[src/trunk]: src/sys/arch/powerpc/oea - Tidy up some comments.

[thorpej <thorpej%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/754806c64c14
branches:  trunk
changeset: 959551:754806c64c14
user:      thorpej <thorpej%NetBSD.org@localhost>
date:      Thu Feb 18 16:29:12 2021 +0000

description:
- Tidy up some comments.
- Use correct stack frame linkage everywhere so that if something goes
  wrong, we can get a meaningful back trace.
- Use an additional layer of indirection so that, when we're very
  early in bootstrap, we can just call OpenFirmware directly, rather
  than using our trampoline that saves/restores kernel state.
- Carve out a space for ofwinit() to call into C code to do additional
  initialization.  (This is not done yet.)

diffstat:

 sys/arch/powerpc/oea/ofw_subr.S |  131 ++++++++++++++++++++++++++++-----------
 1 files changed, 93 insertions(+), 38 deletions(-)

diffs (190 lines):

diff -r c966ad258d4d -r 754806c64c14 sys/arch/powerpc/oea/ofw_subr.S
--- a/sys/arch/powerpc/oea/ofw_subr.S   Thu Feb 18 15:13:37 2021 +0000
+++ b/sys/arch/powerpc/oea/ofw_subr.S   Thu Feb 18 16:29:12 2021 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: ofw_subr.S,v 1.13 2021/02/13 01:48:33 thorpej Exp $    */
+/*     $NetBSD: ofw_subr.S,v 1.14 2021/02/18 16:29:12 thorpej Exp $    */
 
 /*
  * Copyright (C) 1995, 1996 Wolfgang Solfrank.
@@ -35,25 +35,43 @@
 #include "opt_ppcarch.h"
 #endif
 
-       .local  firmstk
-       .globl  openfirmware_entry
-       .globl  ofwmsr
-       .local  ofwsrsave
-       .local  OF_buffer
+       /* Stack used to call into OpenFirmware. */
+       .lcomm  firmstk,NBPG,16
+
+       /* Buffer used to pass data to/from OpenFirmware. */
+       .lcomm  OF_buffer,NBPG + 36,4
+
+       /* The OpenFirmware entry point, provided by OpenFirmware. */
+       .lcomm  ofentry,4,4
 
-       .lcomm  firmstk,NBPG,16
-       .lcomm  OF_buffer,NBPG + 36,4
-       .comm   openfirmware_entry,4,4  /* openfirmware entry point */
-       .lcomm  ofwsrsave,64,4          /* openfirmware SR savearea */
-       .comm   ofwmsr,20,4             /* msr & sprg[0-3] used in OF */
-       .comm   ofwreal_incharge,4,4
+       /* Entry trampoline used by openfirmware(). */
+       .lcomm  oftramp,4,4
+
+       /* OpenFirmware SR save area */
+       .lcomm  ofwsrsave,64,4
+
+       /* MSR and SPRG[0-3] used in OpenFirmware */
+       .globl  ofwmsr
+       .comm   ofwmsr,20,4
 
 /*
  * Called by start to save the initial OFW state so we can restore it
  * when call back to OFW.
+ *
+ * We expect the registers to be as for the entry point into the kernel:
+ *
+ *     %r1     Stack provided by OpenFirmware / boot loader
+ *     %r5     OpenFirmware client entry point
+ *
+ * (others -- don't care)
  */
 ENTRY_NOPROFILE(ofwinit)
-#ifdef FIRMWORKSBUGS
+       /* Save return address, Push a stack frame. */
+       mflr    %r0
+       stw     %r0,4(%r1)
+       stwu    %r1,-16(%r1)
+
+#ifdef FIRMWORKSBUGS
        mfmsr   %r0
        andi.   %r0,%r0,PSL_IR|PSL_DR
        beq     1f
@@ -62,25 +80,24 @@
        lis     %r9,ofwreal_incharge@ha
        stw     %r8,ofwreal_incharge@l(%r9)
 
-       mflr    %r30
        bl      _C_LABEL(ofwr_init)
-       mtlr    %r30
 1:
 #endif
-       lis     %r8,openfirmware_entry@ha
-       stw     %r5,openfirmware_entry@l(%r8) /* save client interface handler*/
+
+       lis     %r8,ofentry@ha
+       stw     %r5,ofentry@l(%r8)      /* save client interface handler */
 
