Re: [PATCH] usb: core: Fix bandwidth for devices with invalid wBytesPerInterval

[Michal Pecio]

On Thu, 2 Apr 2026 10:14:00 +0800, Tao Xue wrote:
> As specified in Section 4.14.2 of the xHCI Specification, the xHC
> reserves bandwidth for periodic endpoints according to bInterval and
> wBytesPerInterval (Max ESIT Payload).

For SuperSpeed endpoints, yes.
This follows from USB3 spec 9.6.7.

> Some peripherals report an invalid wBytesPerInterval in their device
> descriptor, which is either 0 or smaller than the actual data length
> transmitted. This issue is observed on ASIX AX88179 series USB 3.0
> Ethernet adapters.

Damn, it really does.

      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0008  1x 8 bytes
        bInterval              11
        bMaxBurst               0
        wBytesPerInterval       0

Any other examples besides AX88179?

> These errors may lead to unexpected behavior on certain USB host
> controllers, causing USB peripherals to malfunction.

Out of curiosity, Bandwidth Overrun Error or something worse?

It's an oversight that these URBs aren't rejected with EMSGSIZE in the
first place. IIRC zero-length interrupt transfers are allowed by USB
specs and a zero-payload endpoint is probably legal per xHCI, but then
submitting non-empty URBs to it is not.

> To address the issue, return max(wBytesPerInterval, max_payload) when
> calculating bandwidth reservation.
> 
> Fixes: 9238f25d5d32 ("USB: xhci: properly set endpoint context fields for periodic eps.")
> Cc: <stable@xxxxxxxxxx>
> Signed-off-by: Tao Xue <xuetao09@xxxxxxxxxx>
> ---
>  drivers/usb/core/usb.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/core/usb.c b/drivers/usb/core/usb.c
> index e9a10a33534c..8f2e05a5a015 100644
> --- a/drivers/usb/core/usb.c
> +++ b/drivers/usb/core/usb.c
> @@ -1125,6 +1125,8 @@ EXPORT_SYMBOL_GPL(usb_free_noncoherent);
>  u32 usb_endpoint_max_periodic_payload(struct usb_device *udev,
>  				      const struct usb_host_endpoint *ep)
>  {
> +	u32 max_payload;
> +
>  	if (!usb_endpoint_xfer_isoc(&ep->desc) &&
>  	    !usb_endpoint_xfer_int(&ep->desc))
>  		return 0;
> @@ -1135,7 +1137,12 @@ u32 usb_endpoint_max_periodic_payload(struct usb_device *udev,
>  			return le32_to_cpu(ep->ssp_isoc_ep_comp.dwBytesPerInterval);
>  		fallthrough;
>  	case USB_SPEED_SUPER:
> -		return le16_to_cpu(ep->ss_ep_comp.wBytesPerInterval);
> +		max_payload = usb_endpoint_maxp(&ep->desc) * (ep->ss_ep_comp.bMaxBurst + 1);
> +		if (usb_endpoint_xfer_isoc(&ep->desc))
> +			return max_t(u32, max_payload * USB_SS_MULT(ep->ss_ep_comp.bmAttributes),
> +					ep->ss_ep_comp.wBytesPerInterval);
> +		else
> +			return max_t(u32, max_payload, ep->ss_ep_comp.wBytesPerInterval);

Obviously a kludge is necessary here to make these abominable devices
work reliably with xHCI, but OTOH exceeding wBytesPerInterval violates
USB3 9.6.7 and it's unclear if all devices would be happy.

There are devices which define such odd isochronous alt settings with
apparent intent to allow fine-grained bandwidth reservation:

        wMaxPacketSize     0x0400  1x 1024 bytes
        bInterval               1
        bMaxBurst               0
        wBytesPerInterval     512

        wMaxPacketSize     0x0400  1x 1024 bytes
        bInterval               1
        bMaxBurst               0
        wBytesPerInterval    1024

        wMaxPacketSize     0x0400  1x 1024 bytes
        bInterval               1
        bMaxBurst               1  # 2 packets per interval
        wBytesPerInterval    1536

Isochronous drivers use this function to size their URBs or select the
right altsetting for given bandwidth. UVC has obeyed wBytesPerInterval
since forever with no apparent issues and UAC has recently been patched
to work like that too with no issues so far AFAIK.

Maybe start with something specific to the known bogus hardware, i.e.
interrupt endpoint with one packet and zero payload? In such case
it's high chance that the device actually meant it to be wMaxPacket.

>  	default:
>  		if (usb_endpoint_is_hs_isoc_double(udev, ep))
>  			return le32_to_cpu(ep->eusb2_isoc_ep_comp.dwBytesPerInterval);
> -- 
> 2.17.1
>