• 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
irq.c 17.2 KB