Re: [PATCH v3 1/9] s390: vfio_ap: link the vfio_ap devices to the vfio_ap bus subsystem

Pierre Morel
2019-02-14T15:47:30Z

On 14/02/2019 15:54, Cornelia Huck wrote:
> On Thu, 14 Feb 2019 14:51:01 +0100
> Pierre Morel <pmorel@linux.ibm.com> wrote:
>
>> Libudev relies on having a subsystem link for non-root devices. To
>> avoid libudev (and potentially other userspace tools) choking on the
>> matrix device let us introduce a vfio_ap bus and with that the vfio_ap
>> bus subsytem, and make the matrix device reside within it.
>
> How does libudev choke on this? It feels wrong to introduce a bus that
> basically does nothing...

Christian answered this in another thread, so I don't answer.

>
>>
>> We restrict the number of allowed devices to a single one.
>>
...snip...

>> +++ 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.

>
>>
>> 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?

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