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 int hwreg_present(volatile void *regp)
30 {
31 int ret = 0;
32 unsigned long flags;
33 long save_sp, save_vbr;
34 long tmp_vectors[3];
35
36 local_irq_save(flags);
37 __asm__ __volatile__ (
38 "movec %/vbr,%2\n\t"
39 "movel #Lberr1,%4@(8)\n\t"
40 "movec %4,%/vbr\n\t"
41 "movel %/sp,%1\n\t"
42 "moveq #0,%0\n\t"
43 "tstb %3@\n\t"
44 "nop\n\t"
45 "moveq #1,%0\n"
46 "Lberr1:\n\t"
47 "movel %1,%/sp\n\t"
48 "movec %2,%/vbr"
49 : "=&d" (ret), "=&r" (save_sp), "=&r" (save_vbr)
50 : "a" (regp), "a" (tmp_vectors)
51 );
52 local_irq_restore(flags);
53
54 return ret;
55 }
56 EXPORT_SYMBOL(hwreg_present);
57
58 /* Basically the same, but writes a value into a word register, protected
59 * by a bus error handler. Returns 1 if successful, 0 otherwise.
60 */
61
62 int hwreg_write(volatile void *regp, unsigned short val)
63 {
64 int ret;
65 unsigned long flags;
66 long save_sp, save_vbr;
67 long tmp_vectors[3];
68
69 local_irq_save(flags);
70 __asm__ __volatile__ (
71 "movec %/vbr,%2\n\t"
72 "movel #Lberr2,%4@(8)\n\t"
73 "movec %4,%/vbr\n\t"
74 "movel %/sp,%1\n\t"
75 "moveq #0,%0\n\t"
76 "movew %5,%3@\n\t"
77 "nop\n\t"
78 /*
79 * If this nop isn't present, 'ret' may already be loaded
80 * with 1 at the time the bus error happens!
81 */
82 "moveq #1,%0\n"
83 "Lberr2:\n\t"
84 "movel %1,%/sp\n\t"
85 "movec %2,%/vbr"
86 : "=&d" (ret), "=&r" (save_sp), "=&r" (save_vbr)
87 : "a" (regp), "a" (tmp_vectors), "g" (val)
88 );
89 local_irq_restore(flags);
90
91 return ret;
92 }
93 EXPORT_SYMBOL(hwreg_write);
94