1 // SPDX-License-Identifier: GPL-2.0
2 /* Tests for presence or absence of hardware registers.
3  * This code was originally in atari/config.c, but I noticed
4  * that it was also in drivers/nubus/nubus.c and I wanted to
5  * use it in hp300/config.c, so it seemed sensible to pull it
6  * out into its own file.
7  *
8  * The test is for use when trying to read a hardware register
9  * that isn't present would cause a bus error. We set up a
10  * temporary handler so that this doesn't kill the kernel.
11  *
12  * There is a test-by-reading and a test-by-writing; I present
13  * them here complete with the comments from the original atari
14  * config.c...
15  *                -- PMM <pmaydell@chiark.greenend.org.uk>, 05/1998
16  */
17 
18 /* This function tests for the presence of an address, specially a
19  * hardware register address. It is called very early in the kernel
20  * initialization process, when the VBR register isn't set up yet. On
21  * an Atari, it still points to address 0, which is unmapped. So a bus
22  * error would cause another bus error while fetching the exception
23  * vector, and the CPU would do nothing at all. So we needed to set up
24  * a temporary VBR and a vector table for the duration of the test.
25  */
26 
27 #include <linux/module.h>
28 
29 #include <asm/hwtest.h>
30 
hwreg_present(volatile void * regp)31 int hwreg_present(volatile void *regp)
32 {
33 	int ret = 0;
34 	unsigned long flags;
35 	long save_sp, save_vbr;
36 	long tmp_vectors[3];
37 
38 	local_irq_save(flags);
39 	__asm__ __volatile__ (
40 		"movec %/vbr,%2\n\t"
41 		"movel #Lberr1,%4@(8)\n\t"
42 		"movec %4,%/vbr\n\t"
43 		"movel %/sp,%1\n\t"
44 		"moveq #0,%0\n\t"
45 		"tstb %3@\n\t"
46 		"nop\n\t"
47 		"moveq #1,%0\n"
48 	"Lberr1:\n\t"
49 		"movel %1,%/sp\n\t"
50 		"movec %2,%/vbr"
51 		: "=&d" (ret), "=&r" (save_sp), "=&r" (save_vbr)
52 		: "a" (regp), "a" (tmp_vectors)
53 	);
54 	local_irq_restore(flags);
55 
56 	return ret;
57 }
58 EXPORT_SYMBOL(hwreg_present);
59 
60 /* Basically the same, but writes a value into a word register, protected
61  * by a bus error handler. Returns 1 if successful, 0 otherwise.
62  */
63 
hwreg_write(volatile void * regp,unsigned short val)64 int hwreg_write(volatile void *regp, unsigned short val)
65 {
66 	int ret;
67 	unsigned long flags;
68 	long save_sp, save_vbr;
69 	long tmp_vectors[3];
70 
71 	local_irq_save(flags);
72 	__asm__ __volatile__ (
73 		"movec %/vbr,%2\n\t"
74 		"movel #Lberr2,%4@(8)\n\t"
75 		"movec %4,%/vbr\n\t"
76 		"movel %/sp,%1\n\t"
77 		"moveq #0,%0\n\t"
78 		"movew %5,%3@\n\t"
79 		"nop\n\t"
80 		/*
81 		 * If this nop isn't present, 'ret' may already be loaded
82 		 * with 1 at the time the bus error happens!
83 		 */
84 		"moveq #1,%0\n"
85 	"Lberr2:\n\t"
86 		"movel %1,%/sp\n\t"
87 		"movec %2,%/vbr"
88 		: "=&d" (ret), "=&r" (save_sp), "=&r" (save_vbr)
89 		: "a" (regp), "a" (tmp_vectors), "g" (val)
90 	);
91 	local_irq_restore(flags);
92 
93 	return ret;
94 }
95 EXPORT_SYMBOL(hwreg_write);
96 
97