I hate to dig this back up, but I just realized another slightly more
friendly way of doing this would be to use the atomic gcc builtins (like
__sync_and_and_fetch, __sync_or_and_fetch, etc.). Do we support these
functions, if not how hard would it be to add them. It would seem that this
would be the most portable code between different gcc targets (obviously it
wouldn't port outside gcc, though you could fake the __sync functions for
other targets).
On Tue, Sep 4, 2012 at 11:19 AM, Peter Bigot <big...@acm.org> wrote:
> On Tue, Sep 4, 2012 at 11:52 AM, JMGross <msp...@grossibaer.de> wrote:
> > ----- Ursprüngliche Nachricht -----
> >>True, but that won't matter. Compilers do not usually track the state
> >>of status bits over a longer range than one instruction pattern.
> >
> > There's more in the status register than just the status bits.
> > There are the LPM bits too. Besides CPUOFF, which is of
> > course never set when the CPU is running, there is the control
> > for XT1 or DCO. And in 3x/5x family, one of them controls the FLL.
> > Just overwriting the status register may unintendedly reactivate the
> > FLL, that might have been deactivated inside the atomic block,
> > with desastrous results. And the others might have impact on
> > current consumption.
> > Well, one could declare it as a coding bug to deactivate the FLL inside
> > an atomic block, but I prefer not creating dependencies where I can
> avoid it.
> > And if the atomic block is a timing calibration function, it might be
> > necessary.
> >
> > How does the latest MCPGCC handle this on funcitons with critical
> > attribute?
>
> The "critical" function attribute in mspgcc saves and restores the
> entire status register. Any changes within the function are
> discarded. Use of "biX_status_register_on_exit()" may or may not
> work; I haven't tried it, and wouldn't count on it if it did.
>
> As you point out, there are reasons why somebody might want to control
> SCG0, and the fact that __set_interrupt_status() might overwrite that
> is a concern. When implementing the intrinsic I asked on E2E how it
> behaved on other platforms, and was told it restored all the bits. I
> considered alternatives that would save only GIE, but decided that
> doing so wasn't worth the hassle and loss in performance and
> compatibility. Besides, a benefit of explicitly using
> __set_interrupt_status() and __get_interrupt_status() is that the
> application gets full control of what bits are restored. Any
> intentional changes to the live status register can be replicated in
> the saved state before restoring it. (Though really, if you're going
> to play those games, you should probably use __read_status_register()
> and __write_status_register() instead; it's unfortunate that IAR and
> CCS chose a name that implies only the interrupt status is
> manipulated.)
>
> In the MSP430 infrastructure environment I'm developing (both
> standalone and for FreeRTOS) I allow the application to control what
> bits are cleared on LPM exit, and ensure any infrastructure routines
> such as ISRs that manipulate the status register don't clear SCG0 when
> they shouldn't. This was specifically a result of the severity of the
> UCS errata in the 5xx family and the importance of controlling FLL
> behavior via SCG0 to avoid them.
>
> Peter
>
>
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