linux-kernel - [PATCH] DEC: Limit PMAX memory probing to R3k systems

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Date:   Fri, 4 Mar 2022 20:16:23 +0000 (GMT)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>
cc:     Jan-Benedict Glaw <jbglaw@...-owl.de>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Ralf Baechle <ralf@...ux-mips.org>, linux-mips@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: [PATCH] DEC: Limit PMAX memory probing to R3k systems

Recent tightening of the opcode table in binutils so as to consistently 
disallow the assembly or disassembly of CP0 instructions not supported 
by the processor architecture chosen has caused a regression like below:

arch/mips/dec/prom/locore.S: Assembler messages:
arch/mips/dec/prom/locore.S:29: Error: opcode not supported on this processor: r4600 (mips3) `rfe'

in a piece of code used to probe for memory with PMAX DECstation models, 
which have non-REX firmware.  Those computers always have an R2000 CPU 
and consequently the exception handler used in memory probing uses the 
RFE instruction, which those processors use.

While adding 64-bit support this code was correctly excluded for 64-bit 
configurations, however it should have also been excluded for irrelevant 
32-bit configurations.  Do this now then, and only enable PMAX memory 
probing for R3k systems.

Reported-by: Jan-Benedict Glaw <jbglaw@...-owl.de>
Reported-by: Sudip Mukherjee <sudipm.mukherjee@...il.com>
Signed-off-by: Maciej W. Rozycki <macro@...am.me.uk>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@...r.kernel.org # v2.6.12+
---
Hi,

 I'm assuming this won't go back beyond commit 2a11c8ea20bf ("kconfig: 
Introduce IS_ENABLED(), IS_BUILTIN() and IS_MODULE()") or any backport 
will have to be rewritten to avoid IS_ENABLED.

 The original actual change named to fix ought to be commit dd82ef87e4c9 
("PROM interface rework to support a 64-bit kernel.") from the LMO repo: 
<https://git.kernel.org/pub/scm/linux/kernel/git/ralf/linux.git/commit/?id=dd82ef87e4c9>, 
which introduced the `prom_is_rex' macro, which guards this code.  Said 
commit predates the history of our main repository though.

 This change has actually been verified at runtime with a PMIN system 
(effectively a PMAX, but with a slower R2000 CPU) and a 4MAX+ system (an 
R4400SC-based machine), and naturally throughout the three possible build 
configurations: R3k, R4k/32-bit, R4k/64-bit.

 It took longer than expected, but oh well...  Sorry for the inconvenience 
caused.

 Please apply,

  Maciej
---
 arch/mips/dec/prom/Makefile      |    2 +-
 arch/mips/include/asm/dec/prom.h |   15 +++++----------
 2 files changed, 6 insertions(+), 11 deletions(-)

linux-dec-locore-r3000.diff
Index: linux-macro/arch/mips/dec/prom/Makefile
===================================================================
--- linux-macro.orig/arch/mips/dec/prom/Makefile
+++ linux-macro/arch/mips/dec/prom/Makefile
@@ -6,4 +6,4 @@

 lib-y			+= init.o memory.o cmdline.o identify.o console.o

-lib-$(CONFIG_32BIT)	+= locore.o
+lib-$(CONFIG_CPU_R3000)	+= locore.o
Index: linux-macro/arch/mips/include/asm/dec/prom.h
===================================================================
--- linux-macro.orig/arch/mips/include/asm/dec/prom.h
+++ linux-macro/arch/mips/include/asm/dec/prom.h
@@ -43,16 +43,11 @@
  */
 #define REX_PROM_MAGIC		0x30464354

-#ifdef CONFIG_64BIT
-
-#define prom_is_rex(magic)	1	/* KN04 and KN05 are REX PROMs.  */
-
-#else /* !CONFIG_64BIT */
-
-#define prom_is_rex(magic)	((magic) == REX_PROM_MAGIC)
-
-#endif /* !CONFIG_64BIT */
-
+/* KN04 and KN05 are REX PROMs, so only do the check for R3k systems.  */
+static inline bool prom_is_rex(u32 magic)
+{
+	return !IS_ENABLED(CONFIG_CPU_R3000) || magic == REX_PROM_MAGIC;
+}

 /*
  * 3MIN/MAXINE PROM entry points for DS5000/1xx's, DS5000/xx's and