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 #include "mac.h"
  42
  43 volatile __u8 *via1, *via2;
  44 int rbv_present;
  45 int via_alt_mapping;
  46 EXPORT_SYMBOL(via_alt_mapping);
  47 static __u8 rbv_clear;
  48
  49 /*
  50  * Globals for accessing the VIA chip registers without having to
  51  * check if we're hitting a real VIA or an RBV. Normally you could
  52  * just hit the combined register (ie, vIER|rIER) but that seems to
  53  * break on AV Macs...probably because they actually decode more than
  54  * eight address bits. Why can't Apple engineers at least be
  55  * _consistently_ lazy?                          - 1999-05-21 (jmt)
  56  */
  57
  58 static int gIER,gIFR,gBufA,gBufB;
  59
  60 /*
  61  * On Macs with a genuine VIA chip there is no way to mask an individual slot
  62  * interrupt. This limitation also seems to apply to VIA clone logic cores in
  63  * Quadra-like ASICs. (RBV and OSS machines don't have this limitation.)
  64  *
  65  * We used to fake it by configuring the relevant VIA pin as an output
  66  * (to mask the interrupt) or input (to unmask). That scheme did not work on
  67  * (at least) the Quadra 700. A NuBus card's /NMRQ signal is an open-collector
  68  * circuit (see Designing Cards and Drivers for Macintosh II and Macintosh SE,
  69  * p. 10-11 etc) but VIA outputs are not (see datasheet).
  70  *
  71  * Driving these outputs high must cause the VIA to source current and the
  72  * card to sink current when it asserts /NMRQ. Current will flow but the pin
  73  * voltage is uncertain and so the /NMRQ condition may still cause a transition
  74  * at the VIA2 CA1 input (which explains the lost interrupts). A side effect
  75  * is that a disabled slot IRQ can never be tested as pending or not.
  76  *
  77  * Driving these outputs low doesn't work either. All the slot /NMRQ lines are
  78  * (active low) OR'd together to generate the CA1 (aka "SLOTS") interrupt (see
  79  * The Guide To Macintosh Family Hardware, 2nd edition p. 167). If we drive a
  80  * disabled /NMRQ line low, the falling edge immediately triggers a CA1
  81  * interrupt and all slot interrupts after that will generate no transition
  82  * and therefore no interrupt, even after being re-enabled.
  83  *
  84  * So we make the VIA port A I/O lines inputs and use nubus_disabled to keep
  85  * track of their states. When any slot IRQ becomes disabled we mask the CA1
  86  * umbrella interrupt. Only when all slot IRQs become enabled do we unmask
  87  * the CA1 interrupt. It must remain enabled even when cards have no interrupt
  88  * handler registered. Drivers must therefore disable a slot interrupt at the
  89  * device before they call free_irq (like shared and autovector interrupts).
  90  *
  91  * There is also a related problem when MacOS is used to boot Linux. A network
  92  * card brought up by a MacOS driver may raise an interrupt while Linux boots.
  93  * This can be fatal since it can't be handled until the right driver loads
  94  * (if such a driver exists at all). Apparently related to this hardware
  95  * limitation, "Designing Cards and Drivers", p. 9-8, says that a slot
  96  * interrupt with no driver would crash MacOS (the book was written before
  97  * the appearance of Macs with RBV or OSS).
  98  */
  99
 100 static u8 nubus_disabled;
 101
 102 void via_debug_dump(void);
 103 static void via_nubus_init(void);
 104
 105 /*
 106  * Initialize the VIAs
 107  *
 108  * First we figure out where they actually _are_ as well as what type of
 109  * VIA we have for VIA2 (it could be a real VIA or an RBV or even an OSS.)
 110  * Then we pretty much clear them out and disable all IRQ sources.