+       /*
+        * Call directly into OpenFirmware until we're ready to use
+        * the trampoline.
+        */
+       lis     %r8,oftramp@ha
+       stw     %r5,oftramp@l(%r8)
+
+       /* Save the MSR that OpenFirmware is using. */
        mfmsr   %r0
-/*
- * XXX
- * doing this here instead of later on in ofwoea_initppc() after setting
- * up the console and such makes my PowerBook 3400c hang.
- * Probably just another OF 2.0 weirdness
- */
-       /*li    %r8,PSL_IP*/
-       /*andc  %r0,%r0,%r8*/                   /* make sure PSL_IP is off */
        lis     %r9,ofwmsr@ha
-       stwu    %r0,ofwmsr@l(%r9)               /* save initial MSR value */
+       stwu    %r0,ofwmsr@l(%r9)
 
        mfsprg0 %r0                             /* save SPRGs */
        stw     %r0,4(%r9)
@@ -96,28 +113,42 @@
        lis     %r9,_C_LABEL(OF_buf)@ha
        stw     %r8,_C_LABEL(OF_buf)@l(%r9)
 
+       /* XXX Do other stuff in C code. */
+
+       /*
+        * Jump off the trampoline for all subsequent calls
+        * into OpenFirmware.
+        */
+       lis     %r5,_C_LABEL(openfirmware_trampoline)@ha
+       addi    %r5,%r5,_C_LABEL(openfirmware_trampoline)@l
+       lis     %r8,oftramp@ha
+       stw     %r5,oftramp@l(%r8)
+
+       /* Retrieve LR, pop stack frame. */
+       addi    %r1,%r1,16
+       lwz     %r0,4(%r1)
+       mtlr    %r0
+
        blr
 
 /*
- * OpenFirmware entry point
+ * OpenFirmware trampoline.  We are already on the OpenFirmware stack.
  */
-       .text
-ENTRY(openfirmware)
+ENTRY_NOPROFILE(openfirmware_trampoline)
        mflr    %r0
        stw     %r0,4(%r1)              /* save return address */
 
        /*
-        * Switch to OpenFirmware stack.
+        * Push stack frame and save area:
         *
-        * -48 == -16 to stack old SP and align, -32 for save area
+        * [SP+8 save area]
+        * [SP+4 slot for saved LR in callee]
+        * [SP+0 saved SP]
         */
-       lis     %r7,firmstk+NBPG-48@ha
-       addi    %r7,%r7,firmstk+NBPG-48@l
-       stw     %r1,32(%r7)             /* stash away prev stack pointer */
-       mr      %r1,%r7
+       stwu    %r1,-48(%r1)
 
-       lis     %r4,openfirmware_entry@ha       /* get firmware entry point */
-       lwz     %r4,openfirmware_entry@l(%r4)
+       lis     %r4,ofentry@ha          /* get firmware entry point */
+       lwz     %r4,ofentry@l(%r4)
        mtlr    %r4
 
        mfsprg0 %r5                     /* save current sprg0 (curcpu) */
@@ -211,7 +242,31 @@
        lwz     %r5,28(%r1)
        mtsprg3 %r5
 
-       lwz     %r1,32(%r1)             /* restore previous stack pointer */
+       addi    %r1,%r1,48              /* pop stack frame and save area */
        lwz     %r0,4(%r1)              /* return address */
        mtlr    %r0
        blr
+
+/*
+ * Call into OpenFirmware.
+ */
+ENTRY_NOPROFILE(openfirmware)
+       mflr    %r0
+       stw     %r0,4(%r1)              /* save return address */
+
+       /* Switch to OpenFirmware stack. */
+       lis     %r7,firmstk+NBPG-16@ha
+       addi    %r7,%r7,firmstk+NBPG-16@l
+       stw     %r1,0(%r7)              /* stash away prev stack pointer */
+       mr      %r1,%r7
+
+       lis     %r4,oftramp@ha
+       lwz     %r4,oftramp@l(%r4)
+
+       mtctr   %r4
+       bctrl
+
+       lwz     %r1,0(%r1)              /* restore previous stack pointer */
+       lwz     %r0,4(%r1)              /* return address */
+       mtlr    %r0
+       blr