1 = 0x0;
f2 = 0xffffffff; /* -1 */

The above statement will correctly return true.

Compared to the below statement which returns false.

	return f1 > f2;

We test seqno wraps in Xe by setting our initial seqno to -127, again if
you send this patch to our CI any test which sends more than 127 job on
queue will likely fail.

Matt

> 
> Because the only thing the flag still does is do this lower_32 check in
> fence_is_later.
> 
> P.
> 
> > 
> > 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
> > > 
>