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

[Juergen Gross <jgross@suse.com>]

On 09/11/2015 04:41 PM, Ian Campbell wrote:
> On Fri, 2015-09-11 at 16:18 +0200, Juergen Gross wrote:
>> On 09/11/2015 04:09 PM, Ian Campbell wrote:
>>> On Fri, 2015-09-11 at 15:55 +0200, Juergen Gross wrote:
>>>> 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
>>>
>>> Thanks for the info. So an "address" in the USB protocol is actually a
>>> "path" and "hostbus" is an implementation dependent shorthand for all
>>> but
>>> the last link in that path.
>>
>> I'm not sure in which direction you are looking. "address" is a path.
>> A path is normally a list of ports starting at the host and walking
>> through all hubs until you reach the device. The "bus" is the root
>> of that path. So the number of buses the host knows of is the number
>> of USB host adapters without any hub.
>
> OK, I thought I understood but the above suggests not.
>
> In USB speak, the address is a list of port numbers, which you follow from
> the host bus which is the root.
>
> In Linux speak a "bus" is actually each hub along that path.

No. Each hub is just a port which happens to have more ports behind it.

> Let me try a worked example and see if I've got it right. Lets take this
> topology:
>
> ROOT0
>   |-PORT0 ----+--HUB1
>   |-PORT1-,   |-PORT0 -- DEVICE A
>           |   `-PORT1 -- DEVICE B
>           |
>           `--HUB2
>               |-PORT0 -- DEVICE C
>               `-PORT1 -- HUB3
>                           |-PORT0 -- DEVICE D
>                           `-PORT1 -x
>
> ROOT1 -- ... other stuff
>
> In the USB protocol there are two buses corresponding to ROOT0 and ROOT1.
>
> So in the protocol the address of DEVICE D on the bus associated with ROOT0
> is [1,1,0], that is PORT1 on ROOT0 => PORT1 on HUB2 => PORT0 on HUB3.
>
> DEVICE A is [0,0] on the bus associated with ROOT0, similarly.
>
> In the Linux numbering scheme each ROOTn or HUBn is given a bus number,
> somewhat arbitrarily (although I'm sure there is a scheme by which they
> allocated). So perhaps:
>
> ROOT0==BUS1

Correct.

> HUB1==BUS2

No, Just Bus1-Port0 or Bus1:Devnum1

> HUB2==BUS2

Bus1-Port1 or Bus1:Devnum2

> HUB3==BUS4

Bus1-Port1.Port1 or Bus1:Devnum3

> ROOT1==BUS42

Bus2

> And in this scheme the address is hostbus+hostaddr, so DEVICE D is [3,0],
> that is hostbus==3==HUB3, and port 0. And DEVICE A is [2,0]

Device D: Bus1-Port1.Port1.Port0 or e.g. Bus1:Devnum4
Device A: Bus1-Port0.Port0 or e.g. Bus1:Devnum5

> Is that right?
>
>> One bus can have up to 31 ports.
>
> So the answer is that hostaddr can be 5 bits?

5*8 (7 hubs and a device at the last level) == 40, that's the 1 trillion
I suggested before. Things are a little bit more complicated. A devnum
in a bus is never assigned twice. So when you plug in a device, it might
get devnum 6. Unplug it and replug it again will lead to devnum 7.

>>   In theory I think up to 7 cascaded
>> hubs are possible, but I don't think the resulting theoretical maximum
>> of about 1 trillion devices on a bus is to be considered. :-)
>
> And this suggests that in principal a Linux hostbus could be 5*7 bits == 35
> bits, maybe. Or at least that any USB address can be encoded in that many
> bits.

Busnum can grow to arbitrary values. A new USB bus detected will get a
new bus number. Removing it and adding it again will again use a new
number.


Juergen