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 18:36:46 +0100
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.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 14/02/2019 17:57, Cornelia Huck wrote:
> 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?

...yes -(

> 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?)

On device_register there were no bus, so the core just do not look for a 
driver and this field was nor tested nor overwritten.

> 
>> 
>>> 
>>>> 
>>>> 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.
> 

:) definitively right
thanks, this will simplify the code in the next version.
I will take the patch away from this series to get the way to stable as 
Christian requested.

Regards,
Pierre

-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany