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SubjectRE: [RFC net-next v1 1/3] vfio/mdev: Inherit dma masks of parent device
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> -----Original Message-----
> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Friday, March 8, 2019 4:33 PM
> To: Parav Pandit <parav@mellanox.com>
> Cc: netdev@vger.kernel.org; linux-kernel@vger.kernel.org;
> michal.lkml@markovi.net; davem@davemloft.net;
> gregkh@linuxfoundation.org; Jiri Pirko <jiri@mellanox.com>;
> kwankhede@nvidia.com; Vu Pham <vuhuong@mellanox.com>; Yuval Avnery
> <yuvalav@mellanox.com>; jakub.kicinski@netronome.com;
> kvm@vger.kernel.org
> Subject: Re: [RFC net-next v1 1/3] vfio/mdev: Inherit dma masks of parent
> device
>
> On Fri, 8 Mar 2019 16:07:54 -0600
> Parav Pandit <parav@mellanox.com> wrote:
>
> > Inherit dma mask of parent device in child mdev devices, so that
> > protocol stack can use right dma mask while doing dma mappings.
> >
> > Signed-off-by: Parav Pandit <parav@mellanox.com>
> > ---
> > drivers/vfio/mdev/mdev_core.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/drivers/vfio/mdev/mdev_core.c
> > b/drivers/vfio/mdev/mdev_core.c index 0212f0e..9b8bdc9 100644
> > --- a/drivers/vfio/mdev/mdev_core.c
> > +++ b/drivers/vfio/mdev/mdev_core.c
> > @@ -315,6 +315,10 @@ int mdev_device_create(struct kobject *kobj,
> struct device *dev, uuid_le uuid)
> > mdev->dev.parent = dev;
> > mdev->dev.bus = &mdev_bus_type;
> > mdev->dev.release = mdev_device_release;
> > + mdev->dev.dma_mask = dev->dma_mask;
> > + mdev->dev.dma_parms = dev->dma_parms;
> > + mdev->dev.coherent_dma_mask = dev->coherent_dma_mask;
> > +
> > dev_set_name(&mdev->dev, "%pUl", uuid.b);
> >
> > ret = device_register(&mdev->dev);
>
> This seems like a rather large assumption and none of the existing mdev
> drivers even make use of DMA ops.
So its non-harmful anyway.

> Why shouldn't this be done in mdev_parent_ops.create? Thanks,
>
Struct device should be setup correctly before calling device_register().
That is the sane way to access device_register() API.
Doing this under mdev_parent_ops.create() will do it after device_register().
If you want to make it optional, mdev_register_device() can pass a bool flag to setup dma params or not which can be applied conditionally in mdev_device_create() before device_register().

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