On 10/20/25 13:18, Matthew Brost wrote: > 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 <[email protected]> >>>> --- >>>> 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.
Yeah, exactly that's why this flag is needed for quite a lot of things. Question is what is missing in the documentation to make that clear? Regards, Christian. > > 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 >>>> >>
