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

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Dec 08, 2021 at 08:35:49AM -0700, Dave Jiang wrote:
> 
> On 12/8/2021 6:13 AM, Jason Gunthorpe wrote:
> > On Tue, Dec 07, 2021 at 05:47:14AM -0800, Jacob Pan wrote:
> > > In-kernel DMA should be managed by DMA mapping API. The existing kernel
> > > PASID support is based on the SVA machinery in SVA lib that is intended
> > > for user process SVA. The binding between a kernel PASID and kernel
> > > mapping has many flaws. See discussions in the link below.
> > > 
> > > This patch utilizes iommu_enable_pasid_dma() to enable DSA to perform DMA
> > > requests with PASID under the same mapping managed by DMA mapping API.
> > > In addition, SVA-related bits for kernel DMA are removed. As a result,
> > > DSA users shall use DMA mapping API to obtain DMA handles instead of
> > > using kernel virtual addresses.
> > Er, shouldn't this be adding dma_map/etc type calls?
> > 
> > You can't really say a driver is using the DMA API without actually
> > calling the DMA API..
> > 
> > > +	/*
> > > +	 * Try to enable both in-kernel and user DMA request with PASID.
> > > +	 * PASID is supported unless both user and kernel PASID are
> > > +	 * supported. Do not fail probe here in that idxd can still be
> > > +	 * used w/o PASID or IOMMU.
> > > +	 */
> > > +	if (iommu_dev_enable_feature(dev, IOMMU_DEV_FEAT_SVA) ||
> > > +		idxd_enable_system_pasid(idxd)) {
> > > +		dev_warn(dev, "Failed to enable PASID\n");
> > > +	} else {
> > > +		set_bit(IDXD_FLAG_PASID_ENABLED, &idxd->flags);
> > >   	}
> > Huh? How can the driver keep going if PASID isn't supported? I thought
> > the whole point of this was because the device cannot do DMA without
> > PASID at all?
> 
> There are 2 types of WQ supported with the DSA devices. A dedicated WQ type
> and a shared WQ type. The dedicated WQ type can support DMA with and without
> PASID. The shared wq type must have a PASID to operate. The driver can
> support dedicated WQ only without PASID usage when there is no PASID
> support.

Can you add to the cover letter why does the kernel require to use the
shared WQ?

Jason