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    • David Howells's avatar
      [PATCH] FRV: Use virtual interrupt disablement · 28baebae
      David Howells authored
      Make the FRV arch use virtual interrupt disablement because accesses to the
      processor status register (PSR) are relatively slow and because we will
      soon have the need to deal with multiple interrupt controls at the same
      time (separate h/w and inter-core interrupts).
      
      The way this is done is to dedicate one of the four integer condition code
      registers (ICC2) to maintaining a virtual interrupt disablement state
      whilst inside the kernel.  This uses the ICC2.Z flag (Zero) to indicate
      whether the interrupts are virtually disabled and the ICC2.C flag (Carry)
      to indicate whether the interrupts are physically disabled.
      
      ICC2.Z is set to indicate interrupts are virtually disabled.  ICC2.C is set
      to indicate interrupts are physically enabled.  Under normal running
      conditions Z==0 and C==1.
      
      Disabling interrupts with local_irq_disable() doesn't then actually
      physically disable interrupts - it merely sets ICC2.Z to 1.  Should an
      interrupt then happen, the exception prologue will note ICC2.Z is set and
      branch out of line using one instruction (an unlikely BEQ).  Here it will
      physically disable interrupts and clear ICC2.C.
      
      When it comes time to enable interrupts (local_irq_enable()), this simply
      clears the ICC2.Z flag and invokes a trap #2 if both Z and C flags are
      clear (the HI integer condition).  This can be done with the TIHI
      conditional trap instruction.
      
      The trap then physically reenables interrupts and sets ICC2.C again.  Upon
      returning the interrupt will be taken as interrupts will then be enabled.
      Note that whilst processing the trap, the whole exceptions system is
      disabled, and so an interrupt can't happen till it returns.
      
      If no pending interrupt had happened, ICC2.C would still be set, the HI
      condition would not be fulfilled, and no trap will happen.
      
      Saving interrupts (local_irq_save) is simply a matter of pulling the ICC2.Z
      flag out of the CCR register, shifting it down and masking it off.  This
      gives a result of 0 if interrupts were enabled and 1 if they weren't.
      
      Restoring interrupts (local_irq_restore) is then a matter of taking the
      saved value mentioned previously and XOR'ing it against 1.  If it was one,
      the result will be zero, and if it was zero the result will be non-zero.
      This result is then used to affect the ICC2.Z flag directly (it is a
      condition code flag after all).  An XOR instruction does not affect the
      Carry flag, and so that bit of state is unchanged.  The two flags can then
      be sampled to see if they're both zero using the trap (TIHI) as for the
      unconditional reenablement (local_irq_enable).
      
      This patch also:
      
       (1) Modifies the debugging stub (break.S) to handle single-stepping crossing
           into the trap #2 handler and into virtually disabled interrupts.
      
       (2) Removes superseded fixup pointers from the second instructions in the trap
           tables (there's no a separate fixup table for this).
      
       (3) Declares the trap #3 vector for use in .org directives in the trap table.
      
       (4) Moves irq_enter() and irq_exit() in do_IRQ() to avoid problems with
           virtual interrupt handling, and removes the duplicate code that has now
           been folded into irq_exit() (softirq and preemption handling).
      
       (5) Tells the compiler in the arch Makefile that ICC2 is now reserved.
      
       (6) Documents the in-kernel ABI, including the virtual interrupts.
      
       (7) Renames the old irq management functions to different names.
      Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
      Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
      28baebae