[PATCH] FRV: Use virtual interrupt disablement

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: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 234 insertions, 0 deletions

diff --git a/Documentation/fujitsu/frv/kernel-ABI.txt b/Documentation/fujitsu/frv/kernel-ABI.txt
new file mode 100644
index 000000000000..0ed9b0a779bc
--- /dev/null
+++ b/Documentation/fujitsu/frv/kernel-ABI.txt
@@ -0,0 +1,234 @@
+                                 =================================
+                                 INTERNAL KERNEL ABI FOR FR-V ARCH
+                                 =================================
+
+The internal FRV kernel ABI is not quite the same as the userspace ABI. A number of the registers
+are used for special purposed, and the ABI is not consistent between modules vs core, and MMU vs
+no-MMU.
+
+This partly stems from the fact that FRV CPUs do not have a separate supervisor stack pointer, and
+most of them do not have any scratch registers, thus requiring at least one general purpose
+register to be clobbered in such an event. Also, within the kernel core, it is possible to simply
+jump or call directly between functions using a relative offset. This cannot be extended to modules
+for the displacement is likely to be too far. Thus in modules the address of a function to call
+must be calculated in a register and then used, requiring two extra instructions.
+
+This document has the following sections:
+
+ (*) System call register ABI
+ (*) CPU operating modes
+ (*) Internal kernel-mode register ABI
+ (*) Internal debug-mode register ABI
+ (*) Virtual interrupt handling
+
+
+========================
+SYSTEM CALL REGISTER ABI
+========================
+
+When a system call is made, the following registers are effective:
+
+        REGISTERS       CALL                    RETURN
+        =============== ======================= =======================
+        GR7             System call number      Preserved
+        GR8             Syscall arg #1          Return value
+        GR9-GR13        Syscall arg #2-6        Preserved
+
+
+===================
+CPU OPERATING MODES
+===================
+
+The FR-V CPU has three basic operating modes. In order of increasing capability:
+
+  (1) User mode.
+
+      Basic userspace running mode.
+
+  (2) Kernel mode.
+
+      Normal kernel mode. There are many additional control registers available that may be
+      accessed in this mode, in addition to all the stuff available to user mode. This has two
+      submodes:
+
+      (a) Exceptions enabled (PSR.T == 1).
+
+          Exceptions will invoke the appropriate normal kernel mode handler. On entry to the
+          handler, the PSR.T bit will be cleared.
+
+      (b) Exceptions disabled (PSR.T == 0).
+
+          No exceptions or interrupts may happen. Any mandatory exceptions will cause the CPU to
+          halt unless the CPU is told to jump into debug mode instead.
+
+  (3) Debug mode.
+
+      No exceptions may happen in this mode. Memory protection and management exceptions will be
+      flagged for later consideration, but the exception handler won't be invoked. Debugging traps
+      such as hardware breakpoints and watchpoints will be ignored. This mode is entered only by
+      debugging events obtained from the other two modes.
+
+      All kernel mode registers may be accessed, plus a few extra debugging specific registers.
+
+
+=================================
+INTERNAL KERNEL-MODE REGISTER ABI
+=================================
+
+There are a number of permanent register assignments that are set up by entry.S in the exception
+prologue. Note that there is a complete set of exception prologues for each of user->kernel
+transition and kernel->kernel transition. There are also user->debug and kernel->debug mode
+transition prologues.
+
+
+        REGISTER        FLAVOUR USE
+        =============== ======= ====================================================
+        GR1                     Supervisor stack pointer
+        GR15                    Current thread info pointer
+        GR16                    GP-Rel base register for small data
+        GR28                    Current exception frame pointer (__frame)
+        GR29                    Current task pointer (current)
+        GR30                    Destroyed by kernel mode entry
+        GR31            NOMMU   Destroyed by debug mode entry
+        GR31            MMU     Destroyed by TLB miss kernel mode entry
+        CCR.ICC2                Virtual interrupt disablement tracking
+        CCCR.CC3                Cleared by exception prologue (atomic op emulation)
+        SCR0            MMU     See mmu-layout.txt.
+        SCR1            MMU     See mmu-layout.txt.
+        SCR2            MMU     Save for EAR0 (destroyed by icache insns in debug mode)
+        SCR3            MMU     Save for GR31 during debug exceptions
+        DAMR/IAMR       NOMMU   Fixed memory protection layout.
+        DAMR/IAMR       MMU     See mmu-layout.txt.
+
+
+Certain registers are also used or modified across function calls:
+
+        REGISTER        CALL                            RETURN
+        =============== =============================== ===============================
+        GR0             Fixed Zero                      -
+        GR2             Function call frame pointer
+        GR3             Special                         Preserved
+        GR3-GR7         -                               Clobbered
+        GR8             Function call arg #1            Return value (or clobbered)
+        GR9             Function call arg #2            Return value MSW (or clobbered)
+        GR10-GR13       Function call arg #3-#6         Clobbered
+        GR14            -                               Clobbered
+        GR15-GR16       Special                         Preserved
+        GR17-GR27       -                               Preserved
+        GR28-GR31       Special                         Only accessed explicitly
+        LR              Return address after CALL       Clobbered
+        CCR/CCCR        -                               Mostly Clobbered
+
+
+================================
+INTERNAL DEBUG-MODE REGISTER ABI
+================================
+
+This is the same as the kernel-mode register ABI for functions calls. The difference is that in
+debug-mode there's a different stack and a different exception frame. Almost all the global
+registers from kernel-mode (including the stack pointer) may be changed.