 111  */
 112
 113 void __init via_init(void)
 114 {
 115         via1 = (void *)VIA1_BASE;
 116         pr_debug("VIA1 detected at %p\n", via1);
 117
 118         if (oss_present) {
 119                 via2 = NULL;
 120                 rbv_present = 0;
 121         } else {
 122                 switch (macintosh_config->via_type) {
 123
 124                 /* IIci, IIsi, IIvx, IIvi (P6xx), LC series */
 125
 126                 case MAC_VIA_IICI:
 127                         via2 = (void *)RBV_BASE;
 128                         pr_debug("VIA2 (RBV) detected at %p\n", via2);
 129                         rbv_present = 1;
 130                         if (macintosh_config->ident == MAC_MODEL_LCIII) {
 131                                 rbv_clear = 0x00;
 132                         } else {
 133                                 /* on most RBVs (& unlike the VIAs), you   */
 134                                 /* need to set bit 7 when you write to IFR */
 135                                 /* in order for your clear to occur.       */
 136                                 rbv_clear = 0x80;
 137                         }
 138                         gIER = rIER;
 139                         gIFR = rIFR;
 140                         gBufA = rSIFR;
 141                         gBufB = rBufB;
 142                         break;
 143
 144                 /* Quadra and early MacIIs agree on the VIA locations */
 145
 146                 case MAC_VIA_QUADRA:
 147                 case MAC_VIA_II:
 148                         via2 = (void *) VIA2_BASE;
 149                         pr_debug("VIA2 detected at %p\n", via2);
 150                         rbv_present = 0;
 151                         rbv_clear = 0x00;
 152                         gIER = vIER;
 153                         gIFR = vIFR;
 154                         gBufA = vBufA;
 155                         gBufB = vBufB;
 156                         break;
 157
 158                 default:
 159                         panic("UNKNOWN VIA TYPE");
 160                 }
 161         }
 162
 163 #ifdef DEBUG_VIA
 164         via_debug_dump();
 165 #endif
 166
 167         /*
 168          * Shut down all IRQ sources, reset the timers, and
 169          * kill the timer latch on VIA1.
 170          */
 171
 172         via1[vIER] = 0x7F;
 173         via1[vIFR] = 0x7F;
 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[vT1CL] = 0;
 229                 via2[vT1CH] = 0;
 230                 via2[vT2CL] = 0;
 231                 via2[vT2CH] = 0;
 232                 via2[vACR] &= ~0xC0; /* setup T1 timer with no PB7 output */
 233                 via2[vACR] &= ~0x03; /* disable port A & B latches */
 234         }
 235
 236         via_nubus_init();
 237
 238         /* Everything below this point is VIA2 only... */
 239
 240         if (rbv_present)
 241                 return;
 242
 243         /*
 244          * Set vPCR for control line interrupts.
 245          *
 246          * CA1 (SLOTS IRQ), CB1 (ASC IRQ): negative edge trigger.
 247          *
 248          * Macs with ESP SCSI have a negative edge triggered SCSI interrupt.
 249          * Testing reveals that PowerBooks do too. However, the SE/30
 250          * schematic diagram shows an active high NCR5380 IRQ line.
 251          */
 252
 253         pr_debug("VIA2 vPCR is 0x%02X\n", via2[vPCR]);
 254         if (macintosh_config->via_type == MAC_VIA_II) {
 255                 /* CA2 (SCSI DRQ), CB2 (SCSI IRQ): indep. input, pos. edge */
 256                 via2[vPCR] = 0x66;
 257         } else {
 258                 /* CA2 (SCSI DRQ), CB2 (SCSI IRQ): indep. input, neg. edge */
 259                 via2[vPCR] = 0x22;
 260         }
 261 }
 262
 263 /*
 264  * Debugging dump, used in various places to see what's going on.
 265  */
 266
 267 void via_debug_dump(void)
 268 {
 269         printk(KERN_DEBUG "VIA1: DDRA = 0x%02X DDRB = 0x%02X ACR = 0x%02X\n",
 270                 (uint) via1[vDirA], (uint) via1[vDirB], (uint) via1[vACR]);
 271         printk(KERN_DEBUG "         PCR = 0x%02X  IFR = 0x%02X IER = 0x%02X\n",
 272                 (uint) via1[vPCR], (uint) via1[vIFR], (uint) via1[vIER]);
 273         if (!via2)
 274                 return;
 275         if (rbv_present) {
 276                 printk(KERN_DEBUG "VIA2:  IFR = 0x%02X  IER = 0x%02X\n",
 277                         (uint) via2[rIFR], (uint) via2[rIER]);
 278                 printk(KERN_DEBUG "      SIFR = 0x%02X SIER = 0x%02X\n",
 279                         (uint) via2[rSIFR], (uint) via2[rSIER]);
 280         } else {
 281                 printk(KERN_DEBUG "VIA2: DDRA = 0x%02X DDRB = 0x%02X ACR = 0x%02X\n",
 282                         (uint) via2[vDirA], (uint) via2[vDirB],
 283                         (uint) via2[vACR]);
 284                 printk(KERN_DEBUG "         PCR = 0x%02X  IFR = 0x%02X IER = 0x%02X\n",
 285                         (uint) via2[vPCR],
 286                         (uint) via2[vIFR], (uint) via2[vIER]);
 287         }
 288 }
 289
 290 /*
 291  * Flush the L2 cache on Macs that have it by flipping
 292  * the system into 24-bit mode for an instant.
