Re: r8712u driver - on ARM

[Christian Lamparter <chunkeey@googlemail.com>]

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