2007/5/6, Alan Cox <[EMAIL PROTECTED]>: > > However, whatever policy the buffer uses, the fundamental point it's that > > when I flush the input buffer I should be sure that each byte read > > after the flush is *new* (current) data and not old one. This because > > Define "new" and "old" in this case. I don't believe you can give a > precise definition or that such a thing is physically possible. >
You are right. In the case of buffers storing the *last* N bytes received we always get "new" data. To be precise if we get a byte each T seconds we can read data N*T seconds old (worst case). This is a bound, known, and perfectly acceptable jitter. Let see instead the case a not flushable buffer in the chain (inside the PC) uses the policy of storing the *first* N bytes received. That is the case for the flip buffer in mainline. In this case we can read bytes also one hour old, if we read the first byte one hour after the external device begun to send data. This is an unbound, unknown and thus unacceptable error in a measurement (think if we were acquiring a physical "realtime" sampled quantity). That was exactly my original problem causing data corruption in case of multi-byte data. Paul Fulghum patches fix this problem for serial port and for the cdc-acm driver making the "secondary buffers" flushable. So, in general, all the buffers (in the host) that store the first received N bytes (as policy) should be flushable in order to don't read (potentially very) "old" data after we made a flush. I understand that some internal HW buffer can be not flushable via software, but in this case I doubt the buffer use the store-the-first-N-bytes policy. And even if some particular device does this, than it's *its* fault, not linux. To conclude, the store the *last* N bytes received seems a more reasonable policy for the input buffers managed by the kernel. If the other policy is used (as tty does), then the kernel should flush all the buffers he *can* physically flush, remaining inside the host computer of course. Hope my English is sufficiently clear. Regards, ~ Antonio ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ Linux-usb-users@lists.sourceforge.net To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-users
