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Subject: Re: [PATCH V2 3/5] vDPA: introduce vDPA bus
To:     Jason Gunthorpe <jgg@mellanox.com>
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <ebaea825-5432-65e2-2ab3-720a8c4030e7@redhat.com>
Date:   Thu, 13 Feb 2020 22:58:44 +0800
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On 2020/2/13 =E4=B8=8B=E5=8D=889:41, Jason Gunthorpe wrote:
> On Thu, Feb 13, 2020 at 11:34:10AM +0800, Jason Wang wrote:
>
>>>    You have dev, type or
>>> class to choose from. Type is rarely used and doesn't seem to be used
>>> by vdpa, so class seems the right choice
>>>
>>> Jason
>> Yes, but my understanding is class and bus are mutually exclusive. So =
we
>> can't add a class to a device which is already attached on a bus.
> While I suppose there are variations, typically 'class' devices are
> user facing things and 'bus' devices are internal facing (ie like a
> PCI device)


Though all vDPA devices have the same programming interface, but the=20
semantic is different. So it looks to me that use bus complies what=20
class.rst said:

"

Each device class defines a set of semantics and a programming interface
that devices of that class adhere to. Device drivers are the
implementation of that programming interface for a particular device on
a particular bus.

"


>
> So why is this using a bus? VDPA is a user facing object, so the
> driver should create a class vhost_vdpa device directly, and that
> driver should live in the drivers/vhost/ directory.


This is because we want vDPA to be generic for being used by different=20
drivers which is not limited to vhost-vdpa. E.g in this series, it=20
allows vDPA to be used by kernel virtio drivers. And in the future, we=20
will probably introduce more drivers in the future.


>
> For the PCI VF case this driver would bind to a PCI device like
> everything else
>
> For our future SF/ADI cases the driver would bind to some
> SF/ADI/whatever device on a bus.


All these driver will still be bound to their own bus (PCI or other).=20
And what the driver needs is to present a vDPA device to virtual vDPA=20
bus on top.

Thanks

>
> I don't see a reason for VDPA to be creating busses..
>
> Jason
>