sh: provide unified syscall trap compatible with all SH models

Historically SH-2 Linux (and originally uClinux) used a syscall
calling convention incompatible with the established SH-3/4 Linux ABI.
This choice was made because the trap range used by the existing ABI,
0x10-0x17, overlaps with the hardware exception/interrupt trap range
reserved by SH-2, and in particular, with the SH-2A divide-by-zero and
division-overflow exceptions.

Despite the documented syscall convention using the low bits of the
trap number to signal the number of arguments the kernel should
expect, no version of the kernel has ever used this information, nor
is it useful; all of the registers need to be saved anyway. Therefore,
it is possible to pick a new trap number, 0x1f, that is both supported
by all existing SH-3/4 kernels and unassigned as a hardware trap in
the SH-2 range. This makes it possible to produce SH-2 application
binaries that are forwards-compatible with running on SH-3/4 kernels
and to treat SH as a unified platform with varying ISA support levels
rather than multiple gratuitously-incompatible platforms.

This patch adjusts the range checking SH-2 and SH-2A kernels make for
the syscall trap to accept the range 0x1f-0x2f rather than just
0x20-0x2f. As a result, trap 0x1f now acts as a syscall for all SH
models.

Signed-off-by: Rich Felker <dalias@libc.org>

diff --git a/arch/sh/kernel/cpu/sh2/entry.S b/arch/sh/kernel/cpu/sh2/entry.S
index c8a4331d9b8d7398e98df914178bb6ed4aed1eb9..a1505956ef28b4f0fd694cc4d1c609281b782811 100644 (file)
--- a/arch/sh/kernel/cpu/sh2/entry.S
+++ b/arch/sh/kernel/cpu/sh2/entry.S
@@ -144,9 +144,9 @@ ENTRY(exception_handler)
        mov     #64,r8
        cmp/hs  r8,r9
        bt      interrupt_entry ! vec >= 64 is interrupt
-       mov     #32,r8
+       mov     #31,r8
        cmp/hs  r8,r9
-       bt      trap_entry      ! 64 > vec >= 32  is trap
+       bt      trap_entry      ! 64 > vec >= 31  is trap
 
        mov.l   4f,r8
        mov     r9,r4
@@ -178,9 +178,9 @@ interrupt_entry:
 
 trap_entry:
        mov     #0x30,r8
-       cmp/ge  r8,r9           ! vector 0x20-0x2f is systemcall
+       cmp/ge  r8,r9           ! vector 0x1f-0x2f is systemcall
        bt      1f
-       add     #-0x10,r9       ! convert SH2 to SH3/4 ABI
+       mov     #0x1f,r9        ! convert to unified SH2/3/4 trap number
 1:     
        shll2   r9                      ! TRA
        bra     system_call     ! jump common systemcall entry

diff --git a/arch/sh/kernel/cpu/sh2a/entry.S b/arch/sh/kernel/cpu/sh2a/entry.S
index 222742ddc0d6a656ae16b3e5eb9ae5ba0ddb3691..da77a8ef4696e2dacb7e4bbf9bb4ccac8139c7a1 100644 (file)
--- a/arch/sh/kernel/cpu/sh2a/entry.S
+++ b/arch/sh/kernel/cpu/sh2a/entry.S
@@ -109,9 +109,9 @@ ENTRY(exception_handler)
        mov     #64,r8
        cmp/hs  r8,r9
        bt      interrupt_entry ! vec >= 64 is interrupt
-       mov     #32,r8
+       mov     #31,r8
        cmp/hs  r8,r9
-       bt      trap_entry      ! 64 > vec >= 32  is trap
+       bt      trap_entry      ! 64 > vec >= 31  is trap
 
        mov.l   4f,r8
        mov     r9,r4
@@ -143,9 +143,9 @@ interrupt_entry:
 
 trap_entry:
        mov     #0x30,r8
-       cmp/ge  r8,r9           ! vector 0x20-0x2f is systemcall
+       cmp/ge  r8,r9           ! vector 0x1f-0x2f is systemcall
        bt      1f
-       add     #-0x10,r9       ! convert SH2 to SH3/4 ABI
+       mov     #0x1f,r9        ! convert to unified SH2/3/4 trap number
 1:     
        shll2   r9                      ! TRA
        bra     system_call     ! jump common systemcall entry

diff --git a/arch/sh/kernel/entry-common.S b/arch/sh/kernel/entry-common.S
index 13047a4facd2eb442866d751dc0c84fa35062063..c001f782c5f1c6320c89011d1bc3627f10bfb3ea 100644 (file)
--- a/arch/sh/kernel/entry-common.S
+++ b/arch/sh/kernel/entry-common.S
@@ -268,20 +268,29 @@ debug_trap:
  *     Syscall #: R3
  *     Arguments #0 to #3: R4--R7
  *     Arguments #4 to #6: R0, R1, R2
- *     TRA: (number of arguments + ABI revision) x 4
+ *     TRA: See following table.
  *
- * This code also handles delegating other traps to the BIOS/gdb stub
- * according to:
- *
- * Trap number
  * (TRA>>2)    Purpose
  * --------    -------
  * 0x00-0x0f   original SH-3/4 syscall ABI (not in general use).
  * 0x10-0x1f   general SH-3/4 syscall ABI.
- * 0x20-0x2f   syscall ABI for SH-2 parts.
+ *      0x1f   unified SH-2/3/4 syscall ABI (preferred).
+ * 0x20-0x2f   original SH-2 syscall ABI.
  * 0x30-0x3f   debug traps used by the kernel.
  * 0x40-0xff   Not supported by all parts, so left unhandled.
  *
+ * For making system calls, any trap number in the range for the
+ * given cpu model may be used, but the unified trap number 0x1f is
+ * preferred for compatibility with all models.
+ *
+ * The low bits of the trap number were once documented as matching
+ * the number of arguments, but they were never actually used as such
+ * by the kernel. SH-2 originally used its own separate trap range
+ * because several hardware exceptions fell in the range used for the
+ * SH-3/4 syscall ABI.
+ *
+ * This code also handles delegating other traps to the BIOS/gdb stub.
+ *
  * Note: When we're first called, the TRA value must be shifted
  * right 2 bits in order to get the value that was used as the "trapa"
  * argument.