RE: [RFC net-next v1 1/3] vfio/mdev: Inherit dma masks of parent device

[Parav Pandit]

> -----Original Message-----
> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Friday, March 8, 2019 4:33 PM
> To: Parav Pandit <parav@xxxxxxxxxxxx>
> Cc: netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> michal.lkml@xxxxxxxxxxx; davem@xxxxxxxxxxxxx;
> gregkh@xxxxxxxxxxxxxxxxxxx; Jiri Pirko <jiri@xxxxxxxxxxxx>;
> kwankhede@xxxxxxxxxx; Vu Pham <vuhuong@xxxxxxxxxxxx>; Yuval Avnery
> <yuvalav@xxxxxxxxxxxx>; jakub.kicinski@xxxxxxxxxxxxx;
> kvm@xxxxxxxxxxxxxxx
> 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@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > ---
> >  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().