Aerospace & Defense Technology - June 2021 - 24

Digital Design
Irrelevant:
Architecture
Specification
Integration
The implementation has no features in common with the architecture specification
(so the question of conformance does not arise).
Consistent:
The implementation has some features in common with the architecture specification,
and those common features are implemented in accordance with the specification.
However, some features in the architecture specification are not implemented, and the
implementation has other features that are not covered by the specification.
Compliant:
Some features in the architecture specification are not implemented, but all features
implemented are covered by the specification, and in accordance with it.
Conformant:
All the features in the architecture specification are implemented in accordance with the
specification, but some more features are implemented that are not in accordance with it.
Fully Conformant:
There is full correspondence between architecture specification and implementation. All
specified features are implemented in accordance with the specification, and there are no
features implemented that are not covered by the specification.
Non-Conformant:
Any of the above in which some features in the architecture specification are implemented
not in accordance with the specification.
of any changes to the base package,
which should be introduced only
through a managed change-control
process.
The traditional application of formal
test tools is illustrated in the ISO 26262
'V' model diagram. A team facing the
task of building a standards-compliant
application on an open-source operating
system will be required to follow a
more pragmatic approach not only because
the code already exists, but because
the code itself may well be the
only available detailed " design document. "
In
general, for a library or a driver,
that might be a practical proposition.
But for a safety-critical application, will
such a development team really want to
spend their time reverse engineering an
operating system?
Figure 3. The Open Group's illustration of architecture compliance and conformance.
A Pragmatic Solution for Today
In a proof-of-concept development,
the use of Linux in some form or another
is almost inevitable. What happens
after that depends primarily on
timescales. How long will the development
take? And when will ELISA be
ready?
Figure 4. Impact analysis of changing requirements with the LDRA tool suite.
and incrementally, within a missioncritical
project. However, this does not
mean that OSS cannot be used at all.
Linux is an open-source package no
different from any other externally de24
Cov
veloped
software package (e.g. a library
or driver) in that it should be considered
as Software of Unknown Pedigree
(or SOUP) with a full V&V process applied
to it-and reapplied in the event
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ToC
Portability is one of the major advantages
of any Portable Operation
System Interface (POSIX) compliant
development. Even when application
development moves from a sandbox
" hack it and see " approach to a formally
documented, compliant development
lifecycle, that makes it practical
for even an extended development
team to continue to leverage Linux.
This includes performing all the verification
and validation activity demanded
by the functional safety standard
of choice, including re quire ments
tracing, the applications of coding
standards, unit test (Figure 2), and
structural coverage analysis.
Eventually the product has to be
readied for market. If ELISA is deployed
by then, the road ahead is clear. If not,
there are several POSIX conformant,
commercially available RTOS such as
those from QNX and Lynx that are certified
for use. In principle, it is then simple
to recompile the application with a
single licence for a commercial RTOS of
choice, re-run the dynamic analysis
Aerospace & Defense Technology, June 2021
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Aerospace & Defense Technology - June 2021

Table of Contents for the Digital Edition of Aerospace & Defense Technology - June 2021
