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

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Message-ID: <7b53f502aa0eaee4ffe4350621ddbcbfaafcad06.camel@mailbox.org>
Date: Mon, 20 Oct 2025 10:16:23 +0200
From: Philipp Stanner <phasta@...lbox.org>
To: Matthew Brost <matthew.brost@...el.com>, 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, 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,
doesn't the C standard guarantee that the higher, unused 32 bits will
be 0?

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
> >