arch/m68k/mac/via.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  *      6522 Versatile Interface Adapter (VIA)
   4  *
   5  *      There are two of these on the Mac II. Some IRQs are vectored
   6  *      via them as are assorted bits and bobs - eg RTC, ADB.
   7  *
   8  * CSA: Motorola seems to have removed documentation on the 6522 from
   9  * their web site; try
  10  *     http://nerini.drf.com/vectrex/other/text/chips/6522/
  11  *     http://www.zymurgy.net/classic/vic20/vicdet1.htm
  12  * and
  13  *     http://193.23.168.87/mikro_laborversuche/via_iobaustein/via6522_1.html
  14  * for info.  A full-text web search on 6522 AND VIA will probably also
  15  * net some usefulness. <cananian@alumni.princeton.edu> 20apr1999
  16  *
  17  * Additional data is here (the SY6522 was used in the Mac II etc):
  18  *     http://www.6502.org/documents/datasheets/synertek/synertek_sy6522.pdf
  19  *     http://www.6502.org/documents/datasheets/synertek/synertek_sy6522_programming_reference.pdf
  20  *
  21  * PRAM/RTC access algorithms are from the NetBSD RTC toolkit version 1.08b
  22  * by Erik Vogan and adapted to Linux by Joshua M. Thompson (funaho@jurai.org)
  23  *
  24  */
  25
  26 #include <linux/clocksource.h>
  27 #include <linux/types.h>
  28 #include <linux/kernel.h>
  29 #include <linux/mm.h>
  30 #include <linux/delay.h>
  31 #include <linux/init.h>
  32 #include <linux/module.h>
  33 #include <linux/irq.h>
  34
  35 #include <asm/macintosh.h>
  36 #include <asm/macints.h>
  37 #include <asm/mac_via.h>
  38 #include <asm/mac_psc.h>
  39 #include <asm/mac_oss.h>
  40
  41 volatile __u8 *via1, *via2;
  42 int rbv_present;
  43 int via_alt_mapping;
  44 EXPORT_SYMBOL(via_alt_mapping);
  45 static __u8 rbv_clear;
  46
  47 /*
  48  * Globals for accessing the VIA chip registers without having to
  49  * check if we're hitting a real VIA or an RBV. Normally you could
  50  * just hit the combined register (ie, vIER|rIER) but that seems to
  51  * break on AV Macs...probably because they actually decode more than
  52  * eight address bits. Why can't Apple engineers at least be
  53  * _consistently_ lazy?                          - 1999-05-21 (jmt)
  54  */
  55
  56 static int gIER,gIFR,gBufA,gBufB;
  57
  58 /*
  59  * On Macs with a genuine VIA chip there is no way to mask an individual slot
  60  * interrupt. This limitation also seems to apply to VIA clone logic cores in
  61  * Quadra-like ASICs. (RBV and OSS machines don't have this limitation.)
  62  *
  63  * We used to fake it by configuring the relevant VIA pin as an output
  64  * (to mask the interrupt) or input (to unmask). That scheme did not work on
  65  * (at least) the Quadra 700. A NuBus card's /NMRQ signal is an open-collector
  66  * circuit (see Designing Cards and Drivers for Macintosh II and Macintosh SE,
  67  * p. 10-11 etc) but VIA outputs are not (see datasheet).
  68  *
  69  * Driving these outputs high must cause the VIA to source current and the
  70  * card to sink current when it asserts /NMRQ. Current will flow but the pin
  71  * voltage is uncertain and so the /NMRQ condition may still cause a transition
  72  * at the VIA2 CA1 input (which explains the lost interrupts). A side effect
  73  * is that a disabled slot IRQ can never be tested as pending or not.
