Clarification, I don't have the RT patch applied, just CONFIG_PREEMPT from Linus's kernel. Reviewing my notes from several years ago, I ran into a similar issue and the RT patch fixed it, so now to figure out the best way to get a recent RT patched ARM kernel ... Cliff On Mon, Mar 9, 2009 at 5:34 PM, Cliff Brake <cliff.brake@xxxxxxxxx> wrote: > Hello, > > I have an application where I do the following: > > - PXA270 CPU > - communicating over serial ports on 40ms intervals > - communication scheme is very simple in that the linux system sends a > packet every 40ms and expects a response back within the 40ms window > - currently using 2.6.27 with CONFIG_PREEMPT > > If I set the application comm thread to real time priority, the > sending works very well. There is very little jitter in the 40ms send > timing. However, it seems that the system has trouble receiving the > response in a timely fashion. I often observe that it takes at least > 10ms for the application to receive the response data after the data > has appeared on the rs232 bus. Is the RT patch something that would > help speed up the serial receive response time, or is there something > else going on that I am missing? The system is fairly busy with > graphical processing, but the sending is always right on schedule. > > It seems that interrupts may be involved with this problem as when > sending, the send data is smaller than the FIFO size, and is probably > put into the fifo during the write, where with the read, an interrupt > needs to fire, etc. Does all the kernel code that handles the serial > data automatically run at the calling application's RT priority, or is > there additional work required to make everything that touches the > data RT? > > I'm also trying to figure out how well RT will work for CAN > communications. Once again we have the same problem -- a RT user > space application needs to send and receive data with strict timing. > Which, I can make a RT thread in user space run well (toggle a gpio > with very little jitter), how does this translate to all the kernel > components involved in the socketcan stack (networking, etc) that need > to touch the data? > > Thanks, > Cliff > > -- > ======================= > Cliff Brake > http://bec-systems.com > -- ======================= Cliff Brake http://bec-systems.com -- 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
