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Message ID: 289     Entry time: 07 Aug 2006
Author: Stefan Ritt 
Topic: Info 
Subject: New multi-threaded midas slow control system 
Multi-threaded slow control system

The Midas slow control system has been modified to support multi-threaded slow control front-ends. Each device gets it's own thread in the front-end, which has several advantages:

- the communication of all devices runs in parallel and therefor is much faster
- slow devices cannot block any more the front-end. Response times to run transitions etc. become therefore much faster.

This modification requires some minor modifications in the existing class and device drivers.

Dropping of CMD_xxx_ALL commands

The slow control commands CMD_SET_ALL, CMD_GET_ALL, CMD_SET_CURRENT_LIMIT_ALL, CMD_GET_CURRENT_LIMIT_ALL, etc. have been dropped. They were there to accomodate some slow devices, which sometimes works a bit faster if all channels are set or read at once. Since the inter-thread communication scheme implemented now does only allow passing one channel at a time, the "ALL" functions cannot be supported any more. On the other hand this is not such an issue any more, since slow devices are handled now in parallel, speeding up things considereably.

The command have been removed from midas.h and from all device and class drivers coming with the midas distribution. If you have your own drivers, just delete the sections wich use these commands.

Calling the device driver inside the class driver

The device drivers have now to be called differently in the class driver. The reason for that is that in a multi-threaded front-end, there is only one central device driver dispatcher, which communicates with the individual device driver threads. The device drivers do not need to be modified, but all existing class drivers need modification, if they are going to be run in a multi-threaded front-end. Old class drivers which are not used in a multi-threaded front-end do not to be modified.

Following modifications are necessary:

  • Remove following line:
    #define DRIVER(_i) ...

  • Find all lines containing
    DRIVER(i)(CMD_xxx, info->dd_info[i], ...)

    and replace them with
    device_driver(info->driver[i], CMD_xxx, ...)

    note that info->dd_info[i] is not passed any more. Instead, you pass info->driver[i]. Pleae note that the arguments passed after CMD_xxx are not checked by the compiler, since they are a variable argument list. Any error there will not produce a compiler warning, but will just crash the front-end.

  • Find the line with
    status = pequipment->driver[i].dd(CMD_INIT, hKey, &pequipment->driver[i].dd_info,

    and replace it with
    status = device_driver(&pequipment->driver[i], CMD_INIT, hKey);

  • Find the line with
    pequipment->driver[i].dd(CMD_EXIT, pequipment->driver[i].dd_info);

    and replace it with
    device_driver(&pequipment->driver[i], CMD_EXIT);

  • Find following lines
    hv_info->driver[i] = pequipment->driver[index].dd;
    hv_info->dd_info[i] = pequipment->driver[index].dd_info;
    hv_info->channel_offset[i] = offset;
    hv_info->flags[i] = pequipment->driver[index].flags;

    and replace them with
    hv_info->driver[i] = &pequipment->driver[index];
    hv_info->channel_offset[i] = offset;

The class drivers multi.c and generic.c can be used as a reference for these modifications.

Implementing CMD_STOP command

For multithread-enabled device drivers it is necessary to support the CMD_STOP command, which is needed to stop all device threads before the actual device gets closed. Following code is necessary:
INT cd_xxx(INT cmd, EQUIPMENT * pequipment)
   INT i, status;

   switch (cmd) {
   case CMD_INIT:

   case CMD_STOP:
      for (i = 0; pequipment->driver[i].dd != NULL &&
                  pequipment->driver[i].flags & DF_MULTITHREAD ; i++)
         status = device_driver(&pequipment->driver[i], CMD_STOP);

   case CMD_IDLE:

   return status;

Enabling multi-thread support

To turn on multi-thread support for a device, the flag DF_MULTITHREAD must be used in the front-end user code device driver list, such as
DEVICE_DRIVER multi_driver[] = {
   {"Input", nulldev, 2, null, DF_INPUT | DF_MULTITHREAD},
   {"Output", nulldev, 2, null, DF_OUTPUT | DF_MULTITHREAD},
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