1// SPDX-License-Identifier: GPL-2.0-or-later
2/*
3 * A udbg backend which logs messages and reads input from in memory
4 * buffers.
5 *
6 * The console output can be read from memcons_output which is a
7 * circular buffer whose next write position is stored in memcons.output_pos.
8 *
9 * Input may be passed by writing into the memcons_input buffer when it is
10 * empty. The input buffer is empty when both input_pos == input_start and
11 * *input_start == '\0'.
12 *
13 * Copyright (C) 2003-2005 Anton Blanchard and Milton Miller, IBM Corp
14 * Copyright (C) 2013 Alistair Popple, IBM Corp
15 */
16
17#include <linux/kernel.h>
18#include <asm/barrier.h>
19#include <asm/page.h>
20#include <asm/processor.h>
21#include <asm/udbg.h>
22
23struct memcons {
24	char *output_start;
25	char *output_pos;
26	char *output_end;
27	char *input_start;
28	char *input_pos;
29	char *input_end;
30};
31
32static char memcons_output[CONFIG_PPC_MEMCONS_OUTPUT_SIZE];
33static char memcons_input[CONFIG_PPC_MEMCONS_INPUT_SIZE];
34
35struct memcons memcons = {
36	.output_start = memcons_output,
37	.output_pos = memcons_output,
38	.output_end = &memcons_output[CONFIG_PPC_MEMCONS_OUTPUT_SIZE],
39	.input_start = memcons_input,
40	.input_pos = memcons_input,
41	.input_end = &memcons_input[CONFIG_PPC_MEMCONS_INPUT_SIZE],
42};
43
44static void memcons_putc(char c)
45{
46	char *new_output_pos;
47
48	*memcons.output_pos = c;
49	wmb();
50	new_output_pos = memcons.output_pos + 1;
51	if (new_output_pos >= memcons.output_end)
52		new_output_pos = memcons.output_start;
53
54	memcons.output_pos = new_output_pos;
55}
56
57static int memcons_getc_poll(void)
58{
59	char c;
60	char *new_input_pos;
61
62	if (*memcons.input_pos) {
63		c = *memcons.input_pos;
64
65		new_input_pos = memcons.input_pos + 1;
66		if (new_input_pos >= memcons.input_end)
67			new_input_pos = memcons.input_start;
68		else if (*new_input_pos == '\0')
69			new_input_pos = memcons.input_start;
70
71		*memcons.input_pos = '\0';
72		wmb();
73		memcons.input_pos = new_input_pos;
74		return c;
75	}
76
77	return -1;
78}
79
80static int memcons_getc(void)
81{
82	int c;
83
84	while (1) {
85		c = memcons_getc_poll();
86		if (c == -1)
87			cpu_relax();
88		else
89			break;
90	}
91
92	return c;
93}
94
95void __init udbg_init_memcons(void)
96{
97	udbg_putc = memcons_putc;
98	udbg_getc = memcons_getc;
99	udbg_getc_poll = memcons_getc_poll;
100}
101