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Re: A timer driver for Cortex-M0+... it rarely doesn't work

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On 27/04/17 09:37, pozz wrote:
> Il 27/04/2017 02:17, John Speth ha scritto:
>> First you need to make ticks_high volatile.  It doesn't make any sense
>> to me why you'd cast it volatile in one execution context but not the
>> other.  If it's shared between two execution contexts, the compiler
>> needs to know that.

"volatile" does not mean that a variable is shared between two execution
contexts - it is neither necessary nor sufficient to make such sharing work.

To get shared accesses right, you need to be sure of what is accessed at
what times - it is the /accesses/ to the variable that need to be
volatile.  Marking a variable "volatile" is simply a shortcut for saying
that /all/ accesses to it must be volatile accesses.

> 
> _ticks_high is accessed in TimerSet(), TimerExpired() and the ISR
> TC0_Handler().  Timerset() and TimerExpired() reads _ticks_high by
> calling ticks() function.  So _ticks_high is accessed only in two
> points: ticks() and TC0_Handler().
> 
> ticks() is called during normal background flow (not interrupt), so the
> access to _ticks_high must be volatile.
> TC0_Handler() is the ISR, so I think a volatile access to _ticks_high is
> not necessary (the ISR can't be interrupted).
> 

Yes, the volatile accesses here are fine.  Omitting the volatile for
_ticks_high does not give any benefits or disadvantages, because you are
only doing a single read and write of the variable anyway - there is
very little room for the compiler to optimise.  The compiler can move
the non-volatile accesses to _ticks_high to after the clearing of the
overflow flag, maybe saving a cycle or two if it is a Cortex M7 that can
benefit from more sophisticated instruction scheduling.  But otherwise,
it does not matter one way or the other.

> 
>> It sounds like you have a concurrent access problem that you need to
>> protect against.  If it was me, I'd disable interrupts instead of your
>> trick.
> 
> I liked this trick because avoids disabling interrupts. TimerExpired()
> is often called.
> 
> I found the post (from Wouter van Ooijen) where this trick is suggested:
> https://groups.google.com/d/msg/comp.arch.embedded/9d8I5FFbmX4/6yL33UR92F8J
> 
> Don Y suggested other improvements in the same thread.

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A timer driver for Cortex-M0+... it rarely doesn't work pozz <pozzugno@gmail.com> - 2017-04-27 00:55 +0200
  Re: A timer driver for Cortex-M0+... it rarely doesn't work Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP> - 2017-04-26 23:24 +0000
    Re: A timer driver for Cortex-M0+... it rarely doesn't work pozz <pozzugno@gmail.com> - 2017-04-27 09:32 +0200
      Re: A timer driver for Cortex-M0+... it rarely doesn't work Jack <jack4747@gmail.com> - 2017-04-27 01:15 -0700
  Re: A timer driver for Cortex-M0+... it rarely doesn't work John Speth <johnspeth@yahoo.com> - 2017-04-26 17:17 -0700
    Re: A timer driver for Cortex-M0+... it rarely doesn't work pozz <pozzugno@gmail.com> - 2017-04-27 09:37 +0200
      Re: A timer driver for Cortex-M0+... it rarely doesn't work David Brown <david.brown@hesbynett.no> - 2017-04-27 10:13 +0200
        Re: A timer driver for Cortex-M0+... it rarely doesn't work John Speth <johnspeth@yahoo.com> - 2017-04-27 10:24 -0700
          Re: A timer driver for Cortex-M0+... it rarely doesn't work David Brown <david.brown@hesbynett.no> - 2017-04-27 22:04 +0200
  Re: A timer driver for Cortex-M0+... it rarely doesn't work David Brown <david.brown@hesbynett.no> - 2017-04-27 10:01 +0200
    Re: A timer driver for Cortex-M0+... it rarely doesn't work David Brown <david.brown@hesbynett.no> - 2017-04-27 10:27 +0200
      Re: A timer driver for Cortex-M0+... it rarely doesn't work pozz <pozzugno@gmail.com> - 2017-04-27 11:53 +0200
        Re: A timer driver for Cortex-M0+... it rarely doesn't work Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2017-05-01 10:09 +0200
          Re: A timer driver for Cortex-M0+... it rarely doesn't work pozz <pozzugno@gmail.com> - 2017-05-02 09:03 +0200
            Re: A timer driver for Cortex-M0+... it rarely doesn't work Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2017-05-04 10:06 +0200
    Re: A timer driver for Cortex-M0+... it rarely doesn't work pozz <pozzugno@gmail.com> - 2017-04-27 11:38 +0200
      Re: A timer driver for Cortex-M0+... it rarely doesn't work David Brown <david.brown@hesbynett.no> - 2017-04-27 12:54 +0200
        Re: A timer driver for Cortex-M0+... it rarely doesn't work pozz <pozzugno@gmail.com> - 2017-04-27 13:51 +0200
          Re: A timer driver for Cortex-M0+... it rarely doesn't work David Brown <david.brown@hesbynett.no> - 2017-04-27 15:39 +0200
            Re: A timer driver for Cortex-M0+... it rarely doesn't work pozz <pozzugno@gmail.com> - 2017-04-28 09:24 +0200
              Re: A timer driver for Cortex-M0+... it rarely doesn't work David Brown <david.brown@hesbynett.no> - 2017-04-28 11:05 +0200
                Re: A timer driver for Cortex-M0+... it rarely doesn't work pozz <pozzugno@gmail.com> - 2017-04-28 12:18 +0200
                Re: A timer driver for Cortex-M0+... it rarely doesn't work David Brown <david.brown@hesbynett.no> - 2017-04-28 13:10 +0200
                ARM developing under Linux [was Re: A timer driver for Cortex-M0+... it rarely doesn't work] pozz <pozzugno@gmail.com> - 2017-05-02 09:14 +0200
                Re: ARM developing under Linux [was Re: A timer driver for Cortex-M0+... it rarely doesn't work] David Brown <david.brown@hesbynett.no> - 2017-05-02 11:34 +0200
                Re: ARM developing under Linux [was Re: A timer driver for Cortex-M0+... it rarely doesn't work] pozz <pozzugno@gmail.com> - 2017-05-03 09:04 +0200
                Re: ARM developing under Linux [was Re: A timer driver for Cortex-M0+... it rarely doesn't work] David Brown <david.brown@hesbynett.no> - 2017-05-03 10:59 +0200
  Re: A timer driver for Cortex-M0+... it rarely doesn't work Dimiter_Popoff <dp@tgi-sci.com> - 2017-04-27 11:02 +0300
    Re: A timer driver for Cortex-M0+... it rarely doesn't work David Brown <david.brown@hesbynett.no> - 2017-04-27 11:06 +0200
      Re: A timer driver for Cortex-M0+... it rarely doesn't work Dimiter_Popoff <dp@tgi-sci.com> - 2017-04-27 14:20 +0300
        Re: A timer driver for Cortex-M0+... it rarely doesn't work David Brown <david.brown@hesbynett.no> - 2017-04-27 16:16 +0200
      Re: A timer driver for Cortex-M0+... it rarely doesn't work Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP> - 2017-04-27 12:03 +0000
        Re: A timer driver for Cortex-M0+... it rarely doesn't work David Brown <david.brown@hesbynett.no> - 2017-04-27 16:18 +0200

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