Experimental Physics and
Industrial Control System

Ralph Lange <[email protected]> · Tue, 26 May 2015 14:46:01 +0200

Hi Florian,

Yet another thing to keep in mind is the level at which you want locking

of things to happen.

Usually (in the blocking case), a port driver locks its port, does the

IO, then unlocks its port.

Some years ago at BESSY we were thinking how our CAN-Bus driver would

fit the ASYN model. CANopen compliant, i.e. many nodes per bus (~30),

many variables per node (~15), and some types of indexed variables with

many values per variable (~120).

The only thing that we wanted to be locked (between request and reply)

was the variable, to avoid sending commands on two different indexes of

the same variable without waiting for an answer.

Something like that does not fit the ASYN model well. For each fully

populated bus segment you would basically end up with ~450 instances of

a blocking "variable" port driver, all talking to the same non-blocking

lower level "bus" port driver. Starting 450 threads per bus made this

look very unattractive.

In the end we kept our patchwork non-ASYN driver, as (for this and other

reasons) the rewriting effort was not justified well enough.

~Ralph