linux-kernel - RE: query on DWC3

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Date:	Sat, 13 Dec 2014 23:51:41 +0000
From:	Paul Zimmerman <Paul.Zimmerman@...opsys.com>
To:	sundeep subbaraya <sundeep.lkml@...il.com>,
	"balbi@...com" <balbi@...com>
CC:	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: query on DWC3

> From: linux-usb-owner@...r.kernel.org [mailto:linux-usb-owner@...r.kernel.org] On Behalf Of sundeep subbaraya
> Sent: Friday, December 12, 2014 9:13 PM
> 
> Hi Felipe,
> 
> In DWC3 driver, for three stage Control OUT transfer there is a check:
> 
>    else if (!IS_ALIGNED(req->request.length, dep->endpoint.maxpacket)
> 
>              && (dep->number == 0)) {.
>     }
> 
> I understand that we check for alignment of sizes and if not we
> prepare trb with maxpacket and enable interrupt on short packet. In
> databook I could not find anything related to this, it talks only
> about addresses should be aligned. In Control transfer programming
> model there is no difference between Control OUT or IN transfer, if
> three stage transfer - prepare trb with length wLength. Initially I
> followed this and not able to receive data on EP0 OUT.:( .After adding
> the above check it is working. Please help me to understand why we do
> this?

Hi Sundeep,

Please see section 8.2.3.3 "Buffer Size Rules and Zero-Length Packets"
in the databook:

	For OUT endpoints, the following rules apply:

	- The total size of a Buffer Descriptor must be a multiple of
	  MaxPacketSize

The wording may be a little confusing, it actually means that the size
of the data buffer for OUT endpoints must be a multiple of MaxPacketSize.
Section 8.2.5 states it more clearly:

	- An OUT transfer’s transfer size (Total TRB buffer allocation)
	  must be a multiple of MaxPacketSize even if software is expecting
        a fixed non-multiple of MaxPacketSize transfer from the Host.

This rule applies to all OUT endpoint types, including Control endpoints.

-- 
Paul