CPM command processing

[Leon Pollak]

My code is as following:

#define WAIT_LOOPS	200
void DoCommandCPM(int Command, int Channel) {
volatile int i, lvl;
	rtems_interrupt_disable(lvl);
	for(i = 0; i < WAIT_LOOPS; i++)
		if (!(CPU->CP_CR & CP_CMND_FLAG)) break;
	if (i == WAIT_LOOPS)
		MsgFatal("CmndCPM ");
	CPU->CP_CR = CP_CMND_FLAG | Command | Channel;
	//The UM states that CPM needs 120 cycles.
	for(i = 0; i < WAIT_LOOPS; i++)
		if (!(CPU->CP_CR & CP_CMND_FLAG)) break;
	if (i == WAIT_LOOPS)
		MsgFatal("CmndCPM ");
	rtems_interrupt_enable (lvl);
}


On Monday, 14 בJanuary 2008, you wrote:
> My usual 2-cents:
>
> Busy-waiting with interrupts disabled is evil.
> Such a loop *must* IMHO implement a timeout
> so that the worst-case impact on latency other
> tasks have to suffer from is guaranteed.
>
> -- Till
>
> Leon Pollak wrote:
> > Hello, all.
> >
> > Both files
> > ..lib/libcpu/powerpc/mpc8260/cp.c
> > ..lib/libcpu/powerpc/mpc860/cp.c
> > contain the same code:
> >
> > void m8xx_cp_execute_cmd( uint32_t   command )
> > {
> >   uint16_t   lvl;
> >
> >   rtems_interrupt_disable(lvl);
> >   while (m8260.cpcr & M8260_CR_FLG) {
> >     continue;
> >   }
> >
> >   m8260.cpcr = command | M8260_CR_FLG;
> >   rtems_interrupt_enable (lvl);
> > }
> >
> > The problem with this code is (based on my experience with both CPU's)
> > that it does not check the result of a command execution. Although
> > Motorola's specification states that CPM should finish the command
> > processing in about 20 cycles, my 13 years experience shows that there
> > are cases when the command is not executed and the flag bit remains "1".
> >
> > Therefore, I swap the operators in my code:
> >
> >   m8260.cpcr = command | M8260_CR_FLG;
> >
> >   while (m8260.cpcr & M8260_CR_FLG) {
> >     continue;
> >   }
> >
> > and cause system reset if it remains "1" for too long.
> >
> > I have no explanation to this, because after reset the system returns to
> > the same exactly sequence and now it passes. We have such problem once on
> > each 20-25 units based on MPC8260, but I remember this to occur on MPC860
> > units too.
> >
> > Hope this will be useful...



-- 
            Dr.Leon M.Pollak
                Director
       PLR Information Systems Ltd.
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