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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 > > > >