Re: [PATCH v3] usb: core: hub: Re-guess ep0 maxpacket size after reset

[Michal Pecio <michal.pecio@gmail.com>]

On Mon, 9 Nov 2026 11:05:30 -0500, Alan Stern wrote:
> > diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
> > index 199d3f535a89..d86bbba1457c 100644
> > --- a/drivers/usb/core/hub.c
> > +++ b/drivers/usb/core/hub.c
> > @@ -4969,31 +4969,29 @@ hub_port_init(struct usb_hub *hub, struct usb_device *udev, int port1,
> >  	}
> >  	oldspeed = udev->speed;
> >  
> > -	if (initial) {
> > -		/* USB 2.0 section 5.5.3 talks about ep0 maxpacket ...
> > -		 * it's fixed size except for full speed devices.
> > +	/* USB 2.0 section 5.5.3 talks about ep0 maxpacket ...
> > +	 * it's fixed size except for full speed devices.
> > +	 */
> > +	switch (udev->speed) {
> > +	case USB_SPEED_SUPER_PLUS:
> > +	case USB_SPEED_SUPER:
> > +		udev->ep0.desc.wMaxPacketSize = cpu_to_le16(512);
> > +		break;
> > +	case USB_SPEED_HIGH:		/* fixed at 64 */
> > +		udev->ep0.desc.wMaxPacketSize = cpu_to_le16(64);
> > +		break;
> > +	case USB_SPEED_FULL:		/* 8, 16, 32, or 64 */
> > +		/* to determine the ep0 maxpacket size, try to read
> > +		 * the device descriptor to get bMaxPacketSize0 and
> > +		 * then correct our initial guess.
> >  		 */
> > -		switch (udev->speed) {
> > -		case USB_SPEED_SUPER_PLUS:
> > -		case USB_SPEED_SUPER:
> > -			udev->ep0.desc.wMaxPacketSize = cpu_to_le16(512);
> > -			break;
> > -		case USB_SPEED_HIGH:		/* fixed at 64 */
> > -			udev->ep0.desc.wMaxPacketSize = cpu_to_le16(64);
> > -			break;
> > -		case USB_SPEED_FULL:		/* 8, 16, 32, or 64 */
> > -			/* to determine the ep0 maxpacket size, try to read
> > -			 * the device descriptor to get bMaxPacketSize0 and
> > -			 * then correct our initial guess.
> > -			 */
> > -			udev->ep0.desc.wMaxPacketSize = cpu_to_le16(64);
> > -			break;
> > -		case USB_SPEED_LOW:		/* fixed at 8 */
> > -			udev->ep0.desc.wMaxPacketSize = cpu_to_le16(8);
> > -			break;
> > -		default:
> > -			goto fail;
> > -		}
> > +		udev->ep0.desc.wMaxPacketSize = cpu_to_le16(64);
> > +		break;
> > +	case USB_SPEED_LOW:		/* fixed at 8 */
> > +		udev->ep0.desc.wMaxPacketSize = cpu_to_le16(8);
> > +		break;
> > +	default:
> > +		goto fail;
> >  	}  
> 
> The usb_ep0_reinit() call belongs here, right after 
> udev->ep0.desc.wMaxPacketSize is set.
> 
> >  
> >  	speed = usb_speed_string(udev->speed);
> > @@ -5062,6 +5060,9 @@ hub_port_init(struct usb_hub *hub, struct usb_device *udev, int port1,
> >  				goto fail;
> >  			}
> >  
> > +			/* Sync the guessed ep0 maxpacket size to hardware if needed */
> > +			usb_ep0_reinit(udev);  
> 
> Not here.  And not inside the retry loop -- ep0_reinit should be
> called only once since the maxpacket value is changed only once.

Sashiko pointed out that xhci-hcd may not perform ep0_reinit() properly
until address_device() or enable_device() is called.

So it's either this kludge, or dependency on an xhci-hcd patch which
didn't exist until today:

https://lore.kernel.org/linux-usb/20261109112846.04bfeb0f.michal.pecio@gmail.com/

Regards,
Michal