MIPS: Expose Loongson CPUCFG availability via HWCAP

The point is to allow userspace to probe for CPUCFG without possibly
triggering invalid instructions. In addition to that, future Loongson
feature bits could all be stuffed into CPUCFG bit fields (or "leaves"
in x86-speak) if Loongson does not make mistakes, so ELF HWCAP bits are
conserved.

Userspace can determine native CPUCFG availability by checking the LCSRP
(Loongson CSR Present) bit in CPUCFG output after seeing CPUCFG bit in
HWCAP. Native CPUCFG always sets the LCSRP bit, as CPUCFG is part of the
Loongson CSR ASE, while the emulation intentionally leaves this bit
clear.

The other existing Loongson-specific HWCAP bits are, to my best
knowledge, unused, as

(1) they are fairly recent additions,
(2) Loongson never back-ported the patch into their kernel fork, and
(3) Loongson's existing installed base rarely upgrade, if ever;

However, they are still considered userspace ABI, hence unfortunately
unremovable. But hopefully at least we could stop adding new Loongson
HWCAP bits in the future.

Cc: Paul Burton <paulburton@kernel.org>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Huacai Chen <chenhc@lemote.com>
Signed-off-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

diff --git a/arch/mips/include/uapi/asm/hwcap.h b/arch/mips/include/uapi/asm/hwcap.h
index 1ade1daa49210713c53bdd20af2564eeb628e3cd..b7e02bdc19859c8834982238c694a71112661bd4 100644 (file)
--- a/arch/mips/include/uapi/asm/hwcap.h
+++ b/arch/mips/include/uapi/asm/hwcap.h
@@ -17,5 +17,6 @@
 #define HWCAP_LOONGSON_MMI  (1 << 11)
 #define HWCAP_LOONGSON_EXT  (1 << 12)
 #define HWCAP_LOONGSON_EXT2 (1 << 13)
+#define HWCAP_LOONGSON_CPUCFG (1 << 14)
 
 #endif /* _UAPI_ASM_HWCAP_H */

diff --git a/arch/mips/loongson64/cpucfg-emul.c b/arch/mips/loongson64/cpucfg-emul.c
index c16023a13379415fc21e40cf6941cee51748a57c..ca75f07252df54e566daa2c08c15bf146306c34b 100644 (file)
--- a/arch/mips/loongson64/cpucfg-emul.c
+++ b/arch/mips/loongson64/cpucfg-emul.c
@@ -4,6 +4,7 @@
 #include <linux/types.h>
 #include <asm/cpu.h>
 #include <asm/cpu-info.h>
+#include <asm/elf.h>
 
 #include <loongson_regs.h>
 #include <cpucfg-emul.h>
@@ -128,7 +129,7 @@ void loongson3_cpucfg_synthesize_data(struct cpuinfo_mips *c)
 
        /* CPUs with CPUCFG support don't need to synthesize anything. */
        if (cpu_has_cfg())
-               return;
+               goto have_cpucfg_now;
 
        c->loongson3_cpucfg_data[0] = 0;
        c->loongson3_cpucfg_data[1] = 0;
@@ -217,4 +218,10 @@ void loongson3_cpucfg_synthesize_data(struct cpuinfo_mips *c)
        patch_cpucfg_sel1(c);
        patch_cpucfg_sel2(c);
        patch_cpucfg_sel3(c);
+
+have_cpucfg_now:
+       /* We have usable CPUCFG now, emulated or not.
+        * Announce CPUCFG availability to userspace via hwcap.
+        */
+       elf_hwcap |= HWCAP_LOONGSON_CPUCFG;
 }