 293  */
 294
 295 void via_l2_flush(int writeback)
 296 {
 297         unsigned long flags;
 298
 299         local_irq_save(flags);
 300         via2[gBufB] &= ~VIA2B_vMode32;
 301         via2[gBufB] |= VIA2B_vMode32;
 302         local_irq_restore(flags);
 303 }
 304
 305 /*
 306  * Initialize VIA2 for Nubus access
 307  */
 308
 309 static void __init via_nubus_init(void)
 310 {
 311         /* unlock nubus transactions */
 312
 313         if ((macintosh_config->adb_type != MAC_ADB_PB1) &&
 314             (macintosh_config->adb_type != MAC_ADB_PB2)) {
 315                 /* set the line to be an output on non-RBV machines */
 316                 if (!rbv_present)
 317                         via2[vDirB] |= 0x02;
 318
 319                 /* this seems to be an ADB bit on PMU machines */
 320                 /* according to MkLinux.  -- jmt               */
 321                 via2[gBufB] |= 0x02;
 322         }
 323
 324         /*
 325          * Disable the slot interrupts. On some hardware that's not possible.
 326          * On some hardware it's unclear what all of these I/O lines do.
 327          */
 328
 329         switch (macintosh_config->via_type) {
 330         case MAC_VIA_II:
 331         case MAC_VIA_QUADRA:
 332                 pr_debug("VIA2 vDirA is 0x%02X\n", via2[vDirA]);
 333                 break;
 334         case MAC_VIA_IICI:
 335                 /* RBV. Disable all the slot interrupts. SIER works like IER. */
 336                 via2[rSIER] = 0x7F;
 337                 break;
 338         }
 339 }
 340
 341 void via_nubus_irq_startup(int irq)
 342 {
 343         int irq_idx = IRQ_IDX(irq);
 344
 345         switch (macintosh_config->via_type) {
 346         case MAC_VIA_II:
 347         case MAC_VIA_QUADRA:
 348                 /* Make the port A line an input. Probably redundant. */
 349                 if (macintosh_config->via_type == MAC_VIA_II) {
 350                         /* The top two bits are RAM size outputs. */
 351                         via2[vDirA] &= 0xC0 | ~(1 << irq_idx);
 352                 } else {
 353                         /* Allow NuBus slots 9 through F. */
 354                         via2[vDirA] &= 0x80 | ~(1 << irq_idx);
 355                 }
 356                 fallthrough;
 357         case MAC_VIA_IICI:
 358                 via_irq_enable(irq);
 359                 break;
 360         }
 361 }
 362
 363 void via_nubus_irq_shutdown(int irq)
 364 {
 365         switch (macintosh_config->via_type) {
 366         case MAC_VIA_II:
 367         case MAC_VIA_QUADRA:
 368                 /* Ensure that the umbrella CA1 interrupt remains enabled. */
 369                 via_irq_enable(irq);
 370                 break;
 371         case MAC_VIA_IICI:
 372                 via_irq_disable(irq);
 373                 break;
 374         }
 375 }
 376
 377 /*
 378  * The generic VIA interrupt routines (shamelessly stolen from Alan Cox's
 379  * via6522.c :-), disable/pending masks added.
