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| -rw-r--r-- | Documentation/fujitsu/frv/kernel-ABI.txt | 234 |
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diff --git a/Documentation/fujitsu/frv/kernel-ABI.txt b/Documentation/fujitsu/frv/kernel-ABI.txt new file mode 100644 index 00000000000..0ed9b0a779b --- /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. |