linux-kernel - Re: [PATCH] usb: core: allow zero packet flag for interrupt urbs

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Date:	Fri, 18 Jul 2014 10:39:41 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Amit Virdi <amit.virdi@...com>
cc:	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"rdunlap@...radead.org" <rdunlap@...radead.org>,
	"ming.lei@...onical.com" <ming.lei@...onical.com>,
	"hdegoede@...hat.com" <hdegoede@...hat.com>,
	"hsi-ss-sw-devel@...ts.codex.cro.st.com" 
	<hsi-ss-sw-devel@...ts.codex.cro.st.com>
Subject: Re: [PATCH] usb: core: allow zero packet flag for interrupt urbs

On Fri, 18 Jul 2014, Amit Virdi wrote:

> On 7/17/2014 8:25 PM, Alan Stern wrote:
> > I can't say this is actually wrong, but have you ever encountered a
> > situation where this would be needed?  How often does anyone need to do
> > a multi-packet transfer over an interrupt endpoint?
> 
> Honestly, I haven't found any such real device yet. I did this change 
> while I was going through the code when adding support for interrupt 
> transfers in gadget zero. I'm a no expert, but the spec says it should 
> be supported so this code should be added.

I just remembered, the HID spec requires that all reports (except the
longest) end with a short packet.  And since output reports are allowed
to be sent over an Interrupt-OUT endpoint, we have to accept the
URB_ZERO_PACKET flag for interrupt URBs.

> However, I messed up a little in this patch. It should have been
> ---
>          case USB_ENDPOINT_XFER_ISOC:
>                  allowed |= URB_ISO_ASAP;
>                  break;
> +       case USB_ENDPOINT_XFER_INT:
> +               if (is_out)
> +                       allowed |= URB_ZERO_PACKET;
> +               else
> +                       allowed |= URB_SHORT_NOT_OK;
> +               break;
>          }
>          allowed &= urb->transfer_flags;
> 
> ---
> Otherwise, it sets zero packet flag for control out transfers too.
> 
> I'll send a V2 with this change if you agree to setting of the zero 
> packet flag for interrupt transfers.

How about this change instead?

 			URB_FREE_BUFFER);
 	switch (xfertype) {
 	case USB_ENDPOINT_XFER_BULK:
+	case USB_ENDPOINT_XFER_INT:
 		if (is_out)
 			allowed |= URB_ZERO_PACKET;
 		/* FALLTHROUGH */

Alan Stern

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