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

[Juergen Gross <jgross@suse.com>]

On 09/14/2015 01:12 PM, Ian Jackson wrote:
> Juergen Gross writes ("Re: [Xen-devel] [PATCH V6 3/7] libxl: add pvusb API"):
>> On 09/14/2015 12:36 PM, George Dunlap wrote:
>>> Anyone want to look into the Linux source code to find out how big it
>>> will allow busnum / devnum to grow?
>>
>> drivers/usb/core/hcd.c is using a bitmap to find the next bus number
>> currently not in use. It's size is USB_MAXBUS which in turn has the
>> value 64.
>>
>> choose_devnum() in drivers/usb/core/hub.c is doing a similar job for
>> device numbers. Here the highest number supported is 127.
>
> We are defining an API, which shouldn't involve this kind of
> implementation-grobbling.
>
> At an API level, it seems that this Linux busnum is not documented to
> have any particular number or behaviour or range or anything.  We
> should use the biggest type we can use conveniently.

Agreed.

> Do we need to worry that some bus might have 2^24 unplugs/plugs
> (perhaps in some kind of software emulation) and that we need to use a
> type which can hold a uint32_t or maybe even a uint64_t ?

uint128_t ? ;-)

I think 24 bits should be more than enough. Nobody will accept such huge
numbers without any need: they are to be used by users.


Juergen