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 -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
