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