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

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Date:   Thu, 9 Dec 2021 02:06:27 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     "Jiang, Dave" <dave.jiang@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
CC:     "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Lu Baolu" <baolu.lu@...ux.intel.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, Barry Song <21cnbao@...il.com>,
        "Zanussi, Tom" <tom.zanussi@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: RE: [PATCH 4/4] dmaengine: idxd: Use DMA API for in-kernel DMA with
 PASID

> From: Jiang, Dave <dave.jiang@...el.com>
> 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.