1. November 16, 2012 Gary Stringham gary@garystringham.com
Writing Reusable C Code for Embedded Systems Reusable Code Tests & Concluding Remarks
November 16, 2012 Gary Stringham

2. Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Review of Yesterday
Flexible reusable code:
Compile-, build-, and run-time switches
Self-adapting code
Eliminate the need
Use comments liberally
SysLevelTestDebugPt1.pptx

3. Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Today’s Topics
Testing reusable code
Reusable insights
Review
SysLevelTestDebugPt1.pptx

4. Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Testing Reusable Code
The same thoroughness used to test code for one product applies in testing reusable code for many products
Don’t add support for a switch setting without testing it
Test all switch settings and boundary conditions
Use good engineering judgment to determine an appropriate mix of combinations and permutations of switch settings
SysLevelTestDebugPt1.pptx

5. Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Test the CPP Checks
Here is an example check from Tuesday
Run a test with SCAN_SCALAR undefined
Run a test with FEATURE_A defined and not FEATURE_B
#ifndef SCAN_SCALAR
# error SCAN_SCALAR left undefined
#endif
#if defined(FEATURE_A) && !defined(FEATURE_B)
# error Feature B should be defined when A is
SysLevelTestDebugPt1.pptx

6. Test the Switches On and Off
Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Test the Switches On and Off
Make sure the code is tested with switches turned on and with switches turned off
Helps ensure that the correct pieces of code were compiled in or out as appropriate
#if FEATURE_X == MY_TRUE
// Some code
#elif FEATURE_X == MY_FALSE
// Other code
#else
# error Bad CPP coding
#endif
SysLevelTestDebugPt1.pptx

7. Testing Switch Combinations
Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Testing Switch Combinations
Features A and B are somehow related, so test all four combinations of their on and off settings
But testing the FEATURE_A’s on and off in combination with each of many possible integers for SCAN_SCALAR is probably not necessary
#ifndef SCAN_SCALAR
# error SCAN_SCALAR left undefined
#endif
#if defined(FEATURE_A) && !defined(FEATURE_B)
# error Feature B should be defined when A is
SysLevelTestDebugPt1.pptx

8. Test Features Switches
Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Test Features Switches
Test features, not products (yet)
Thoroughly test the feature
Test features’ dependency switches
Test feature A on hardware X
Test feature A on hardware Y (supported?)
These are similar to unit-level tests
Feature-level tests apply for all products
A feature modified for a product needs modified feature-level tests
SysLevelTestDebugPt1.pptx

9. Test Setup Switching Logic
Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Test Setup Switching Logic
Product and hardware switches exist only at the “beginning” and turn on appropriate features
Feature switches exist throughout code
Test to ensure appropriate feature switch setup
Use #warn or #warning if compilers support it
SysLevelTestDebugPt1.pptx

10. Test Setup Switching Logic
Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Test Setup Switching Logic
#if defined (PROD_GOOFY)
# warn Setting up product Goofy
# define SCAN_SCALAR 3
#elif defined (PROD_SMILEY)
# warn Setting up product Smiley
# define SCAN_SCALAR 2
# define FEATURE_MAILBOX
#endif
#ifdef FEATURE_MAILBOX
# warn FEATURE_MAILBOX turned on
#else
# warn FEATURE_MAILBOX turned off
#error End of Switching Test – Remove when done
SysLevelTestDebugPt1.pptx

11. Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Testing Products
After thoroughly testing features, then configure for a product and run tests
These are system-level tests
When a new product comes along, adjust the switches as necessary then conduct system-level tests for the new product
If a new product requires a modified feature, the feature-level test should be modified and passing before product system-level tests are run
SysLevelTestDebugPt1.pptx

12. Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Reusable Insights
Own the same code for at least three products
Collaborate with hardware engineers
SysLevelTestDebugPt1.pptx

13. Own the Same Code for Three Products
Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Own the Same Code for Three Products
You don’t know what you need the first time you own the (new or existing) code
By the third product, you know where changes are required and are motivated to make it easier
Work yourself out of the porting job
There’s no motivation to improve the code if it is written once and thrown away
Document the porting steps so the next owner can continue
SysLevelTestDebugPt1.pptx

14. Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
We’re Not Stupid
“… a module isn’t really reusable till it has been reused three times. … That’s not because we’re stupid; it’s simply because domain analysis is hard.” Jack Ganssle, The Embedded Muse, #167
SysLevelTestDebugPt1.pptx

15. Collaborate with Hardware Engineers
Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Collaborate with Hardware Engineers
Work with the hardware team in their designs
Minimize changes, reducing switches required
Keep bits and registers in the same location
Use version numbers at the chip and block level
Practice these especially with FPGAs that change frequently
Work on eliminating product-specific switches
This effort takes time, years
SysLevelTestDebugPt1.pptx

16. Book: Hardware/Firmware Interface Design
Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Book: Hardware/Firmware Interface Design
garystringham.com/hwfwbook
How to design hardware to ease firmware development
Many concepts support reusable code
SysLevelTestDebugPt1.pptx

17. Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Conclusion
Use switches to allow the same code to support multiple products
Use what makes sense for your application and environment
Note the pitfall areas and avoid them
It is an evolutionary process that takes time
But it is worth it
Best-case examples of porting my code:
If I do it, 1 hour
If others do it, 4 hours
SysLevelTestDebugPt1.pptx

18. Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Further Reading
The Embedded Muse, Jack Ganssle, Issues 167, 168, and 170
Lessons Learned on Five Large-Scale System Developments, section titled “Excessive Faith in Code Reuse,” Nat Ozarin, IEEE Instrumentation & Measurement Magazine, Feb, 2008, pgs 18-23
One at a Time, Please, Stephen C. Dewhurst, semantics.org/publications/01_08_oneatatime.pdf
SysLevelTestDebugPt1.pptx

19. Remember…
Proper application of common and flexible techniques produces effective reusable code

20. Introduction: What You Need to Know about System Testing & Debugging
3/19/2012
Questions?
SysLevelTestDebugPt1.pptx