netdev - Re: [PATCH 5/5] vdpasim: vDPA device simulator

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Date:   Tue, 4 Feb 2020 08:52:04 -0400
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Zhu Lingshan <lingshan.zhu@...ux.intel.com>, mst@...hat.com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        tiwei.bie@...el.com, maxime.coquelin@...hat.com,
        cunming.liang@...el.com, zhihong.wang@...el.com,
        rob.miller@...adcom.com, xiao.w.wang@...el.com,
        haotian.wang@...ive.com, lingshan.zhu@...el.com,
        eperezma@...hat.com, lulu@...hat.com, parav@...lanox.com,
        kevin.tian@...el.com, stefanha@...hat.com, rdunlap@...radead.org,
        hch@...radead.org, aadam@...hat.com, jakub.kicinski@...ronome.com,
        jiri@...lanox.com, shahafs@...lanox.com, hanand@...inx.com,
        mhabets@...arflare.com
Subject: Re: [PATCH 5/5] vdpasim: vDPA device simulator

On Tue, Feb 04, 2020 at 04:28:27PM +0800, Jason Wang wrote:
> 
> On 2020/2/4 下午4:21, Zhu Lingshan wrote:
> > > +static const struct dma_map_ops vdpasim_dma_ops = {
> > > +    .map_page = vdpasim_map_page,
> > > +    .unmap_page = vdpasim_unmap_page,
> > > +    .alloc = vdpasim_alloc_coherent,
> > > +    .free = vdpasim_free_coherent,
> > > +};
> > > +
> > 
> > Hey Jason,
> > 
> > IMHO, it would be nice if dma_ops of the parent device could be re-used.
> > vdpa_device is expecting to represent a physical device except this
> > simulator, however, there are not enough information in vdpa_device.dev
> > to indicating which kind physical device it attached to. Namely
> > get_arch_dma_ops(struct bus type) can not work on vdpa_device.dev. Then
> > it seems device drivers need to implement a wrap of dma_ops of parent
> > devices. Can this work be done in the vdpa framework since it looks like
> > a common task? Can "vd_dev->vdev.dev.parent = vdpa->dev->parent;" in
> > virtio_vdpa_probe() do the work?
> > 
> > Thanks,
> > BR
> > Zhu Lingshan
> 
> 
> Good catch.
> 
> I think we can.

IMHO you need to specify some 'dma_device', not try and play tricks
with dma_ops, or assuming the parent is always the device used for
dma.

Jason