  74  *
  75  * Driving these outputs low doesn't work either. All the slot /NMRQ lines are
  76  * (active low) OR'd together to generate the CA1 (aka "SLOTS") interrupt (see
  77  * The Guide To Macintosh Family Hardware, 2nd edition p. 167). If we drive a
  78  * disabled /NMRQ line low, the falling edge immediately triggers a CA1
  79  * interrupt and all slot interrupts after that will generate no transition
  80  * and therefore no interrupt, even after being re-enabled.
  81  *
  82  * So we make the VIA port A I/O lines inputs and use nubus_disabled to keep
  83  * track of their states. When any slot IRQ becomes disabled we mask the CA1
  84  * umbrella interrupt. Only when all slot IRQs become enabled do we unmask
  85  * the CA1 interrupt. It must remain enabled even when cards have no interrupt
  86  * handler registered. Drivers must therefore disable a slot interrupt at the
  87  * device before they call free_irq (like shared and autovector interrupts).
  88  *
  89  * There is also a related problem when MacOS is used to boot Linux. A network
  90  * card brought up by a MacOS driver may raise an interrupt while Linux boots.
  91  * This can be fatal since it can't be handled until the right driver loads
  92  * (if such a driver exists at all). Apparently related to this hardware
  93  * limitation, "Designing Cards and Drivers", p. 9-8, says that a slot
  94  * interrupt with no driver would crash MacOS (the book was written before
  95  * the appearance of Macs with RBV or OSS).
  96  */
  97
  98 static u8 nubus_disabled;
  99
 100 void via_debug_dump(void);
 101 static void via_nubus_init(void);
 102
 103 /*
 104  * Initialize the VIAs
 105  *
 106  * First we figure out where they actually _are_ as well as what type of
 107  * VIA we have for VIA2 (it could be a real VIA or an RBV or even an OSS.)
 108  * Then we pretty much clear them out and disable all IRQ sources.
 109  */
 110
 111 void __init via_init(void)
 112 {
 113         via1 = (void *)VIA1_BASE;
 114         pr_debug("VIA1 detected at %p\n", via1);
 115
 116         if (oss_present) {
 117                 via2 = NULL;
 118                 rbv_present = 0;
 119         } else {
 120                 switch (macintosh_config->via_type) {
 121
 122                 /* IIci, IIsi, IIvx, IIvi (P6xx), LC series */
 123
 124                 case MAC_VIA_IICI:
 125                         via2 = (void *)RBV_BASE;
 126                         pr_debug("VIA2 (RBV) detected at %p\n", via2);
 127                         rbv_present = 1;
 128                         if (macintosh_config->ident == MAC_MODEL_LCIII) {
 129                                 rbv_clear = 0x00;
 130                         } else {
 131                                 /* on most RBVs (& unlike the VIAs), you   */
 132                                 /* need to set bit 7 when you write to IFR */
 133                                 /* in order for your clear to occur.       */
 134                                 rbv_clear = 0x80;
 135                         }
 136                         gIER = rIER;
 137                         gIFR = rIFR;
 138                         gBufA = rSIFR;
 139                         gBufB = rBufB;
 140                         break;
 141
 142                 /* Quadra and early MacIIs agree on the VIA locations */
 143
 144                 case MAC_VIA_QUADRA:
 145                 case MAC_VIA_II:
 146                         via2 = (void *) VIA2_BASE;
 147                         pr_debug("VIA2 detected at %p\n", via2);
 148                         rbv_present = 0;
 149                         rbv_clear = 0x00;
 150                         gIER = vIER;
 151                         gIFR = vIFR;
 152                         gBufA = vBufA;
 153                         gBufB = vBufB;
 154                         break;
 155
 156                 default:
 157                         panic("UNKNOWN VIA TYPE");
 158                 }
 159         }
 160
 161 #ifdef DEBUG_VIA
 162         via_debug_dump();
 163 #endif
 164
 165         /*
 166          * Shut down all IRQ sources, reset the timers, and
 167          * kill the timer latch on VIA1.