+
+        REGISTER        FLAVOUR USE
+        =============== ======= ====================================================
+        GR1                     Debug stack pointer
+        GR16                    GP-Rel base register for small data
+        GR31                    Current debug exception frame pointer (__debug_frame)
+        SCR3            MMU     Saved value of GR31
+
+
+Note that debug mode is able to interfere with the kernel's emulated atomic ops, so it must be
+exceedingly careful not to do any that would interact with the main kernel in this regard. Hence
+the debug mode code (gdbstub) is almost completely self-contained. The only external code used is
+the sprintf family of functions.
+
+Futhermore, break.S is so complicated because single-step mode does not switch off on entry to an
+exception. That means unless manually disabled, single-stepping will blithely go on stepping into
+things like interrupts. See gdbstub.txt for more information.
+
+
+==========================
+VIRTUAL INTERRUPT HANDLING
+==========================
+
+Because accesses to the PSR is so slow, and to disable interrupts we have to access it twice (once
+to read and once to write), we don't actually disable interrupts at all if we don't have to. What
+we do instead is use the ICC2 condition code flags to note virtual disablement, such that if we
+then do take an interrupt, we note the flag, really disable interrupts, set another flag and resume
+execution at the point the interrupt happened. Setting condition flags as a side effect of an
+arithmetic or logical instruction is really fast. This use of the ICC2 only occurs within the
+kernel - it does not affect userspace.
+
+The flags we use are:
+
+ (*) CCR.ICC2.Z [Zero flag]
+
+     Set to virtually disable interrupts, clear when interrupts are virtually enabled. Can be
+     modified by logical instructions without affecting the Carry flag.
+
+ (*) CCR.ICC2.C [Carry flag]
+
+     Clear to indicate hardware interrupts are really disabled, set otherwise.
+
+
+What happens is this:
+
+ (1) Normal kernel-mode operation.
+
+        ICC2.Z is 0, ICC2.C is 1.
+
+ (2) An interrupt occurs. The exception prologue examines ICC2.Z and determines that nothing needs
+     doing. This is done simply with an unlikely BEQ instruction.
+
+ (3) The interrupts are disabled (local_irq_disable)
+
+        ICC2.Z is set to 1.
+
+ (4) If interrupts were then re-enabled (local_irq_enable):
+
+        ICC2.Z would be set to 0.
+
+     A TIHI #2 instruction (trap #2 if condition HI - Z==0 && C==0) would be used to trap if
+     interrupts were now virtually enabled, but physically disabled - which they're not, so the
+     trap isn't taken. The kernel would then be back to state (1).
+
+ (5) An interrupt occurs. The exception prologue examines ICC2.Z and determines that the interrupt
+     shouldn't actually have happened. It jumps aside, and there disabled interrupts by setting
+     PSR.PIL to 14 and then it clears ICC2.C.
+
+ (6) If interrupts were then saved and disabled again (local_irq_save):
+
+        ICC2.Z would be shifted into the save variable and masked off (giving a 1).
+
+        ICC2.Z would then be set to 1 (thus unchanged), and ICC2.C would be unaffected (ie: 0).
+
+ (7) If interrupts were then restored from state (6) (local_irq_restore):
+
+        ICC2.Z would be set to indicate the result of XOR'ing the saved value (ie: 1) with 1, which
+        gives a result of 0 - thus leaving ICC2.Z set.
+
+        ICC2.C would remain unaffected (ie: 0).
+
+     A TIHI #2 instruction would be used to again assay the current state, but this would do
+     nothing as Z==1.
+
+ (8) If interrupts were then enabled (local_irq_enable):
+
+        ICC2.Z would be cleared. ICC2.C would be left unaffected. Both flags would now be 0.
+
+     A TIHI #2 instruction again issued to assay the current state would then trap as both Z==0
+     [interrupts virtually enabled] and C==0 [interrupts really disabled] would then be true.
+
+ (9) The trap #2 handler would simply enable hardware interrupts (set PSR.PIL to 0), set ICC2.C to
+     1 and return.
+
+(10) Immediately upon returning, the pending interrupt would be taken.
+
+(11) The interrupt handler would take the path of actually processing the interrupt (ICC2.Z is
+     clear, BEQ fails as per step (2)).
+
+(12) The interrupt handler would then set ICC2.C to 1 since hardware interrupts are definitely
+     enabled - or else the kernel wouldn't be here.
+
+(13) On return from the interrupt handler, things would be back to state (1).
+
+This trap (#2) is only available in kernel mode. In user mode it will result in SIGILL.