Hi,

I'm writing an application that sends a lot of short (about 10 bytes each) frames over serial line (there would be a maximum of 300 frames/second), and after each query it collects the answer of similar length.
Asked devices are replying almost immediately (about 1 ms max).
However, despite the baud rate (the same is in 19200 bps and 230400 bps), there's always a ~20ms latency between the end of write() and first received bytes.
When using tcdrain() all this delay is exactly on this function, and when I don't use tcdrain, but start waiting for answer immediately after write(), the delay also occurs.
I've tried waiting for answer with both select() syscall and looped non-blocking read()s.
The second method with gettimeofday() measuring also revealed that linux doesn't switch the task:
write() takes about 200µs, and subsequent reads always take 0 or 30 µs, and until there's some bytes received, all this numbers sum up to the total of about ~20ms.
So I suppose that the latency is because of serial driver but in kernel sources I can't find anything that may be connected with this.

Do you have any ideas how to minimize this latency on 2.6.14 linux kernel?

I'm using Artila M501 with AT91RM9200 @180 MHz, internal AT91 USART.

Thanks,
Adrian Matoga