 168          */
 169
 170         via1[vIER] = 0x7F;
 171         via1[vIFR] = 0x7F;
 172         via1[vT1LL] = 0;
 173         via1[vT1LH] = 0;
 174         via1[vT1CL] = 0;
 175         via1[vT1CH] = 0;
 176         via1[vT2CL] = 0;
 177         via1[vT2CH] = 0;
 178         via1[vACR] &= ~0xC0; /* setup T1 timer with no PB7 output */
 179         via1[vACR] &= ~0x03; /* disable port A & B latches */
 180
 181         /*
 182          * SE/30: disable video IRQ
 183          */
 184
 185         if (macintosh_config->ident == MAC_MODEL_SE30) {
 186                 via1[vDirB] |= 0x40;
 187                 via1[vBufB] |= 0x40;
 188         }
 189
 190         switch (macintosh_config->adb_type) {
 191         case MAC_ADB_IOP:
 192         case MAC_ADB_II:
 193         case MAC_ADB_PB1:
 194                 /*
 195                  * Set the RTC bits to a known state: all lines to outputs and
 196                  * RTC disabled (yes that's 0 to enable and 1 to disable).
 197                  */
 198                 via1[vDirB] |= VIA1B_vRTCEnb | VIA1B_vRTCClk | VIA1B_vRTCData;
 199                 via1[vBufB] |= VIA1B_vRTCEnb | VIA1B_vRTCClk;
 200                 break;
 201         }
 202
 203         /* Everything below this point is VIA2/RBV only... */
 204
 205         if (oss_present)
 206                 return;
 207
 208         if ((macintosh_config->via_type == MAC_VIA_QUADRA) &&
 209             (macintosh_config->adb_type != MAC_ADB_PB1) &&
 210             (macintosh_config->adb_type != MAC_ADB_PB2) &&
 211             (macintosh_config->ident    != MAC_MODEL_C660) &&
 212             (macintosh_config->ident    != MAC_MODEL_Q840)) {
 213                 via_alt_mapping = 1;
 214                 via1[vDirB] |= 0x40;
 215                 via1[vBufB] &= ~0x40;
 216         } else {
 217                 via_alt_mapping = 0;
 218         }
 219
 220         /*
 221          * Now initialize VIA2. For RBV we just kill all interrupts;
 222          * for a regular VIA we also reset the timers and stuff.
 223          */
 224
 225         via2[gIER] = 0x7F;
 226         via2[gIFR] = 0x7F | rbv_clear;
 227         if (!rbv_present) {
 228                 via2[vT1LL] = 0;
 229                 via2[vT1LH] = 0;
 230                 via2[vT1CL] = 0;
 231                 via2[vT1CH] = 0;
 232                 via2[vT2CL] = 0;
 233                 via2[vT2CH] = 0;
 234                 via2[vACR] &= ~0xC0; /* setup T1 timer with no PB7 output */
 235                 via2[vACR] &= ~0x03; /* disable port A & B latches */
 236         }
 237
 238         via_nubus_init();
 239
 240         /* Everything below this point is VIA2 only... */
 241
 242         if (rbv_present)
 243                 return;
 244
 245         /*
 246          * Set vPCR for control line interrupts.
 247          *
 248          * CA1 (SLOTS IRQ), CB1 (ASC IRQ): negative edge trigger.
 249          *
 250          * Macs with ESP SCSI have a negative edge triggered SCSI interrupt.
 251          * Testing reveals that PowerBooks do too. However, the SE/30
 252          * schematic diagram shows an active high NCR5380 IRQ line.
 253          */
 254
 255         pr_debug("VIA2 vPCR is 0x%02X\n", via2[vPCR]);
 256         if (macintosh_config->via_type == MAC_VIA_II) {
 257                 /* CA2 (SCSI DRQ), CB2 (SCSI IRQ): indep. input, pos. edge */
 258                 via2[vPCR] = 0x66;
 259         } else {
 260                 /* CA2 (SCSI DRQ), CB2 (SCSI IRQ): indep. input, neg. edge */
 261                 via2[vPCR] = 0x22;
 262         }
 263 }
 264
 265 /*
 266  * Debugging dump, used in various places to see what's going on.
