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

[Mathias Nyman]

On 5/6/26 13:31, Michal Pecio wrote:
> 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?

Maybe prefer the hardcoded 0x02 value as spec does it as well

Thanks
Mathias