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

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Message-ID: <Pine.LNX.4.44L0.1407171053330.1517-100000@iolanthe.rowland.org>
Date:	Thu, 17 Jul 2014 10:55:32 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Amit Virdi <amit.virdi@...com>
cc:	linux-usb@...r.kernel.org, <linux-kernel@...r.kernel.org>,
	<gregkh@...uxfoundation.org>, <rdunlap@...radead.org>,
	<ming.lei@...onical.com>, <hdegoede@...hat.com>,
	<hsi-ss-sw-devel@...ts.codex.cro.st.com>
Subject: Re: [PATCH] usb: core: allow zero packet flag for interrupt urbs

On Thu, 17 Jul 2014, Amit Virdi wrote:

> Section 4.4.7.2 of the USB3.0 spec says:
> 	A zero-length data payload is a valid transfer and may be useful for
> 	some implementations.
> 
> So, extend the logic of allowing URB_ZERO_PACKET to interrupt urbs too.
> Otherwise, the kernel throws error of BOGUS transfer flags.
> 
> Signed-off-by: Amit Virdi <amit.virdi@...com>
> ---
>  drivers/usb/core/urb.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/usb/core/urb.c b/drivers/usb/core/urb.c
> index 991386c..a136246 100644
> --- a/drivers/usb/core/urb.c
> +++ b/drivers/usb/core/urb.c
> @@ -460,6 +460,10 @@ int usb_submit_urb(struct urb *urb, gfp_t mem_flags)
>  	case USB_ENDPOINT_XFER_CONTROL:
>  		allowed |= URB_NO_FSBR;	/* only affects UHCI */
>  		/* FALLTHROUGH */
> +	case USB_ENDPOINT_XFER_INT:
> +		if (is_out)
> +			allowed |= URB_ZERO_PACKET;
> +		/* FALLTHROUGH */
>  	default:			/* all non-iso endpoints */
>  		if (!is_out)
>  			allowed |= URB_SHORT_NOT_OK;

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?

Alan Stern

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