linux-kernel - Re: [PATCH] USB: g_file_storage: fix handling zero-length packet

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Date:	Sat, 6 Sep 2008 16:27:17 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	sangsu <sangsu@...il.com>
cc:	dbrownell@...rs.sourceforge.net, <gregkh@...e.de>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] USB: g_file_storage: fix handling zero-length packet 

On Sat, 6 Sep 2008, sangsu wrote:

> According to "CH 5.5.3. Control Transfer Packet Size Constraints",
> Zero-length packet should be sent when the last data payload size
> is equal to the endpoint's MaxPacketSize.

Not so.  5.5.3 actually says this:

	The Data stage of a control transfer from an endpoint to the
	host is complete when the endpoint does one of the following:

	 - Has transferred exactly the amount of data specified during 
	the Setup stage

	 - Transfers a packet with a payload size less than
	wMaxPacketSize or transfers a zero-length packet

This clearly indicates that if the amount of data specified in the
Setup stage is a multiple of the wMaxPacketSize then there should not
be a zero-length packet, because the transfer ends as soon as the last 
wMaxPacketSize packet is sent.

> This patch helps
> g_file_storage send zero-length packet properly.
> 
> This is tested in s3c2440/s5c7329 architectures and works well.
> 
> Signed-off-by: SangSu Park<sangsu@...il.com>
> ---
> driver/usb/gadget/file_storage.c | 3++-
> 1 files changed, 2 insertion(+), 1 deletions(-)
> 
> --- a/drivers/usb/gadget/file_storage.c	2008-09-01 11:13:03.000000000 -0400
> +++ b/drivers/usb/gadget/file_storage.c	2008-09-01 11:16:41.000000000 -0400
> @@ -1463,7 +1463,8 @@
> 	if (rc >= 0 && rc != DELAYED_STATUS) {
> 		rc = min(rc, w_length);
> 		fsg->ep0req->length = rc;
> -		fsg->ep0req->zero = rc < w_length;
> +		fsg->ep0req->zero = rc < w_length
> +			&& (rc % gadget->ep0->maxpacket) == 0;
> 		fsg->ep0req_name = (ctrl->bRequestType & USB_DIR_IN ?
> 				"ep0-in" : "ep0-out");
> 		rc = ep0_queue(fsg);

Firstly, this patch does not do what your description says it does.

Secondly, the patch is not needed.  g_file_storage doesn't have to 
check whether the data size is a multiple of MaxPacketSize because the 
device controller driver already is supposed to make that check.

Alan Stern

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