 267  */
 268
 269 void via_debug_dump(void)
 270 {
 271         printk(KERN_DEBUG "VIA1: DDRA = 0x%02X DDRB = 0x%02X ACR = 0x%02X\n",
 272                 (uint) via1[vDirA], (uint) via1[vDirB], (uint) via1[vACR]);
 273         printk(KERN_DEBUG "         PCR = 0x%02X  IFR = 0x%02X IER = 0x%02X\n",
 274                 (uint) via1[vPCR], (uint) via1[vIFR], (uint) via1[vIER]);
 275         if (!via2)
 276                 return;
 277         if (rbv_present) {
 278                 printk(KERN_DEBUG "VIA2:  IFR = 0x%02X  IER = 0x%02X\n",
 279                         (uint) via2[rIFR], (uint) via2[rIER]);
 280                 printk(KERN_DEBUG "      SIFR = 0x%02X SIER = 0x%02X\n",
 281                         (uint) via2[rSIFR], (uint) via2[rSIER]);
 282         } else {
 283                 printk(KERN_DEBUG "VIA2: DDRA = 0x%02X DDRB = 0x%02X ACR = 0x%02X\n",
 284                         (uint) via2[vDirA], (uint) via2[vDirB],
 285                         (uint) via2[vACR]);
 286                 printk(KERN_DEBUG "         PCR = 0x%02X  IFR = 0x%02X IER = 0x%02X\n",
 287                         (uint) via2[vPCR],
 288                         (uint) via2[vIFR], (uint) via2[vIER]);
 289         }
 290 }
 291
 292 /*
 293  * Flush the L2 cache on Macs that have it by flipping
 294  * the system into 24-bit mode for an instant.
 295  */
 296
 297 void via_flush_cache(void)
 298 {
 299         via2[gBufB] &= ~VIA2B_vMode32;
 300         via2[gBufB] |= VIA2B_vMode32;
 301 }
 302
 303 /*
 304  * Return the status of the L2 cache on a IIci
 305  */
 306
 307 int via_get_cache_disable(void)
 308 {
 309         /* Safeguard against being called accidentally */
 310         if (!via2) {
 311                 printk(KERN_ERR "via_get_cache_disable called on a non-VIA machine!\n");
 312                 return 1;
 313         }
 314
 315         return (int) via2[gBufB] & VIA2B_vCDis;
 316 }
 317
 318 /*
 319  * Initialize VIA2 for Nubus access
 320  */
 321
 322 static void __init via_nubus_init(void)
 323 {
 324         /* unlock nubus transactions */
 325
 326         if ((macintosh_config->adb_type != MAC_ADB_PB1) &&
 327             (macintosh_config->adb_type != MAC_ADB_PB2)) {
 328                 /* set the line to be an output on non-RBV machines */
 329                 if (!rbv_present)
 330                         via2[vDirB] |= 0x02;
 331
 332                 /* this seems to be an ADB bit on PMU machines */
 333                 /* according to MkLinux.  -- jmt               */
 334                 via2[gBufB] |= 0x02;
 335         }
 336
 337         /*
 338          * Disable the slot interrupts. On some hardware that's not possible.
 339          * On some hardware it's unclear what all of these I/O lines do.
