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Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ] > > Hi again Shivank, > > I just got some time to resume testing this on Intel Sapphire Rapids and > something caught my attention, below > > On Tue, Apr 28, 2026 at 03:50:49PM +0000, Shivank Garg wrote: >> +static int submit_dma_transfers(struct dma_work *work) >> +{ >> + struct scatterlist *sg_src, *sg_dst; >> + struct dma_async_tx_descriptor *tx; >> + unsigned long flags = DMA_CTRL_ACK; >> + dma_cookie_t cookie; >> + int i; >> + >> + atomic_set(&work->pending, 1); >> + >> + sg_src = work->src_sgt->sgl; >> + sg_dst = work->dst_sgt->sgl; >> + for_each_sgtable_dma_sg(work->src_sgt, sg_src, i) { >> + if (i == work->src_sgt->nents - 1) >> + flags |= DMA_PREP_INTERRUPT; >> + >> + tx = dmaengine_prep_dma_memcpy(work->chan, >> + sg_dma_address(sg_dst), >> + sg_dma_address(sg_src), >> + sg_dma_len(sg_src), flags); >> + if (!tx) { >> + atomic_set(&work->pending, 0); >> + return -EIO; >> + } >> + >> + if (i == work->src_sgt->nents - 1) { >> + tx->callback = dma_completion_callback; >> + tx->callback_param = work; >> + } >> + > > Here, you are submitting the descriptors one after the other and only > the last descriptor has a callback, which in theory sounds correct as > you expect the DMA engine to complete the descriptors in the same order > they were submitted. However, in reality that's not really gauranteed. > > Intel DSA in particular can complete descriptors out of order. That > means, the last descriptor submitted may not necessarily be the last > descriptors that completes. In that case, you will return in > folios_copy_dma() before the copy truly completes for all the folios. > > For correctness, we have to add a callback to every descriptor and > initialize work->pending to the number of descriptors submitted then > every time a descriptor completes, you call atomic_dec(&work->pending) > and only complete the completion the moment it reaches zero. > > Btw, waiting for an interrupt adds massive scheduling overhead. If we > also add the logic above, it'll get even worse. In my measurements, this > can easily add up to 6ms, by which CPU page copy have easily completed > the entire copy, which again adds to the list of latency concerns I > raised in my other reply. Thanks Karim for catching this. I was not aware that descriptor chaining was not applicable for DSA. Going forward, implementing the device_prep_dma_memcpy_sg() fixes this broken assumption. So client will issue single transaction and see single completion for whole batch. The ordering/correctness will become provider's responsibility. Thanks, Shivank