Re: Advice, tasking and hardware

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 25/05/2016 23:24, patrick@spellingbeewinnars.org wrote:

> Do you have any examples of tasking calling low level functions?

Just call it as any other subprogram.

> Spark doesn't use tasking, are there many dangers in using tasking with hardware?

Dangers to the hardware? (:-))

Pre-/postcondition based proofs (SPARK's business) are pretty much 
orthogonal to tasking issues. Schedulability and other tasking stuff has 
a non-functional aspect whereas proofs are about functional issues. If 
you are concerned about static analysis regarding tasks, see Ravenscar 
profile.

> Would it be wise to wrap C driver calls in a protected object or are
> most drivers in Linux thread safe and suitable for being called from
> different threads?

Driver calls are thread safe. It does not mean that they are safe from 
the higher level semantics point of view. E.g. it is safe to delete a 
file in one process and open it in another, safe at the filesystem 
level, but unsafe at the application level.

Anyway, you never call driver operations from a protected action because 
it is a "potentially blocking" call. Even if not blocking in the sense 
of being asynchronous to the CPU/memory bus cycles, accessing hardware 
might be quite slow, e.g. reading/writing a hardware port, dual-ported 
memory access etc. A protected action is considered logically instant.

If you want interlocking for doing blocking calls use a mutex etc. A 
mutex implementation based on protected objects is two actions: Seize 
and Release.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de