Re: [PATCH V6 3/7] libxl: add pvusb API

[Juergen Gross <jgross@suse.com>]

On 09/11/2015 03:26 PM, Ian Campbell wrote:
> On Thu, 2015-09-10 at 23:42 -0600, Chun Yan Liu wrote:
>>
>>> Do these fields have any particular size requirements arising from e.g. the
>>> USB spec or from possible dom0 implementations?
>>>
>>> If they have a well defined fixed size from a USB spec then maybe we
>>> could
>>> use the appropriate fixed size types?
>>
>> Di> dn't see the size limitation. In Linux kernel code, busnum and devnum (here
>> 'hostbus, hostaddr') are both 'int' type.
>
> Is that a Linux-specific implementation detail or a fundamental property of
> USB? We should be designing the interface around Linux implementation
> details. It seems like something in the USB spec ought to define precisely
> the number of bits in both a bus number and a device address within that
> bus.

The USB spec is only about _the_ bus. How many buses a host can
operate and how they are numbered is outside the USB spec.

Devices are addressed via their ports in the USB protocol. devnum
is a unique index for a device on the bus, the USB protocol equivalent
is a list of ports of:
- 1 member in case of direct attached devices
- multiple members in case of hubs between bus and device

>
> Note also that integer in the libxl IDL is signed 24 bits.
>
>>   And idProduct and idVendor are 'u16'.
>
> That's a USB spec thing, I think, so int16 in the IDL seems appropriate.

Correct.


Juergen