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[vT1CL] = 0;
 173         via1[vT1CH] = 0;
 174         via1[vT2CL] = 0;
 175         via1[vT2CH] = 0;
 176         via1[vACR] &= ~0xC0; /* setup T1 timer with no PB7 output */
 177         via1[vACR] &= ~0x03; /* disable port A & B latches */
 178
 179         /*
 180          * SE/30: disable video IRQ
 181          */
 182
 183         if (macintosh_config->ident == MAC_MODEL_SE30) {
 184                 via1[vDirB] |= 0x40;
 185                 via1[vBufB] |= 0x40;
 186         }
 187
 188         switch (macintosh_config->adb_type) {
 189         case MAC_ADB_IOP:
 190         case MAC_ADB_II:
 191         case MAC_ADB_PB1:
 192                 /*
 193                  * Set the RTC bits to a known state: all lines to outputs and
 194                  * RTC disabled (yes that's 0 to enable and 1 to disable).
 195                  */
 196                 via1[vDirB] |= VIA1B_vRTCEnb | VIA1B_vRTCClk | VIA1B_vRTCData;
 197                 via1[vBufB] |= VIA1B_vRTCEnb | VIA1B_vRTCClk;
 198                 break;
 199         }
 200
 201         /* Everything below this point is VIA2/RBV only... */
 202
 203         if (oss_present)
 204                 return;
 205
 206         if ((macintosh_config->via_type == MAC_VIA_QUADRA) &&
 207             (macintosh_config->adb_type != MAC_ADB_PB1) &&
 208             (macintosh_config->adb_type != MAC_ADB_PB2) &&
 209             (macintosh_config->ident    != MAC_MODEL_C660) &&
 210             (macintosh_config->ident    != MAC_MODEL_Q840)) {
 211                 via_alt_mapping = 1;
 212                 via1[vDirB] |= 0x40;
 213                 via1[vBufB] &= ~0x40;
 214         } else {
 215                 via_alt_mapping = 0;
 216         }
 217
 218         /*
 219          * Now initialize VIA2. For RBV we just kill all interrupts;
 220          * for a regular VIA we also reset the timers and stuff.
 221          */
 222
 223         via2[gIER] = 0x7F;
 224         via2[gIFR] = 0x7F | rbv_clear;
 225         if (!rbv_present) {
 226                 via2[vT1CL] = 0;
 227                 via2[vT1CH] = 0;
 228                 via2[vT2CL] = 0;
 229                 via2[vT2CH] = 0;
 230                 via2[vACR] &= ~0xC0; /* setup T1 timer with no PB7 output */
 231                 via2[vACR] &= ~0x03; /* disable port A & B latches */
 232         }
 233
 234         via_nubus_init();
 235
 236         /* Everything below this point is VIA2 only... */
 237
 238         if (rbv_present)
 239                 return;
 240
 241         /*
 242          * Set vPCR for control line interrupts.
 243          *
 244          * CA1 (SLOTS IRQ), CB1 (ASC IRQ): negative edge trigger.
 245          *
 246          * Macs with ESP SCSI have a negative edge triggered SCSI interrupt.
 247          * Testing reveals that PowerBooks do too. However, the SE/30
 248          * schematic diagram shows an active high NCR5380 IRQ line.
 249          */
 250
 251         pr_debug("VIA2 vPCR is 0x%02X\n", via2[vPCR]);
 252         if (macintosh_config->via_type == MAC_VIA_II) {
 253                 /* CA2 (SCSI DRQ), CB2 (SCSI IRQ): indep. input, pos. edge */
 254                 via2[vPCR] = 0x66;
 255         } else {
 256                 /* CA2 (SCSI DRQ), CB2 (SCSI IRQ): indep. input, neg. edge */
 257                 via2[vPCR] = 0x22;
 258         }
 259 }
 260
 261 /*
 262  * Debugging dump, used in various places to see what's going on.
