wiki:Developer/Projects/SequencedInitialization

Version 7 (modified by Sh, on May 11, 2010 at 12:40:11 PM) (diff)

/* Work Breakdown */

RTEMSSequencedInitialization

Table of Contents

    Error: Page Projects/SequencedInitialization does not exist

Status: ChrisJohns? has filed PR1253 with a prototype.

Mentor: Chris Johns and Sebastian Huber

Student: Wenjie Zhang

Note: This project was called RTEMS Initialization By Constructor.

Overview

This project adds to RTEMS a Sequencer that calls user defined functions held in an unordered table in a specific order. An example of its use is the RTEMS Managers initialization. The initialisation of the RTEMS Managers is a specific ordered sequence of calls currently hard coded into RTEMS. The sequencing code will take a table of nodes that is un-order, determine the order and make calls to the user provided functions which in this case is the manager initialisation calls.

The solution should be general enough to support RTEMS managers, drivers and BSD sysctl type nodes. The result of this work is expected to impact the TinyRTEMS effort as it would provide a central mechanism to automatically eliminate unused code. If you don't reference a part of the RTEMS API the initialization code would automatically drop out. The dropping of unused code is critical to both the TinyRTEMS effort and to those woh use RTEMS in safety-critical systems which must be validated. The automatic part here refers to the creation of the table of nodes.

A prototype of the work can be found in PR1253. This work combines the call sequencing with the automatic generation of the table of nodes. The solution is not suitable as it uses the word "constructor" which we have since decided should not be used, as well as too much RAM. The ideal solution should use little if any RAM and all nodes and tables be held in ROM type memory. A typical small foot print CPU which would use a TinyRTEMS solution has much more ROM than RAM. Also this is a once off initialization procedure and so using RAM means it is gone from application use.

The automatic table generation uses the same linker technique used to create C++ static object constructor and destructor tables. Consider the RTEMS Message API. In the Message Manager's initialization call's source file you create a {{{static const</code> structure with the specific fields needed by the sequencer code then reference this variable from the source file containing the {{{rtems_message_queue_create</code> code. The application must make a call to {{{rtems_message_queue_create</code> or all the other Message API calls will fail therefore this API needs only one reference from a single source file. The application call of {{{rtems_message_queue_create</code> will pull in its code and as this code references the node in the initialization call's file it is also pulled in along with the initialization code. We also create a variable that is a pointer to the sequencer node and this is placed in a special section using the GCC "attribute" modifier. The linker command file for each BSP must be modified to group all these pointers together in one location in ROM. This creates the table that is passed to the sequencer code.

The sequencer code iterates over the table calling entries in the order specified. The order could be a number or it could be relative. The relative order design makes for a more robust system because you have moved away from specific numbers. The idea here is to allow high level ordering operators, for example "first", "last", "after", "before", "just after", "just before", etc. You can then say "message" is "after" "heap" and the order is determined at runtime. Most system parts are relative not absolute. It may even be possible to allow relative and absolute ordering to be mixed. Priorities are similar. At the end of the day we do not need 256 levels if we only use 4. Typically all we need is to say is this task is higher or lower than another task. Sequencing ca viewed the same way.

Work Breakdown

This project tasks are: # Develop the sequencer code and API with documentation and tests. # Cleanup the linker command files on a per architecture basis to use a general purpose base linker command file and a board specific linker command file. # Update all linker command files to handle the initialization automatic table entries. # Update the RTEMS Classic API, POSIX API and ITRON API initialization calls with the sequencer implementation using automatic table generation. This also includes removal of the hard coded call with a sequencer call. # Initial testing can all be done on SPARC/sis. The modification of other BSP linkcmds should be done with caution but mistakes will be caught by Test Builds of RTEMS by JoelSherrill which include all BSP configurations.

Code and data size measurements should be made before and after the implementation is done on the sparc/sis and arm/rtl22xx_t BSPs. These BSPs are used for code size analysis across releases.

Implementation Alternatives

Definitions

; Input Section : Collection of input items identified by a section name for the linker. ; Output Section : Output part of the linker. ; Linker Set : An input section that contains only identical items with respect to size and layout.

Sequenced initialization requires support by the linker. The general approach is to put certain items identified by an input section into a special output section and mark the begin and end of this section.

FreeBSD Approach

FreeBSD uses a fixed linker set which contains arbitrarily ordered system initialization items. Each item has an order field which is used to sort the items during runtime. The initialization functions will be call with respect to this order.

  • [PRO] The order information is stored in the item and can evaluated on a per item basis.
  • [PRO] Flexible order granularity.
  • [CON] The order information requires space.
  • [CON] Sorting at runtime requires space and time.

Linux Approach

Linux uses several linker sets which contain arbitrarily ordered system initialization items. The linker sets have a hard coded order defined in the linker command file.

  • [PRO] No space overhead for the order information.
  • [PRO] No runtime and space overhead to sort the items.
  • [CON] Very limited order granularity.
  • [CON] Relative huge code section in the linker command file. Hard to maintain.
  • [CON] Order information must be derived from the place in the linker sets. It is impossible to derive the order value specified at compile time.

RTEMS Approach

RTEMS should use a fixed linker set which contains ordered system initialization items. The order will be defined by the input section name. The GNU linker is able to sort input sections by name.

  • [PRO] No space overhead for the order information.
  • [PRO] No runtime and space overhead to sort the items.
  • [PRO] Flexible order granularity.
  • [CON] Depends on GNU linker feature.
  • [CON] Order information must be derived from the place in the linker set. It is impossible to derive the order value specified at compile time.

Open Projects

Implement this.