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

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Date:   Sat, 9 Mar 2019 03:55:43 +0000
From:   Parav Pandit <parav@...lanox.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "michal.lkml@...kovi.net" <michal.lkml@...kovi.net>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        Jiri Pirko <jiri@...lanox.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        Vu Pham <vuhuong@...lanox.com>,
        Yuval Avnery <yuvalav@...lanox.com>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: RE: [RFC net-next v1 1/3] vfio/mdev: Inherit dma masks of parent
 device



> -----Original Message-----
> From: Alex Williamson <alex.williamson@...hat.com>
> Sent: Friday, March 8, 2019 4:33 PM
> To: Parav Pandit <parav@...lanox.com>
> Cc: netdev@...r.kernel.org; linux-kernel@...r.kernel.org;
> michal.lkml@...kovi.net; davem@...emloft.net;
> gregkh@...uxfoundation.org; Jiri Pirko <jiri@...lanox.com>;
> kwankhede@...dia.com; Vu Pham <vuhuong@...lanox.com>; Yuval Avnery
> <yuvalav@...lanox.com>; jakub.kicinski@...ronome.com;
> kvm@...r.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@...lanox.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@...lanox.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().