 340          */
 341
 342         switch (macintosh_config->via_type) {
 343         case MAC_VIA_II:
 344         case MAC_VIA_QUADRA:
 345                 pr_debug("VIA2 vDirA is 0x%02X\n", via2[vDirA]);
 346                 break;
 347         case MAC_VIA_IICI:
 348                 /* RBV. Disable all the slot interrupts. SIER works like IER. */
 349                 via2[rSIER] = 0x7F;
 350                 break;
 351         }
 352 }
 353
 354 void via_nubus_irq_startup(int irq)
 355 {
 356         int irq_idx = IRQ_IDX(irq);
 357
 358         switch (macintosh_config->via_type) {
 359         case MAC_VIA_II:
 360         case MAC_VIA_QUADRA:
 361                 /* Make the port A line an input. Probably redundant. */
 362                 if (macintosh_config->via_type == MAC_VIA_II) {
 363                         /* The top two bits are RAM size outputs. */
 364                         via2[vDirA] &= 0xC0 | ~(1 << irq_idx);
 365                 } else {
 366                         /* Allow NuBus slots 9 through F. */
 367                         via2[vDirA] &= 0x80 | ~(1 << irq_idx);
 368                 }
 369                 /* fall through */
 370         case MAC_VIA_IICI:
 371                 via_irq_enable(irq);
 372                 break;
 373         }
 374 }
 375
 376 void via_nubus_irq_shutdown(int irq)
 377 {
 378         switch (macintosh_config->via_type) {
 379         case MAC_VIA_II:
 380         case MAC_VIA_QUADRA:
 381                 /* Ensure that the umbrella CA1 interrupt remains enabled. */
 382                 via_irq_enable(irq);
 383                 break;
 384         case MAC_VIA_IICI:
 385                 via_irq_disable(irq);
 386                 break;
 387         }
 388 }
 389
 390 /*
 391  * The generic VIA interrupt routines (shamelessly stolen from Alan Cox's
 392  * via6522.c :-), disable/pending masks added.
 393  */
 394
 395 #define VIA_TIMER_1_INT BIT(6)
 396
 397 void via1_irq(struct irq_desc *desc)
 398 {
 399         int irq_num;
 400         unsigned char irq_bit, events;
 401
 402         events = via1[vIFR] & via1[vIER] & 0x7F;
 403         if (!events)
 404                 return;
 405
 406         irq_num = IRQ_MAC_TIMER_1;
 407         irq_bit = VIA_TIMER_1_INT;
 408         if (events & irq_bit) {
 409                 unsigned long flags;
 410
 411                 local_irq_save(flags);
 412                 via1[vIFR] = irq_bit;
 413                 generic_handle_irq(irq_num);
 414                 local_irq_restore(flags);
 415
 416                 events &= ~irq_bit;
 417                 if (!events)
 418                         return;
 419         }
 420
 421         irq_num = VIA1_SOURCE_BASE;
 422         irq_bit = 1;
 423         do {
 424                 if (events & irq_bit) {
 425                         via1[vIFR] = irq_bit;
 426                         generic_handle_irq(irq_num);
 427                 }
 428                 ++irq_num;
 429                 irq_bit <<= 1;
 430         } while (events >= irq_bit);
 431 }
 432
 433 static void via2_irq(struct irq_desc *desc)
 434 {
 435         int irq_num;
 436         unsigned char irq_bit, events;
 437
 438         events = via2[gIFR] & via2[gIER] & 0x7F;
 439         if (!events)
 440                 return;
 441
 442         irq_num = VIA2_SOURCE_BASE;
 443         irq_bit = 1;
 444         do {
 445                 if (events & irq_bit) {
 446                         via2[gIFR] = irq_bit | rbv_clear;
 447                         generic_handle_irq(irq_num);
 448                 }
 449                 ++irq_num;
 450                 irq_bit <<= 1;
 451         } while (events >= irq_bit);
 452 }
 453
 454 /*
 455  * Dispatch Nubus interrupts. We are called as a secondary dispatch by the
 456  * VIA2 dispatcher as a fast interrupt handler.