 380  */
 381
 382 #define VIA_TIMER_1_INT BIT(6)
 383
 384 void via1_irq(struct irq_desc *desc)
 385 {
 386         int irq_num;
 387         unsigned char irq_bit, events;
 388
 389         events = via1[vIFR] & via1[vIER] & 0x7F;
 390         if (!events)
 391                 return;
 392
 393         irq_num = IRQ_MAC_TIMER_1;
 394         irq_bit = VIA_TIMER_1_INT;
 395         if (events & irq_bit) {
 396                 unsigned long flags;
 397
 398                 local_irq_save(flags);
 399                 via1[vIFR] = irq_bit;
 400                 generic_handle_irq(irq_num);
 401                 local_irq_restore(flags);
 402
 403                 events &= ~irq_bit;
 404                 if (!events)
 405                         return;
 406         }
 407
 408         irq_num = VIA1_SOURCE_BASE;
 409         irq_bit = 1;
 410         do {
 411                 if (events & irq_bit) {
 412                         via1[vIFR] = irq_bit;
 413                         generic_handle_irq(irq_num);
 414                 }
 415                 ++irq_num;
 416                 irq_bit <<= 1;
 417         } while (events >= irq_bit);
 418 }
 419
 420 static void via2_irq(struct irq_desc *desc)
 421 {
 422         int irq_num;
 423         unsigned char irq_bit, events;
 424
 425         events = via2[gIFR] & via2[gIER] & 0x7F;
 426         if (!events)
 427                 return;
 428
 429         irq_num = VIA2_SOURCE_BASE;
 430         irq_bit = 1;
 431         do {
 432                 if (events & irq_bit) {
 433                         via2[gIFR] = irq_bit | rbv_clear;
 434                         generic_handle_irq(irq_num);
 435                 }
 436                 ++irq_num;
 437                 irq_bit <<= 1;
 438         } while (events >= irq_bit);
 439 }
 440
 441 /*
 442  * Dispatch Nubus interrupts. We are called as a secondary dispatch by the
 443  * VIA2 dispatcher as a fast interrupt handler.
 444  */
 445
 446 static void via_nubus_irq(struct irq_desc *desc)
 447 {
 448         int slot_irq;
 449         unsigned char slot_bit, events;
 450
 451         events = ~via2[gBufA] & 0x7F;
 452         if (rbv_present)
 453                 events &= via2[rSIER];
 454         else
 455                 events &= ~via2[vDirA];
 456         if (!events)
 457                 return;
 458
 459         do {
 460                 slot_irq = IRQ_NUBUS_F;
 461                 slot_bit = 0x40;
 462                 do {
 463                         if (events & slot_bit) {
 464                                 events &= ~slot_bit;
 465                                 generic_handle_irq(slot_irq);
 466                         }
 467                         --slot_irq;
 468                         slot_bit >>= 1;
 469                 } while (events);
 470
 471                 /* clear the CA1 interrupt and make certain there's no more. */
 472                 via2[gIFR] = 0x02 | rbv_clear;
 473                 events = ~via2[gBufA] & 0x7F;
 474                 if (rbv_present)
 475                         events &= via2[rSIER];
 476                 else
 477                         events &= ~via2[vDirA];
 478         } while (events);
 479 }
 480
 481 /*
 482  * Register the interrupt dispatchers for VIA or RBV machines only.
 483  */
 484
 485 void __init via_register_interrupts(void)
 486 {
 487         if (via_alt_mapping) {
 488                 /* software interrupt */
 489                 irq_set_chained_handler(IRQ_AUTO_1, via1_irq);
 490                 /* via1 interrupt */
 491                 irq_set_chained_handler(IRQ_AUTO_6, via1_irq);
 492         } else {
 493                 irq_set_chained_handler(IRQ_AUTO_1, via1_irq);
 494         }
 495         irq_set_chained_handler(IRQ_AUTO_2, via2_irq);
 496         irq_set_chained_handler(IRQ_MAC_NUBUS, via_nubus_irq);
 497 }
 498
 499 void via_irq_enable(int irq) {
 500         int irq_src     = IRQ_SRC(irq);
 501         int irq_idx     = IRQ_IDX(irq);
 502
 503         if (irq_src == 1) {
 504                 via1[vIER] = IER_SET_BIT(irq_idx);
 505         } else if (irq_src == 2) {
 506                 if (irq != IRQ_MAC_NUBUS || nubus_disabled == 0)
 507                         via2[gIER] = IER_SET_BIT(irq_idx);
 508         } else if (irq_src == 7) {
 509                 switch (macintosh_config->via_type) {
 510                 case MAC_VIA_II:
 511                 case MAC_VIA_QUADRA:
 512                         nubus_disabled &= ~(1 << irq_idx);
 513                         /* Enable the CA1 interrupt when no slot is disabled. */
 514                         if (!nubus_disabled)
 515                                 via2[gIER] = IER_SET_BIT(1);
 516                         break;
 517                 case MAC_VIA_IICI:
 518                         /* On RBV, enable the slot interrupt.
 519                          * SIER works like IER.
