Re: [SystemSafety] Separating critical software modules from non-critical software modules

From: Peter Bishop < >
Date: Wed, 24 Jul 2013 10:33:44 +0100

With no physical separation you need to be very sure that "non-safety" cannot affect safety (overwriting "safe" memory, crashing the system, hogging cpu, comms...)
In nuclear standards, anything in the same box has to be implemented to the level of the most critical function.

Peter Bishop

On 24 July 2013 09:29, Gerry R Creech <grcreech_at_xxxxxx

> Malcolm,
> I agree that for small devices it may be difficult to provide or prove
> separation between safety and non-safety and therefore all needs to be
> considered safety in these cases.
> However, I am also a firm believer that removing complexity increases
> safety. If I can prove, say 50% of the code, cannot affect safety then I
> can focus on the 50% that does and not get distracted on the areas that
> have less effect.
> Just because there is a safety & non-safety section, doesn’t mean that the
> programming style needs to be different, after all even in the non-safety
> section quality is important for any product and the non-safety sections
> obviously need to be in the document structure clearly documented as
> non-safety.
> Once the segregation is in place it has several benefits, for example,
> although all code needs to be reviewed & tested (from a quality point of
> view) why focus on software that could be classed as black channel software
> components where the safety aspect is assured elsewhere, the focus can be
> where it is needed, complexity is reduced and the amount of important
> safety code is reduced.
> This method has the added benefit that proven in use / COTS firmware can
> be used in the non-safety area knowing that it is unlikely to affect the
> safety firmware.
> Best regards,
> Gerry Creech
> From: "Watts Malcolm (AE/ENG11-AU)" <Malcolm.Watts_at_xxxxxx > To: "systemsafety_at_xxxxxx > systemsafety_at_xxxxxx > Date: 24/07/2013 02:01
> Subject: Re: [SystemSafety] Separating critical software modules
> from non-critical software modules
> Sent by: systemsafety-bounces_at_xxxxxx > ------------------------------
> Our exerience in automotive is that it is effectively impossible for most
> automotive products to have the kind of separation José speaks of; for
> example “two separate board groups”. Much of our software (although not
> SIL4 – often SIL2 equivalent) runs on a single micro in a single device.
> Very high integrity product might have 2 independent micros in a single
> enclosure, with some redundancy of function in other devices in the vehicle
> (for example, data redundancy). Many of the micros used do not have
> memory-protection units, and micros may be running only scheduling
> executives, not full operating systems (in the interests of simplicity,
> proven field use, and testability). In this circumstance, it makes the
> most sense (to me) to develop all of the software in the micro to the
> highest integrity level required by any component.
> I share the concerns raised in to Myriam’s post; as a matter of
> practicality, few developers are feasibly able to swap back and forth
> between “safety” and “no-safety” development methodologies (to say nothing
> of the cost and complexity of maintaining two sets of procedures, two sets
> of training, duplicated QA, the complexity of planning and tracking, and so
> on. To answer Myriam’s rhetorical question; no, for me it does not make
> sense that developers can swap back and forward between two different
> mindsets without mistakes, and no, it does not make much sense that
> tightly-coupled modules can be part of significantly different lifecycles
> without adverse effects on interfaces, assumptions, change management and
> quality requirements. [This is the same problem faced when incorporating 3
> rd-party components. There’s a reason that such a high proportion of
> defects are in the interfaces].
> The more conservative approach (taking into account possible changes, and
> mistakes in understanding whether a component or its interface is
> safety-relevant or not, under given circumstances, is to develop all
> software components (in tightly-coupled products typical of automotive) to
> the highest-applicable integrity level.
> The benefit you get (in my opinion) is reduced risk due to unexpected
> interference between modules, reduce risk due to systematic defects,
> reduced risk due to human-factors effects from the developers, reduced cost
> due to consistency, and better/faster impact analysis on change.
> The flip side is increased cost and effort for all components (and their