 457  */
 458
 459 static void via_nubus_irq(struct irq_desc *desc)
 460 {
 461         int slot_irq;
 462         unsigned char slot_bit, events;
 463
 464         events = ~via2[gBufA] & 0x7F;
 465         if (rbv_present)
 466                 events &= via2[rSIER];
 467         else
 468                 events &= ~via2[vDirA];
 469         if (!events)
 470                 return;
 471
 472         do {
 473                 slot_irq = IRQ_NUBUS_F;
 474                 slot_bit = 0x40;
 475                 do {
 476                         if (events & slot_bit) {
 477                                 events &= ~slot_bit;
 478                                 generic_handle_irq(slot_irq);
 479                         }
 480                         --slot_irq;
 481                         slot_bit >>= 1;
 482                 } while (events);
 483
 484                 /* clear the CA1 interrupt and make certain there's no more. */
 485                 via2[gIFR] = 0x02 | rbv_clear;
 486                 events = ~via2[gBufA] & 0x7F;
 487                 if (rbv_present)
 488                         events &= via2[rSIER];
 489                 else
 490                         events &= ~via2[vDirA];
 491         } while (events);
 492 }
 493
 494 /*
 495  * Register the interrupt dispatchers for VIA or RBV machines only.
 496  */
 497
 498 void __init via_register_interrupts(void)
 499 {
 500         if (via_alt_mapping) {
 501                 /* software interrupt */
 502                 irq_set_chained_handler(IRQ_AUTO_1, via1_irq);
 503                 /* via1 interrupt */
 504                 irq_set_chained_handler(IRQ_AUTO_6, via1_irq);
 505         } else {
 506                 irq_set_chained_handler(IRQ_AUTO_1, via1_irq);
 507         }
 508         irq_set_chained_handler(IRQ_AUTO_2, via2_irq);
 509         irq_set_chained_handler(IRQ_MAC_NUBUS, via_nubus_irq);
 510 }
 511
 512 void via_irq_enable(int irq) {
 513         int irq_src     = IRQ_SRC(irq);
 514         int irq_idx     = IRQ_IDX(irq);
 515
 516         if (irq_src == 1) {
 517                 via1[vIER] = IER_SET_BIT(irq_idx);
 518         } else if (irq_src == 2) {
 519                 if (irq != IRQ_MAC_NUBUS || nubus_disabled == 0)
 520                         via2[gIER] = IER_SET_BIT(irq_idx);
 521         } else if (irq_src == 7) {
 522                 switch (macintosh_config->via_type) {
 523                 case MAC_VIA_II:
 524                 case MAC_VIA_QUADRA:
 525                         nubus_disabled &= ~(1 << irq_idx);
 526                         /* Enable the CA1 interrupt when no slot is disabled. */
 527                         if (!nubus_disabled)
 528                                 via2[gIER] = IER_SET_BIT(1);
 529                         break;
 530                 case MAC_VIA_IICI:
 531                         /* On RBV, enable the slot interrupt.
 532                          * SIER works like IER.
