RE: query on DWC3

From: Paul Zimmerman
Date: Sat Dec 13 2014 - 18:51:50 EST

> From: linux-usb-owner@xxxxxxxxxxxxxxx [mailto:linux-usb-owner@xxxxxxxxxxxxxxx] 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 "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

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 transferâs 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.


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