 263  */
 264
 265 void via_debug_dump(void)
 266 {
 267         printk(KERN_DEBUG "VIA1: DDRA = 0x%02X DDRB = 0x%02X ACR = 0x%02X\n",
 268                 (uint) via1[vDirA], (uint) via1[vDirB], (uint) via1[vACR]);
 269         printk(KERN_DEBUG "         PCR = 0x%02X  IFR = 0x%02X IER = 0x%02X\n",
 270                 (uint) via1[vPCR], (uint) via1[vIFR], (uint) via1[vIER]);
 271         if (!via2)
 272                 return;
 273         if (rbv_present) {
 274                 printk(KERN_DEBUG "VIA2:  IFR = 0x%02X  IER = 0x%02X\n",
 275                         (uint) via2[rIFR], (uint) via2[rIER]);
 276                 printk(KERN_DEBUG "      SIFR = 0x%02X SIER = 0x%02X\n",
 277                         (uint) via2[rSIFR], (uint) via2[rSIER]);
 278         } else {
 279                 printk(KERN_DEBUG "VIA2: DDRA = 0x%02X DDRB = 0x%02X ACR = 0x%02X\n",
 280                         (uint) via2[vDirA], (uint) via2[vDirB],
 281                         (uint) via2[vACR]);
 282                 printk(KERN_DEBUG "         PCR = 0x%02X  IFR = 0x%02X IER = 0x%02X\n",
 283                         (uint) via2[vPCR],
 284                         (uint) via2[vIFR], (uint) via2[vIER]);
 285         }
 286 }
 287
 288 /*
 289  * Flush the L2 cache on Macs that have it by flipping
 290  * the system into 24-bit mode for an instant.
 291  */
 292
 293 void via_l2_flush(int writeback)
 294 {
 295         unsigned long flags;
 296
 297         local_irq_save(flags);
 298         via2[gBufB] &= ~VIA2B_vMode32;
 299         via2[gBufB] |= VIA2B_vMode32;
 300         local_irq_restore(flags);
 301 }
 302
 303 /*
 304  * Initialize VIA2 for Nubus access
 305  */
 306
 307 static void __init via_nubus_init(void)
 308 {
 309         /* unlock nubus transactions */
 310
 311         if ((macintosh_config->adb_type != MAC_ADB_PB1) &&
 312             (macintosh_config->adb_type != MAC_ADB_PB2)) {
 313                 /* set the line to be an output on non-RBV machines */
 314                 if (!rbv_present)
 315                         via2[vDirB] |= 0x02;
 316
 317                 /* this seems to be an ADB bit on PMU machines */
 318                 /* according to MkLinux.  -- jmt               */
 319                 via2[gBufB] |= 0x02;
 320         }
 321
 322         /*
 323          * Disable the slot interrupts. On some hardware that's not possible.
 324          * On some hardware it's unclear what all of these I/O lines do.
 325          */
 326
 327         switch (macintosh_config->via_type) {
 328         case MAC_VIA_II:
 329         case MAC_VIA_QUADRA:
 330                 pr_debug("VIA2 vDirA is 0x%02X\n", via2[vDirA]);
 331                 break;
 332         case MAC_VIA_IICI:
 333                 /* RBV. Disable all the slot interrupts. SIER works like IER. */
 334                 via2[rSIER] = 0x7F;
 335                 break;
 336         }
 337 }
 338
 339 void via_nubus_irq_startup(int irq)
 340 {
 341         int irq_idx = IRQ_IDX(irq);
 342
 343         switch (macintosh_config->via_type) {
 344         case MAC_VIA_II:
 345         case MAC_VIA_QUADRA:
 346                 /* Make the port A line an input. Probably redundant. */
 347                 if (macintosh_config->via_type == MAC_VIA_II) {
 348                         /* The top two bits are RAM size outputs. */
 349                         via2[vDirA] &= 0xC0 | ~(1 << irq_idx);
 350                 } else {
 351                         /* Allow NuBus slots 9 through F. */
 352                         via2[vDirA] &= 0x80 | ~(1 << irq_idx);
 353                 }
 354                 fallthrough;
 355         case MAC_VIA_IICI:
 356                 via_irq_enable(irq);
 357                 break;
 358         }
 359 }
 360
 361 void via_nubus_irq_shutdown(int irq)
 362 {
 363         switch (macintosh_config->via_type) {
 364         case MAC_VIA_II:
 365         case MAC_VIA_QUADRA:
 366                 /* Ensure that the umbrella CA1 interrupt remains enabled. */
 367                 via_irq_enable(irq);
 368                 break;
 369         case MAC_VIA_IICI:
 370                 via_irq_disable(irq);
 371                 break;
 372         }
 373 }
 374
 375 /*
 376  * The generic VIA interrupt routines (shamelessly stolen from Alan Cox's
 377  * via6522.c :-), disable/pending masks added.
