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 Post subject: FIQ interrupts and IRQCHIP
PostPosted: Tue Jul 04, 2017 9:11 pm 

Joined: Thu Jun 22, 2017 8:00 pm
Posts: 11
Dear colleagues,

It seems that SAM SoCs have been ported to the "irqchip" subsystem quite a long time ago.

All material I could gather about FIQ interrupts are either considering an older kernel either targeting a different platform.

This classic reference "FIQ Handlers in the ARM Linux Kernel" makes FIQ stuff seem a piece of cake, but targets iMX's.
This other one "SPI 16 / 32 bits_per_word transfers fail" deals with SAMA5D3 and kernel 3.15 in a very low level manner that surely hurts the "irqchip" effort.

Well, I am really confused.

I have tried to follow both approaches to learn something and try to find out which one is going to give me any results but, up to now, I haven't moved as far as I needed.

The low level approach seems to be trying to vainly fight against kernel protection while I haven't been able to make the simple and high level "set_fiq_handler" from free-electrons work for me.

Surely I am missing something.

Could anybody here please point me any working example for FIQ in an Atmel SoC using any 4.x kernel that I could take as basis or give any further directions?

Thanks in advance.

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 Post subject: [SOLVED] FIQ interrupts and IRQCHIP
PostPosted: Thu Jul 06, 2017 7:51 pm 

Joined: Thu Jun 22, 2017 8:00 pm
Posts: 11
I got to make my FIQ work

There are some caveats, though, and I'll share with you..

1) It looks like someone at Atmel forgot to add a dependency for FIQ in "kernel-source/arch/arm/mach-at91/Kconfig".

I haven't performed a thorough investigation and I don't want to blame anyone but all I can say is that my Kconfig had none and I fixed that with:

menuconfig ARCH_AT91
   bool "Atmel SoCs"
   depends on ARCH_MULTI_V4T || ARCH_MULTI_V5 || ARCH_MULTI_V7
   select COMMON_CLK_AT91
   select PINCTRL
   select PINCTRL_AT91
   select SOC_BUS
   select FIQ  <== Added this line.

Withou this, `fiq.c` is not compiled and even the most perfect code won't work.
j4x: Unknown symbol claim_fiq (err 0)
j4x: Unknown symbol enable_fiq (err 0)
j4x: Unknown symbol set_fiq_handler (err 0)

2) Every production code I could find makes FIQ routine in Assembly. I could find a few people telling that it is possible to make it in C but the very few examples I could gather start from a "naked" function and inline-asm the entry and exit, with a small C core in it.

In particular one example found HERE makes it and gives some precious advices.

Nevertheless, I tend to use C just because I need to learn one thing at a time, and ARM Assembly can be left for later.

(To be honest, right now my FIQ handler is just a bunch of bytes in an array compiled from the simplest assembly needed to make it doubtlessly run - it turns on an LED.)

Finally, the code:

unsigned int _fiq_routine_[] = {
    0xe599b000,   // ldr   fp, [r9]
    0xe3a0b001,   // mov   fp, #1
    0xe588b000,   // str   fp, [r8]
    0xe3a0c401,   // mov   r12, #16777216
    0xe58ac030,    // str   r12, [r10, #0x0030]
    0xe25ef004,   // subs  pc, lr, #4

int   fiq_init ( void )
   struct pt_regs    regs;
   u32               dummy;
   int               result = claim_fiq( &fh );

   if ( result )
      pr_alert( "Couldn't claim FIQ: %d.\n", result );
      return result;

   set_fiq_handler( &_fiq_routine_, sizeof( _fiq_routine_ ) );

   local_irq_disable ();

   // load r8 with base address of AIC
   regs.ARM_r8   = ( long ) &aic->iccr; // ioremap'ed ICCR register.
   regs.ARM_r9  = tcb->regs + ATMEL_TC_REG ( TC_CH_NO, SR );

      set_fiq_regs( &regs );

   // Fast Force TC IRQ to be FIQ.
   // - must be done by direct access to AIC registers.
   __raw_writel( IRQ_PID( TC_IRQ_NO ), &aic->ffer );

   if ( 0 == ( IRQ_PID( TC_IRQ_NO ) & dummy ) )
      pr_alert( "Fast Forcing activation failed for PID: %i\n", TC_IRQ_NO );
      return -EIO;

   local_fiq_enable ();
   local_irq_enable ();

   return 0;

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