> integration ?) that could otherwise have been considered
> “non-safety-relevant”. This really is a serious disadvantage of the
> approach. Ignacio mentioned that this may be practical only for small
> teams and “small software”. Does anyone know of any research in this area ?
> Best Regards,
> Mal.
> Mal Watts
> --------------------------------------------------------------------------------------------------------------
> Functional Safety Manager (AE/ENG11-AU)
> Robert Bosch (Australia) Pty. Ltd.
> Automotive Energy and Body Systems,
> Locked Bag 66 - Clayton South, VIC 3169 - AUSTRALIA
> Tel: +61 3 9541-7877 Fax: +61 3 9541-3935
> Malcolm.Watts_at_xxxxxx >
> *From:* systemsafety-bounces_at_xxxxxx > mailto:systemsafety-bounces_at_xxxxxx > *On Behalf Of *José Faria*
> Sent:* Tuesday, 23 July 2013 7:58 PM*
> To:* M Mencke*
> Cc:* systemsafety_at_xxxxxx > Subject:* Re: [SystemSafety] Separating critical software modules from
> non-critical software modules
> Myriam,
> Yes, it is a valid approach. Valid meaning both technically feasible and
> acceptable by certification authorities. As Gerry said, the fundamental
> issue is to demonstrate that the lower SIL level part cannot compromise the
> higher level part.
> In the systems I've worked the basic architecture solution was to have 2
> separate board groups for the SIL4 and SIL0 software. In such a solution,
> you can find the guidance for the safety analysis of the communication
> protocol between the two boards in EN 50159 Annex A.
> Best,
> José
> On Tue, Jul 23, 2013 at 9:21 AM, M Mencke <*menckem_at_xxxxxx > wrote:
> Dear All,
> For any software development project, many software modules are involved,
> where some are defined as safety critical, others are not. For example, in
> railway signaling, communications modules are likely to be defined as
> critical, whereas other modules such as those involving data storage or
> other basic functions are not. An analysis may be performed with the
> objective of demonstrating that the safety critical modules are entirely
> independent from the non critical modules, leading to the conclusion that
> the application of a programming standard for safety critical software is
> only required for those modules defined as safety critical (note the phrase
> “with the objective of demonstrating…”; I would hesitate before drawing the
> conclusion that the analysis really demonstrates what it is supposed to
> demonstrate).
> In my field the EN 50128 would be applied, however, it could be any
> standard for safety critical software. Thus, the software is developed
> applying the standard only to the modules which have been defined as
> “safety critical”. In order to supposedly save time/money, etc., the rest
> of the modules are developed as non-critical software, either as SIL 0
> functions or according to a standard programming standard. My question is
> whether such an approach is really valid, given that the application of a
> safety critical standard does not only involve the application of specific
> language features, it involves an entire development life cycle, and I find
> it difficult to see how the modules defined as “non-critical” then do not
> form part of that life cycle. I’m not saying it is not valid, but I would
> like to know how others see this.
> Additionally, if the same programmers are involved in the programming of
> both critical and non-critical modules, does it really make sense that they
> only pay attention to the features required for safety critical software
> when programming the critical modules, and modify their programming style
> for the rest of the modules (or revert back to their “usual” style)? These
> questions also depend on what you consider as critical, for example, for a
> control system with a HMI, you could only consider communication modules
> critical, however, you need a GUI to display the status of the elements an
> operator has to control correctly. Some operations performed by the
> operator may not have the potential to generate a hazard with a high
> severity level, because there are mitigations in place. However, that
> doesn’t necessarily mean that the software responsible for displaying the
> information should not be programmed according to a safety critical
> standard. I am aware that these questions don’t have an “easy” answer; any
> opinions would be appreciated.
> Kind Regards,
> Myriam.
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systemsafety_at_xxxxxx Received on Wed Jul 24 2013 - 11:33:54 CEST

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