 533                          */
 534                         via2[rSIER] = IER_SET_BIT(irq_idx);
 535                         break;
 536                 }
 537         }
 538 }
 539
 540 void via_irq_disable(int irq) {
 541         int irq_src     = IRQ_SRC(irq);
 542         int irq_idx     = IRQ_IDX(irq);
 543
 544         if (irq_src == 1) {
 545                 via1[vIER] = IER_CLR_BIT(irq_idx);
 546         } else if (irq_src == 2) {
 547                 via2[gIER] = IER_CLR_BIT(irq_idx);
 548         } else if (irq_src == 7) {
 549                 switch (macintosh_config->via_type) {
 550                 case MAC_VIA_II:
 551                 case MAC_VIA_QUADRA:
 552                         nubus_disabled |= 1 << irq_idx;
 553                         if (nubus_disabled)
 554                                 via2[gIER] = IER_CLR_BIT(1);
 555                         break;
 556                 case MAC_VIA_IICI:
 557                         via2[rSIER] = IER_CLR_BIT(irq_idx);
 558                         break;
 559                 }
 560         }
 561 }
 562
 563 void via1_set_head(int head)
 564 {
 565         if (head == 0)
 566                 via1[vBufA] &= ~VIA1A_vHeadSel;
 567         else
 568                 via1[vBufA] |= VIA1A_vHeadSel;
 569 }
 570 EXPORT_SYMBOL(via1_set_head);
 571
 572 int via2_scsi_drq_pending(void)
 573 {
 574         return via2[gIFR] & (1 << IRQ_IDX(IRQ_MAC_SCSIDRQ));
 575 }
 576 EXPORT_SYMBOL(via2_scsi_drq_pending);
 577
 578 /* timer and clock source */
 579
 580 #define VIA_CLOCK_FREQ     783360                /* VIA "phase 2" clock in Hz */
 581 #define VIA_TIMER_CYCLES   (VIA_CLOCK_FREQ / HZ) /* clock cycles per jiffy */
 582
 583 #define VIA_TC             (VIA_TIMER_CYCLES - 2) /* including 0 and -1 */
 584 #define VIA_TC_LOW         (VIA_TC & 0xFF)
 585 #define VIA_TC_HIGH        (VIA_TC >> 8)
 586
 587 static u64 mac_read_clk(struct clocksource *cs);
 588
 589 static struct clocksource mac_clk = {
 590         .name   = "via1",
 591         .rating = 250,
 592         .read   = mac_read_clk,
 593         .mask   = CLOCKSOURCE_MASK(32),
 594         .flags  = CLOCK_SOURCE_IS_CONTINUOUS,
 595 };
 596
 597 static u32 clk_total, clk_offset;
 598
 599 static irqreturn_t via_timer_handler(int irq, void *dev_id)
 600 {
 601         irq_handler_t timer_routine = dev_id;
 602
 603         clk_total += VIA_TIMER_CYCLES;
 604         clk_offset = 0;
 605         timer_routine(0, NULL);
 606
 607         return IRQ_HANDLED;
 608 }
 609
 610 void __init via_init_clock(irq_handler_t timer_routine)
 611 {
 612         if (request_irq(IRQ_MAC_TIMER_1, via_timer_handler, IRQF_TIMER, "timer",
 613                         timer_routine)) {
 614                 pr_err("Couldn't register %s interrupt\n", "timer");
 615                 return;
 616         }
 617
 618         via1[vT1LL] = VIA_TC_LOW;
 619         via1[vT1LH] = VIA_TC_HIGH;
 620         via1[vT1CL] = VIA_TC_LOW;
 621         via1[vT1CH] = VIA_TC_HIGH;
 622         via1[vACR] |= 0x40;
 623
 624         clocksource_register_hz(&mac_clk, VIA_CLOCK_FREQ);
 625 }
 626
 627 static u64 mac_read_clk(struct clocksource *cs)
 628 {
 629         unsigned long flags;
 630         u8 count_high;
 631         u16 count;
 632         u32 ticks;
 633
 634         /*
 635          * Timer counter wrap-around is detected with the timer interrupt flag
 636          * but reading the counter low byte (vT1CL) would reset the flag.
 637          * Also, accessing both counter registers is essentially a data race.
 638          * These problems are avoided by ignoring the low byte. Clock accuracy
 639          * is 256 times worse (error can reach 0.327 ms) but CPU overhead is
 640          * reduced by avoiding slow VIA register accesses.
 641          */
 642
 643         local_irq_save(flags);
 644         count_high = via1[vT1CH];
 645         if (count_high == 0xFF)
 646                 count_high = 0;
 647         if (count_high > 0 && (via1[vIFR] & VIA_TIMER_1_INT))
 648                 clk_offset = VIA_TIMER_CYCLES;
 649         count = count_high << 8;
 650         ticks = VIA_TIMER_CYCLES - count;
 651         ticks += clk_offset + clk_total;
 652         local_irq_restore(flags);
 653
 654         return ticks;
 655 }