RE: [PATCH 4/4] dmaengine: idxd: Use DMA API for in-kernel DMA with PASID

[Tian, Kevin]

> From: Jiang, Dave <dave.jiang@xxxxxxxxx>
> Sent: Thursday, December 9, 2021 8:12 AM
> >>>
> >> Do you mean wq completion record address? It is already using DMA API.
> >> 	wq->compls = dma_alloc_coherent(dev, wq->compls_size,
> >> &wq->compls_addr, GFP_KERNEL);
> >> 	desc->compl_dma = wq->compls_addr + idxd->data->compl_size * i;
> > I would have expected something on the queue submission side too?
> 
> DSA is different than typical DMA devices in the past. Instead of a
> software descriptor ring where the device DMA to fetch the descriptors
> after the software ringing a doorbell or writing a head index, the
> descriptors are submitted directly to the device via a CPU instruction
> (i.e. MOVDIR64B or ENQCMD(S)). The CPU takes the KVA of the 64B
> descriptor and writes to the device atomically. No DMA mapping is
> necessary in this case.
> 

To be accurate, the cpu reads the 64B descriptor from KVA and
then write the descriptor to the device atomically.