On Wed, Mar 26, 2008 at 09:49:46AM -0700, mark gross wrote: > On Wed, Mar 26, 2008 at 09:27:28AM -0700, Greg KH wrote: > > On Wed, Mar 26, 2008 at 08:25:59AM -0700, mark gross wrote: > > > At home I'm playing with a real time amateur robotics application with a > > > micro controller talking over its serial port to a Linux box running an RT > > > kernel and some RT user mode software to suck in the serial port data > > > and do some processing in real time. One example of what I want to do > > > with this type of configuration is to create a type of IMU thing and run > > > the data through a kalman filter to see how good I can do with my > > > hardware. > > > > > > I'll be sending data regularly at data rates between 50 and 500 HZ from > > > the MCU (depending on mode) > > > > > > Anyway, I have the RT code and the system prototyped using the hardware > > > serial port on my desktop and I'm getting rock solid RT delivery times > > > of the data +/- 30usec. While stressing the system in CPU and disk. (I > > > still need to do the test while flood pinging the box) However; when I > > > use my USB dongle instead of the hardware serial port the system falls > > > over WRT real time behavior. > > > > > > I'm just starting to look into the behavior now but has anyone looked at > > > the RT'ness of USB2Serial + USB stack yet? I would like to see what > > > other folks have done. My googleing last night didn't get any useful > > > hits. > > > > I have, and others have in the past, and it's something that you can not > > really do. USB is not "deterministic", and these cheap usb to serial > > devices introduce a very big lag that also is not deterministic. > > > > I'm seeing some of that today :( > > > I wouldn't recommend using USB for anything that you need to be able to > > accuratly control the latency of, it's just not going to work as that is > > not what it was designed for at all. > > Oh, thats a drag. I know bulk has retries, and that on a clean bus they > don't happen (well as far as my hobby use they shouldn't, real > applications should use isochronous streams to avoid them.) > > What is the root cause for the non deterministic behavior? HW or USB > stack? The USB hardware. Go read the spec for the gory details, in short, nothing is guaranteed :( > Is there any reason to think that if I created my own isochronous > USB2Serial adapter and iso-usb-driver that I couldn't get determinism? I strongly doubt it as others have tried and failed in the past. sorry, greg k-h -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
