The big problem is that the kernel does not know what is a "transmission"
it just sees a series of writes to the device. In many cases the
multi-drop or radio systems also need the caller to wait for a
transmission slot either by beacon, by timing or by monitoring the
carrier detect to avoid collisions.
I believe that he is referring to something much simpler than you are
thinking. He is talking about 2-wire RS485 transmissions in which the
RTS signal is used to enable the driver chips just before transmitting
data and disabling (tri-stating) them just after transmitting data.
There are some UARTs that have this behavior built into them.
That seems to best be done in user space as the algorithms are quite
variable and some are complex.
Do we really benefit from having this in kernel ?
The problem that can come up when executing this feature in user-land
(though not exactly common) is when the hardware on the other end
responds to your message faster than your user app was able to detect
that the UART is finished and then toggle RTS. When this happens both
ends are trying to drive the line and you have bus contention, lost data
and possibly damage to the driver chips themselves.
Matt Schulte
Commtech, Inc.
http://www.commtech-fastcom.com
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