 378  */
 379
 380 #define VIA_TIMER_1_INT BIT(6)
 381
 382 void via1_irq(struct irq_desc *desc)
 383 {
 384         int irq_num;
 385         unsigned char irq_bit, events;
 386
 387         events = via1[vIFR] & via1[vIER] & 0x7F;
 388         if (!events)
 389                 return;
 390
 391         irq_num = IRQ_MAC_TIMER_1;
 392         irq_bit = VIA_TIMER_1_INT;
 393         if (events & irq_bit) {
 394                 unsigned long flags;
 395
 396                 local_irq_save(flags);
 397                 via1[vIFR] = irq_bit;
 398                 generic_handle_irq(irq_num);
 399                 local_irq_restore(flags);
 400
 401                 events &= ~irq_bit;
 402                 if (!events)
 403                         return;
 404         }
 405
 406         irq_num = VIA1_SOURCE_BASE;
 407         irq_bit = 1;
 408         do {
 409                 if (events & irq_bit) {
 410                         via1[vIFR] = irq_bit;
 411                         generic_handle_irq(irq_num);
 412                 }
 413                 ++irq_num;
 414                 irq_bit <<= 1;
 415         } while (events >= irq_bit);
 416 }
 417
 418 static void via2_irq(struct irq_desc *desc)
 419 {
 420         int irq_num;
 421         unsigned char irq_bit, events;
 422
 423         events = via2[gIFR] & via2[gIER] & 0x7F;
 424         if (!events)
 425                 return;
 426
 427         irq_num = VIA2_SOURCE_BASE;
 428         irq_bit = 1;
 429         do {
 430                 if (events & irq_bit) {
 431                         via2[gIFR] = irq_bit | rbv_clear;
 432                         generic_handle_irq(irq_num);
 433                 }
 434                 ++irq_num;
 435                 irq_bit <<= 1;
 436         } while (events >= irq_bit);
 437 }
 438
 439 /*
 440  * Dispatch Nubus interrupts. We are called as a secondary dispatch by the
 441  * VIA2 dispatcher as a fast interrupt handler.
 442  */
 443
 444 static void via_nubus_irq(struct irq_desc *desc)
 445 {
 446         int slot_irq;
 447         unsigned char slot_bit, events;
 448
 449         events = ~via2[gBufA] & 0x7F;
 450         if (rbv_present)
 451                 events &= via2[rSIER];
 452         else
 453                 events &= ~via2[vDirA];
 454         if (!events)
 455                 return;
 456
 457         do {
 458                 slot_irq = IRQ_NUBUS_F;
 459                 slot_bit = 0x40;
 460                 do {
 461                         if (events & slot_bit) {
 462                                 events &= ~slot_bit;
 463                                 generic_handle_irq(slot_irq);
 464                         }
 465                         --slot_irq;
 466                         slot_bit >>= 1;
 467                 } while (events);
 468
 469                 /* clear the CA1 interrupt and make certain there's no more. */
 470                 via2[gIFR] = 0x02 | rbv_clear;
 471                 events = ~via2[gBufA] & 0x7F;
 472                 if (rbv_present)
 473                         events &= via2[rSIER];
 474                 else
 475                         events &= ~via2[vDirA];
 476         } while (events);
 477 }
 478
 479 /*
 480  * Register the interrupt dispatchers for VIA or RBV machines only.
 481  */
 482
 483 void __init via_register_interrupts(void)
 484 {
 485         if (via_alt_mapping) {
 486                 /* software interrupt */
 487                 irq_set_chained_handler(IRQ_AUTO_1, via1_irq);
 488                 /* via1 interrupt */
 489                 irq_set_chained_handler(IRQ_AUTO_6, via1_irq);
 490         } else {
 491                 irq_set_chained_handler(IRQ_AUTO_1, via1_irq);
 492         }
 493         irq_set_chained_handler(IRQ_AUTO_2, via2_irq);
 494         irq_set_chained_handler(IRQ_MAC_NUBUS, via_nubus_irq);
 495 }
 496
 497 void via_irq_enable(int irq) {
 498         int irq_src     = IRQ_SRC(irq);
 499         int irq_idx     = IRQ_IDX(irq);
 500
 501         if (irq_src == 1) {
 502                 via1[vIER] = IER_SET_BIT(irq_idx);
 503         } else if (irq_src == 2) {
 504                 if (irq != IRQ_MAC_NUBUS || nubus_disabled == 0)
 505                         via2[gIER] = IER_SET_BIT(irq_idx);
 506         } else if (irq_src == 7) {
 507                 switch (macintosh_config->via_type) {
 508                 case MAC_VIA_II:
 509                 case MAC_VIA_QUADRA:
 510                         nubus_disabled &= ~(1 << irq_idx);
 511                         /* Enable the CA1 interrupt when no slot is disabled. */
 512                         if (!nubus_disabled)
 513                                 via2[gIER] = IER_SET_BIT(1);
 514                         break;
 515                 case MAC_VIA_IICI:
 516                         /* On RBV, enable the slot interrupt.
 517                          * SIER works like IER.
