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@kernel.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_fen... [2] https://elixir.bootlin.com/linux/v6.17.1/source/drivers/gpu/drm/xe/xe_hw_fen...
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