MIPS: emulate CPUCFG instruction on older Loongson64 cores

CPUCFG is the instruction for querying processor characteristics on
newer Loongson processors, much like CPUID of x86. Since the instruction
is supposedly designed to provide a unified way to do feature detection
(without having to, for example, parse /proc/cpuinfo which is too
heavyweight), it is important to provide compatibility for older cores
without native support. Fortunately, most of the fields can be
synthesized without changes to semantics. Performance is not really big
a concern, because feature detection logic is not expected to be
invoked very often in typical userland applications.

The instruction can't be emulated on LOONGSON_2EF cores, according to
FlyGoat's experiments. Because the LWC2 opcode is assigned to other
valid instructions on 2E and 2F, no RI exception is raised for us to
intercept. So compatibility is only extended back furthest to
Loongson-3A1000. Loongson-2K is covered too, as it is basically a remix
of various blocks from the 3A/3B models from a kernel perspective.

This is lightly based on Loongson's work on their Linux 3.10 fork, for
being the authority on the right feature flags to fill in, where things
aren't otherwise discoverable.

Signed-off-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

2 files changed, 72 insertions, 0 deletions

diff --git a/arch/mips/include/asm/cpu-info.h b/arch/mips/include/asm/cpu-info.h
index bce3ea7fff7c..a600670d00e9 100644
--- a/arch/mips/include/asm/cpu-info.h
+++ b/arch/mips/include/asm/cpu-info.h
@@ -105,6 +105,15 @@ struct cpuinfo_mips {
 	unsigned int		gtoffset_mask;
 	unsigned int		guestid_mask;
 	unsigned int		guestid_cache;
+
+#ifdef CONFIG_CPU_LOONGSON3_CPUCFG_EMULATION
+	/* CPUCFG data for this CPU, synthesized at probe time.
+	 *
+	 * CPUCFG select 0 is PRId, 4 and above are unimplemented for now.
+	 * So the only stored values are for CPUCFG selects 1-3 inclusive.
+	 */
+	u32 loongson3_cpucfg_data[3];
+#endif
 } __attribute__((aligned(SMP_CACHE_BYTES)));
 
 extern struct cpuinfo_mips cpu_data[];
diff --git a/arch/mips/include/asm/mach-loongson64/cpucfg-emul.h b/arch/mips/include/asm/mach-loongson64/cpucfg-emul.h
new file mode 100644
index 000000000000..01dc308df7b2
--- /dev/null
+++ b/arch/mips/include/asm/mach-loongson64/cpucfg-emul.h
@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_MACH_LOONGSON64_CPUCFG_EMUL_H_
+#define _ASM_MACH_LOONGSON64_CPUCFG_EMUL_H_
+
+#include <asm/cpu-info.h>
+
+#ifdef CONFIG_CPU_LOONGSON3_CPUCFG_EMULATION
+
+#include <loongson_regs.h>
+
+#define LOONGSON_FPREV_MASK 0x7
+
+void loongson3_cpucfg_synthesize_data(struct cpuinfo_mips *c);
+
+static inline u32 loongson3_cpucfg_read_synthesized(struct cpuinfo_mips *c,
+	__u64 sel)
+{
+	switch (sel) {
+	case LOONGSON_CFG0:
+		return c->processor_id;
+	case LOONGSON_CFG1:
+	case LOONGSON_CFG2:
+	case LOONGSON_CFG3:
+		return c->loongson3_cpucfg_data[sel - 1];
+	case LOONGSON_CFG4:
+	case LOONGSON_CFG5:
+		/* CPUCFG selects 4 and 5 are related to the input clock
+		 * signal.
+		 *
+		 * Unimplemented for now.
+		 */
+		return 0;
+	case LOONGSON_CFG6:
+		/* CPUCFG select 6 is for the undocumented Safe Extension. */
+		return 0;
+	case LOONGSON_CFG7:
+		/* CPUCFG select 7 is for the virtualization extension.
+		 * We don't know if the two currently known features are
+		 * supported on older cores according to the public
+		 * documentation, so leave this at zero.
+		 */
+		return 0;
+	}
+
+	/*
+	 * Return 0 for unrecognized CPUCFG selects, which is real hardware
+	 * behavior observed on Loongson 3A R4.
+	 */
+	return 0;
+}
+#else
+static inline void loongson3_cpucfg_synthesize_data(struct cpuinfo_mips *c)
+{
+}
+
+static inline u32 loongson3_cpucfg_read_synthesized(struct cpuinfo_mips *c,
+	__u64 sel)
+{
+	return 0;
+}
+#endif
+
+#endif /* _ASM_MACH_LOONGSON64_CPUCFG_EMUL_H_ */