Return-path: <linux-wireless-owner@vger.kernel.org>
Received: from mail-fx0-f46.google.com ([209.85.161.46]:64777 "EHLO
	mail-fx0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1756646Ab1I3TGQ (ORCPT
	<rfc822;linux-wireless@vger.kernel.org>);
	Fri, 30 Sep 2011 15:06:16 -0400
Received: by fxe4 with SMTP id 4so3234749fxe.19
        for <linux-wireless@vger.kernel.org>; Fri, 30 Sep 2011 12:06:15 -0700 (PDT)
From: Christian Lamparter <chunkeey@googlemail.com>
To: Ian Jeffray <ian@emobix.co.uk>
Subject: Re: r8712u driver - on ARM
Date: Fri, 30 Sep 2011 21:06:11 +0200
Cc: linux-wireless@vger.kernel.org
References: <4E81ED65.7060703@emobix.co.uk> <4E81F8C8.4010909@lwfinger.net> <4E860CDB.9040207@emobix.co.uk>
In-Reply-To: <4E860CDB.9040207@emobix.co.uk>
MIME-Version: 1.0
Content-Type: Text/Plain;
  charset="iso-8859-1"
Message-Id: <201109302106.12028.chunkeey@googlemail.com> (sfid-20110930_210620_271506_96988452)
Sender: linux-wireless-owner@vger.kernel.org
List-ID: <linux-wireless.vger.kernel.org>

On Friday 30 September 2011 20:39:23 Ian Jeffray wrote:
> On 27/09/2011 17:24, Larry Finger wrote:
> > On 09/27/2011 10:36 AM, Ian Jeffray wrote:
> >> Dear all,
> >>
> >> I'm attempting to get some sense from a Realtek 8191S device on
> >> ARM linux.
> <SNIP>
> >> My host is a TI DaVinci DM8168 (OMAP3+toys). The USB host is not
> >> fantastic, but has been thrashed heavily with other devices and seems
> >> to be reliable in those cases.
> <SNIP>
> >
> > I have not tried this chip on an ARM host as I have no hardware. My
> > testing has been with X86 and PPC - thus I know the endianess is OK, but
> > that is the extent of my platform testing.
> >
> > The first thing I would try is wireshark on another host to verify that
> > packets are actually getting on the air.
> 
> Ok, so I've spent more more days working on this.  The driver appears
> to be fine on an ARM9 S3X board.   I've contacted the TI support
> team about the USB host and applied some patches from them which have
> slightly improved the situation, but it's still horrible - I'm actually
> getting "reliable" scans but only a pathetic 1Mbit or less of data
> throughput.
> 
> I've also tried this driver (and the realtek upstream version) with
> a Blackfin BF527 device, which has roughly the same 'musb' Inventra
> USB host controller... and I see exactly the same problem there!
> The firmware appears to load fine, but scans produce no results.
> 
> I'm not really sure about your comment about wireshark because,
> as far as I understand, a 'iwlist wlan0 scan' command doesn't cause
> any traffic which would be wireshark capturable on another host...
> am I misunderstanding something?
> 
> So it appears as though there may be some general problem with this
> driver and an 'musb' host controller.  Very strange.
> 
> My next course of attack is to try comparing outputs of 'usbmon'
> dumps between hosts which work and hosts which don't... but I can't
> make a great deal of sense looking at them really.
> 
> I'd greatly appreciate any more advice you may be able to give.

Well, I'm not really in r8712u but I can tell you a few things about
carl9170 and musb.

You see I had a similar discussion a while ago:
http://www.spinics.net/lists/linux-wireless/msg68870.html

Apparently, musb can't do DMA with non 4-bytes aligned buffers
and falls back to PIO [which results in a much higher cpu utilization
and really bad usb throughput... so if timing is critical this "fact"
is pretty much a deal-breaker for the combination].

Now, you might ask, why this DMA vs. PIO is so importent... Well,
it's because the network stack has its own alignment ideas and
about the data/skbs. There's a comment in include/linux/skbuff.h
around line 1400 which explains the situation. The upshot is you
can choose between a penatly on the packet processing by the
stack, or a penatly on the driver/hardware site if r8712u does
not implement any additional techniques to counter the issues.

Regards,
	Chr