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 95