 520                          */
 521                         via2[rSIER] = IER_SET_BIT(irq_idx);
 522                         break;
 523                 }
 524         }
 525 }
 526
 527 void via_irq_disable(int irq) {
 528         int irq_src     = IRQ_SRC(irq);
 529         int irq_idx     = IRQ_IDX(irq);
 530
 531         if (irq_src == 1) {
 532                 via1[vIER] = IER_CLR_BIT(irq_idx);
 533         } else if (irq_src == 2) {
 534                 via2[gIER] = IER_CLR_BIT(irq_idx);
 535         } else if (irq_src == 7) {
 536                 switch (macintosh_config->via_type) {
 537                 case MAC_VIA_II:
 538                 case MAC_VIA_QUADRA:
 539                         nubus_disabled |= 1 << irq_idx;
 540                         if (nubus_disabled)
 541                                 via2[gIER] = IER_CLR_BIT(1);
 542                         break;
 543                 case MAC_VIA_IICI:
 544                         via2[rSIER] = IER_CLR_BIT(irq_idx);
 545                         break;
 546                 }
 547         }
 548 }
 549
 550 void via1_set_head(int head)
 551 {
 552         if (head == 0)
 553                 via1[vBufA] &= ~VIA1A_vHeadSel;
 554         else
 555                 via1[vBufA] |= VIA1A_vHeadSel;
 556 }
 557 EXPORT_SYMBOL(via1_set_head);
 558
 559 int via2_scsi_drq_pending(void)
 560 {
 561         return via2[gIFR] & (1 << IRQ_IDX(IRQ_MAC_SCSIDRQ));
 562 }
 563 EXPORT_SYMBOL(via2_scsi_drq_pending);
 564
 565 /* timer and clock source */
 566
 567 #define VIA_CLOCK_FREQ     783360                /* VIA "phase 2" clock in Hz */
 568 #define VIA_TIMER_CYCLES   (VIA_CLOCK_FREQ / HZ) /* clock cycles per jiffy */
 569
 570 #define VIA_TC             (VIA_TIMER_CYCLES - 2) /* including 0 and -1 */
 571 #define VIA_TC_LOW         (VIA_TC & 0xFF)
 572 #define VIA_TC_HIGH        (VIA_TC >> 8)
 573
 574 static u64 mac_read_clk(struct clocksource *cs);
 575
 576 static struct clocksource mac_clk = {
 577         .name   = "via1",
 578         .rating = 250,
 579         .read   = mac_read_clk,
 580         .mask   = CLOCKSOURCE_MASK(32),
 581         .flags  = CLOCK_SOURCE_IS_CONTINUOUS,
 582 };
 583
 584 static u32 clk_total, clk_offset;
 585
 586 static irqreturn_t via_timer_handler(int irq, void *dev_id)
 587 {
 588         clk_total += VIA_TIMER_CYCLES;
 589         clk_offset = 0;
 590         legacy_timer_tick(1);
 591
 592         return IRQ_HANDLED;
 593 }
 594
 595 void __init via_init_clock(void)
 596 {
 597         if (request_irq(IRQ_MAC_TIMER_1, via_timer_handler, IRQF_TIMER, "timer",
 598                         NULL)) {
 599                 pr_err("Couldn't register %s interrupt\n", "timer");
 600                 return;
 601         }
 602
 603         via1[vT1CL] = VIA_TC_LOW;
 604         via1[vT1CH] = VIA_TC_HIGH;
 605         via1[vACR] |= 0x40;
 606
 607         clocksource_register_hz(&mac_clk, VIA_CLOCK_FREQ);
 608 }
 609
 610 static u64 mac_read_clk(struct clocksource *cs)
 611 {
 612         unsigned long flags;
 613         u8 count_high;
 614         u16 count;
 615         u32 ticks;
 616
 617         /*
 618          * Timer counter wrap-around is detected with the timer interrupt flag
 619          * but reading the counter low byte (vT1CL) would reset the flag.
 620          * Also, accessing both counter registers is essentially a data race.
 621          * These problems are avoided by ignoring the low byte. Clock accuracy
 622          * is 256 times worse (error can reach 0.327 ms) but CPU overhead is
 623          * reduced by avoiding slow VIA register accesses.
 624          */
 625
 626         local_irq_save(flags);
 627         count_high = via1[vT1CH];
 628         if (count_high == 0xFF)
 629                 count_high = 0;
 630         if (count_high > 0 && (via1[vIFR] & VIA_TIMER_1_INT))
 631                 clk_offset = VIA_TIMER_CYCLES;
 632         count = count_high << 8;
 633         ticks = VIA_TIMER_CYCLES - count;
 634         ticks += clk_offset + clk_total;
 635         local_irq_restore(flags);
 636
 637         return ticks;
 638 }