MIPS: CPS: Early debug using an ns16550-compatible UART

Provide support for outputting early debug information, in the form of
various register values should an exception occur, during the early
bringup of secondary cores. This code requires an ns16550-compatible
UART accessible from the secondary core, and is written in assembly due
to the environment in which such early exceptions occur where way may
not have a stack, be coherent or even have initialised caches.

[ralf@linux-mips.org: Fix merge conflict.]

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Cc: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: linux-kernel@vger.kernel.org
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/11202/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 202 insertions, 0 deletions

diff --git a/arch/mips/kernel/cps-vec-ns16550.S b/arch/mips/kernel/cps-vec-ns16550.S
new file mode 100644
index 0000000..6d246ad
--- /dev/null
+++ b/arch/mips/kernel/cps-vec-ns16550.S
@@ -0,0 +1,202 @@
+/*
+ * Copyright (C) 2015 Imagination Technologies
+ * Author: Paul Burton <paul.burton@imgtec.com>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation;  either version 2 of the  License, or (at your
+ * option) any later version.
+ */
+
+#include <asm/addrspace.h>
+#include <asm/asm.h>
+#include <asm/asm-offsets.h>
+#include <asm/mipsregs.h>
+#include <asm/regdef.h>
+#include <linux/serial_reg.h>
+
+#define UART_TX_OFS	(UART_TX << CONFIG_MIPS_CPS_NS16550_SHIFT)
+#define UART_LSR_OFS	(UART_LSR << CONFIG_MIPS_CPS_NS16550_SHIFT)
+
+/**
+ * _mips_cps_putc() - write a character to the UART
+ * @a0: ASCII character to write
+ * @t9: UART base address
+ */
+LEAF(_mips_cps_putc)
+1:	lw		t0, UART_LSR_OFS(t9)
+	andi		t0, t0, UART_LSR_TEMT
+	beqz		t0, 1b
+	sb		a0, UART_TX_OFS(t9)
+	jr		ra
+	END(_mips_cps_putc)
+
+/**
+ * _mips_cps_puts() - write a string to the UART
+ * @a0: pointer to NULL-terminated ASCII string
+ * @t9: UART base address
+ *
+ * Write a null-terminated ASCII string to the UART.
+ */
+NESTED(_mips_cps_puts, 0, ra)
+	move		s7, ra
+	move		s6, a0
+
+1:	lb		a0, 0(s6)
+	beqz		a0, 2f
+	jal		_mips_cps_putc
+	PTR_ADDIU	s6, s6, 1
+	b		1b
+
+2:	jr		s7
+	END(_mips_cps_puts)
+
+/**
+ * _mips_cps_putx4 - write a 4b hex value to the UART
+ * @a0: the 4b value to write to the UART
+ * @t9: UART base address
+ *
+ * Write a single hexadecimal character to the UART.
+ */
+NESTED(_mips_cps_putx4, 0, ra)
+	andi		a0, a0, 0xf
+	li		t0, '0'
+	blt		a0, 10, 1f
+	li		t0, 'a'
+	addiu		a0, a0, -10
+1:	addu		a0, a0, t0
+	b		_mips_cps_putc
+	END(_mips_cps_putx4)
+
+/**
+ * _mips_cps_putx8 - write an 8b hex value to the UART
+ * @a0: the 8b value to write to the UART
+ * @t9: UART base address
+ *
+ * Write an 8 bit value (ie. 2 hexadecimal characters) to the UART.
+ */
+NESTED(_mips_cps_putx8, 0, ra)
+	move		s3, ra
+	move		s2, a0
+	srl		a0, a0, 4
+	jal		_mips_cps_putx4
+	move		a0, s2
+	move		ra, s3
+	b		_mips_cps_putx4
+	END(_mips_cps_putx8)
+
+/**
+ * _mips_cps_putx16 - write a 16b hex value to the UART
