Re: [Qemu-devel] [PATCH 1/2] usb-linux: fix device path aka physical port handling

[Gerd Hoffmann <kraxel@redhat.com>]

On 05/11/11 10:52, Markus Armbruster wrote:
> Good stuff, just a few questions.
>
> Gerd Hoffmann<kraxel@redhat.com>  writes:
>
>> The device path isn't just a number.  It specifies the physical port
>> the device is connected to and in case the device is connected via
>> usb hub you'll have two numbers there, like this: "5.1".  The first
>> specifies the root port where the hub is plugged into, the second
>> specifies the port number of the hub where the device is plugged in.
>> With multiple hubs chained the string can become longer.
>
> "5.1"?  Do you mean "5-1"?

No.

> 			snprintf(dev->devpath, sizeof dev->devpath,
> 				"%s.%d", parent->devpath, port1);
                                  ^^^^^

> $ ls /sys/bus/usb/devices
> 1-0:1.0  2-0:1.0  3-0:1.0  4-0:1.0  5-0:1.0  usb1  usb2  usb3  usb4  usb5

Boring, nothing plugged in here ;)

Laptop docking stations often have a usb hub built in.  If you have one 
place your laptop there, then connect a usb device to one of the ports 
of the docking station.  You'll see a file named like this:

   1-5.3

where "1" is the bus number, "5" is the port number of the root hub and 
"3" is the port number of the docking station hub.

>> @@ -79,6 +79,7 @@ typedef int USBScanFunc(void *opaque, int bus_num, int addr, int devpath,
>>   #define USBPROCBUS_PATH "/proc/bus/usb"
>>   #define PRODUCT_NAME_SZ 32
>>   #define MAX_ENDPOINTS 15
>> +#define MAX_PORTLEN 8
>
> Where does 8 come from?

Pulled out of thin air.  Should be enougth, you can build chains with 
three usb hubs (anyone ever did in practice? did the devices still 
work?), getting port addresses like "1.2.3.4", and it still fits in.

> For what it's worth, kernel's struct usb_device
> has char devpath[16].

We can make that 16 too to be on the really safe side.

>> @@ -1672,8 +1674,8 @@ static int usb_host_scan_sys(void *opaque, USBScanFunc *func)
>>               if (!strncmp(de->d_name, "usb", 3)) {
>>                   tmpstr += 3;
>>               }
>
> Sloppy parsing, but that's not your fault.

I think this can be zapped now, the new sscanf will fail on them and 
skip the entries anyway.

>> -            if (sscanf(tmpstr, "%d-%d",&bus_num,&devpath)<  1) {
>> -                goto the_end;
>> +            if (sscanf(tmpstr, "%d-%7[0-9.]",&bus_num, port)<  2) {
>> +                continue;
>>               }
>>
>>               if (!usb_host_read_file(line, sizeof(line), "devnum", de->d_name)) {
>
> The old sscanf() succeeds if at least one item is assigned, i.e. tmpstr
> starts with an integer.  I suspect this is broken for roots.  Consider
> d_name "usb1": tmpstr is "1", sscan() returns 1, and devpath remains
> uninitialized.  It's passed to the func() callback.  Bug?  If yes, the
> commit message should mention it.

Indeed.

> The new sscan() succeeds only if both items are assigned, i.e. tmpstr
> starts with an integer, '-', and up to 7 of [0-9.].  Unlike the old one,
> it fails for roots.  Intentional?

Yes, now the roots are skipped.  You can't assign them anyway, so it is 
pretty pointless to loom at them and to list them in "info usbhost".

cheers,
   Gerd