linux-kernel - Re: [PATCH v3 1/9] s390: vfio_ap: link the vfio_ap devices to the vfio

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Date:   Thu, 14 Feb 2019 17:57:30 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     Pierre Morel <pmorel@...ux.ibm.com>
Cc:     borntraeger@...ibm.com, alex.williamson@...hat.com,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, frankja@...ux.ibm.com, akrowiak@...ux.ibm.com,
        pasic@...ux.ibm.com, david@...hat.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, freude@...ux.ibm.com, mimu@...ux.ibm.com
Subject: Re: [PATCH v3 1/9] s390: vfio_ap: link the vfio_ap devices to the
 vfio_ap bus subsystem

On Thu, 14 Feb 2019 16:47:30 +0100
Pierre Morel <pmorel@...ux.ibm.com> wrote:

> On 14/02/2019 15:54, Cornelia Huck wrote:
> > On Thu, 14 Feb 2019 14:51:01 +0100
> > Pierre Morel <pmorel@...ux.ibm.com> wrote:

> >> +++ b/drivers/s390/crypto/vfio_ap_drv.c
> >> @@ -24,8 +24,9 @@ MODULE_LICENSE("GPL v2");
> >>   
> >>   static struct ap_driver vfio_ap_drv;
> >>   
> >> -static struct device_type vfio_ap_dev_type = {
> >> -	.name = VFIO_AP_DEV_TYPE_NAME,
> >> +struct matrix_driver {
> >> +	struct device_driver drv;
> >> +	int device_count;  
> > 
> > This counter basically ensures that at most one device may bind with
> > this driver... you'd still have that device on the bus, though.  
> 
> yes, this is what is wanted: this driver can only support one device.
> May be another matrix driver can support one or more other devices.
> 
> I should update comment message my be.
> 
> >   
> >>   };
> >>   
> >>   struct ap_matrix_dev *matrix_dev;  
> 
> >>   
> >> -	matrix_dev->device.type = &vfio_ap_dev_type;
> >>   	dev_set_name(&matrix_dev->device, "%s", VFIO_AP_DEV_NAME);
> >>   	matrix_dev->device.parent = root_device;
> >> +	matrix_dev->device.bus = &matrix_bus;
> >>   	matrix_dev->device.release = vfio_ap_matrix_dev_release;
> >> -	matrix_dev->device.driver = &vfio_ap_drv.driver;
> >> +	matrix_dev->vfio_ap_drv = &vfio_ap_drv;  
> > 
> > Can't you get that structure through matrix_dev->device.driver instead
> > when you need it in the function below?  
> 
> Not anymore.
> We have two different drivers and devices
> matrix_drv <-> matrix_dev
> and
> vfio_ap_drv <-> ap_devices
> 
> The driver behind the matrix_dev->dev->driver is matrix_drv
> what is needed here is vfio_ap_drv.

Wait, we had tacked a driver for ap devices unto a matrix device, which
is not on the ap bus? Maybe that's what trips libudev?

(And reading further in the current code, it seems we clear that
structure _after_ the matrix device had been setup, so how can that
even work? Where am I confused?)

> 
> >   
> >>   
> >>   	ret = device_register(&matrix_dev->device);
> >>   	if (ret)
> >>   		goto matrix_reg_err;
> >>   
> >> +	ret = driver_register(&matrix_driver.drv);
> >> +	if (ret)
> >> +		goto matrix_drv_err;
> >> +  
> > 
> > As you already have several structures that can be registered exactly
> > once (the root device, the bus, the driver, ...), you can already be
> > sure that there's only one device on the bus, can't you?  
> 
> hum, no I don't think so, no device can register before this module is 
> loaded, but what does prevent a device to register later from another 
> module?

Not unless you export the interface, I guess.