+ * @a0: the 16b value to write to the UART
+ * @t9: UART base address
+ *
+ * Write a 16 bit value (ie. 4 hexadecimal characters) to the UART.
+ */
+NESTED(_mips_cps_putx16, 0, ra)
+	move		s5, ra
+	move		s4, a0
+	srl		a0, a0, 8
+	jal		_mips_cps_putx8
+	move		a0, s4
+	move		ra, s5
+	b		_mips_cps_putx8
+	END(_mips_cps_putx16)
+
+/**
+ * _mips_cps_putx32 - write a 32b hex value to the UART
+ * @a0: the 32b value to write to the UART
+ * @t9: UART base address
+ *
+ * Write a 32 bit value (ie. 8 hexadecimal characters) to the UART.
+ */
+NESTED(_mips_cps_putx32, 0, ra)
+	move		s7, ra
+	move		s6, a0
+	srl		a0, a0, 16
+	jal		_mips_cps_putx16
+	move		a0, s6
+	move		ra, s7
+	b		_mips_cps_putx16
+	END(_mips_cps_putx32)
+
+#ifdef CONFIG_64BIT
+
+/**
+ * _mips_cps_putx64 - write a 64b hex value to the UART
+ * @a0: the 64b value to write to the UART
+ * @t9: UART base address
+ *
+ * Write a 64 bit value (ie. 16 hexadecimal characters) to the UART.
+ */
+NESTED(_mips_cps_putx64, 0, ra)
+	move		sp, ra
+	move		s8, a0
+	dsrl32		a0, a0, 0
+	jal		_mips_cps_putx32
+	move		a0, s8
+	move		ra, sp
+	b		_mips_cps_putx32
+	END(_mips_cps_putx64)
+
+#define _mips_cps_putxlong _mips_cps_putx64
+
+#else /* !CONFIG_64BIT */
+
+#define _mips_cps_putxlong _mips_cps_putx32
+
+#endif /* !CONFIG_64BIT */
+
+/**
+ * mips_cps_bev_dump() - dump relevant exception state to UART
+ * @a0: pointer to NULL-terminated ASCII string naming the exception
+ *
+ * Write information that may be useful in debugging an exception to the
+ * UART configured by CONFIG_MIPS_CPS_NS16550_*. As this BEV exception
+ * will only be run if something goes horribly wrong very early during
+ * the bringup of a core and it is very likely to be unsafe to perform
+ * memory accesses at that point (cache state indeterminate, EVA may not
+ * be configured, coherence may be disabled) let alone have a stack,
+ * this is all written in assembly using only registers & unmapped
+ * uncached access to the UART registers.
+ */
+LEAF(mips_cps_bev_dump)
+	move		s0, ra
+	move		s1, a0
+
+	li		t9, CKSEG1ADDR(CONFIG_MIPS_CPS_NS16550_BASE)
+
+	PTR_LA		a0, str_newline
+	jal		_mips_cps_puts
+	PTR_LA		a0, str_bev
+	jal		_mips_cps_puts
+	move		a0, s1
+	jal		_mips_cps_puts
+	PTR_LA		a0, str_newline
+	jal		_mips_cps_puts
+	PTR_LA		a0, str_newline
+	jal		_mips_cps_puts
+
+#define DUMP_COP0_REG(reg, name, sz, _mfc0)		\
+	PTR_LA		a0, 8f;				\
+	jal		_mips_cps_puts;			\
+	_mfc0		a0, reg;			\
+	jal		_mips_cps_putx##sz;		\
+	PTR_LA		a0, str_newline;		\
+	jal		_mips_cps_puts;			\
+	TEXT(name)
+
+	DUMP_COP0_REG(CP0_CAUSE,    "Cause:    0x", 32, mfc0)
+	DUMP_COP0_REG(CP0_STATUS,   "Status:   0x", 32, mfc0)
+	DUMP_COP0_REG(CP0_EBASE,    "EBase:    0x", long, MFC0)
+	DUMP_COP0_REG(CP0_BADVADDR, "BadVAddr: 0x", long, MFC0)
+	DUMP_COP0_REG(CP0_BADINSTR, "BadInstr: 0x", 32, mfc0)
+
+	PTR_LA		a0, str_newline
+	jal		_mips_cps_puts
+	jr		s0
+	END(mips_cps_bev_dump)
+
+.pushsection	.data
+str_bev: .asciiz "BEV Exception: "
+str_newline: .asciiz "\r\n"
+.popsection