 518                          */
 519                         via2[rSIER] = IER_SET_BIT(irq_idx);
 520                         break;
 521                 }
 522         }
 523 }
 524
 525 void via_irq_disable(int irq) {
 526         int irq_src     = IRQ_SRC(irq);
 527         int irq_idx     = IRQ_IDX(irq);
 528
 529         if (irq_src == 1) {
 530                 via1[vIER] = IER_CLR_BIT(irq_idx);
 531         } else if (irq_src == 2) {
 532                 via2[gIER] = IER_CLR_BIT(irq_idx);
 533         } else if (irq_src == 7) {
 534                 switch (macintosh_config->via_type) {
 535                 case MAC_VIA_II:
 536                 case MAC_VIA_QUADRA:
 537                         nubus_disabled |= 1 << irq_idx;
 538                         if (nubus_disabled)
 539                                 via2[gIER] = IER_CLR_BIT(1);
 540                         break;
 541                 case MAC_VIA_IICI:
 542                         via2[rSIER] = IER_CLR_BIT(irq_idx);
 543                         break;
 544                 }
 545         }
 546 }
 547
 548 void via1_set_head(int head)
 549 {
 550         if (head == 0)
 551                 via1[vBufA] &= ~VIA1A_vHeadSel;
 552         else
 553                 via1[vBufA] |= VIA1A_vHeadSel;
 554 }
 555 EXPORT_SYMBOL(via1_set_head);
 556
 557 int via2_scsi_drq_pending(void)
 558 {
 559         return via2[gIFR] & (1 << IRQ_IDX(IRQ_MAC_SCSIDRQ));
 560 }
 561 EXPORT_SYMBOL(via2_scsi_drq_pending);
 562
 563 /* timer and clock source */
 564
 565 #define VIA_CLOCK_FREQ     783360                /* VIA "phase 2" clock in Hz */
 566 #define VIA_TIMER_CYCLES   (VIA_CLOCK_FREQ / HZ) /* clock cycles per jiffy */
 567
 568 #define VIA_TC             (VIA_TIMER_CYCLES - 2) /* including 0 and -1 */
 569 #define VIA_TC_LOW         (VIA_TC & 0xFF)
 570 #define VIA_TC_HIGH        (VIA_TC >> 8)
 571
 572 static u64 mac_read_clk(struct clocksource *cs);
 573
 574 static struct clocksource mac_clk = {
 575         .name   = "via1",
 576         .rating = 250,
 577         .read   = mac_read_clk,
 578         .mask   = CLOCKSOURCE_MASK(32),
 579         .flags  = CLOCK_SOURCE_IS_CONTINUOUS,
 580 };
 581
 582 static u32 clk_total, clk_offset;
 583
 584 static irqreturn_t via_timer_handler(int irq, void *dev_id)
 585 {
 586         clk_total += VIA_TIMER_CYCLES;
 587         clk_offset = 0;
 588         legacy_timer_tick(1);
 589
 590         return IRQ_HANDLED;
 591 }
 592
 593 void __init via_init_clock(void)
 594 {
 595         if (request_irq(IRQ_MAC_TIMER_1, via_timer_handler, IRQF_TIMER, "timer",
 596                         NULL)) {
 597                 pr_err("Couldn't register %s interrupt\n", "timer");
 598                 return;
 599         }
 600
 601         via1[vT1CL] = VIA_TC_LOW;
 602         via1[vT1CH] = VIA_TC_HIGH;
 603         via1[vACR] |= 0x40;
 604
 605         clocksource_register_hz(&mac_clk, VIA_CLOCK_FREQ);
 606 }
 607
 608 static u64 mac_read_clk(struct clocksource *cs)
 609 {
 610         unsigned long flags;
 611         u8 count_high;
 612         u16 count;
 613         u32 ticks;
 614
 615         /*
 616          * Timer counter wrap-around is detected with the timer interrupt flag
 617          * but reading the counter low byte (vT1CL) would reset the flag.
 618          * Also, accessing both counter registers is essentially a data race.
 619          * These problems are avoided by ignoring the low byte. Clock accuracy
 620          * is 256 times worse (error can reach 0.327 ms) but CPU overhead is
 621          * reduced by avoiding slow VIA register accesses.
 622          */
 623
 624         local_irq_save(flags);
 625         count_high = via1[vT1CH];
 626         if (count_high == 0xFF)
 627                 count_high = 0;
 628         if (count_high > 0 && (via1[vIFR] & VIA_TIMER_1_INT))
 629                 clk_offset = VIA_TIMER_CYCLES;
 630         count = count_high << 8;
 631         ticks = VIA_TIMER_CYCLES - count;
 632         ticks += clk_offset + clk_total;
 633         local_irq_restore(flags);
 634
 635         return ticks;
 636 }