Re: [PATCH v2 1/3] usb: core: Fix root hub descriptor wBytesPerInterval

[Michal Pecio]

On Wed, 6 May 2026 13:17:46 +0300, Mathias Nyman wrote:
> On 5/4/26 12:13, Michal Pecio wrote:
> > Per USB3 9.6.7, it's "the total number of bytes this endpoint will
> > transfer every service interval". There seems to be no good reason
> > to have wBytesPerInterval < wMaxPacketSize - either one is too low
> > or the other too high. Here, wBytesPerInterval is too low for hubs
> > with more than 15 ports and xHCI spec allows such root hubs.
> >   
> 
> There shouldn't be a USB3 hub with more than 15 ports as
> USB 3.x specification limits USB3 ports to 15.
> 
> See USB 3.2 section 10.15.2.1 Hub Descriptor
> "bNbrPorts:	Number of downstream facing ports that this hub supports. The
> 		maximum number of ports of ports a hub can support is 15."
> 
> 
> Hub driver also fails if hub has more than 15 ports.
> hub.c hub_configure():
> 
>     maxchild = USB_MAXCHILDREN;
>          if (hub_is_superspeed(hdev))
>                  maxchild = min_t(unsigned, maxchild, USB_SS_MAXPORTS);
> 
>          if (hub->descriptor->bNbrPorts > maxchild) {
>                  message = "hub has too many ports!";
>                  ret = -ENODEV;
>                  goto fail;
> 
> ch11.h:#define USB_SS_MAXPORTS           15
> 
> The USB 3.2 specification 10.15.1 'Standard Descriptors for Hub Class' also
> shows hardcoded values of '2' for both the wBytesPerInterval and wMaxPacketSize
> for the hub interrupt endpoint.

Thanks, looks like we should be able to get away with reducing
wMaxPacketSize then. Currently it's calculated to fit USB_MAXCHILDREN
i.e. 31 ports, supposedly dictated by the needs of (obsolete) WUSB.

Any preferences whether to replace USB_MAXCHILDREN with USB_SS_MAXPORTS
in the existing formula for USB3 hubs or just hardcode 0x02?

Regards,
Michal