xref: /linux/arch/m68k/mac/via.c (revision 0ea5c948cb64bab5bc7a5516774eb8536f05aa0d)
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 
via_init(void)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 
via_debug_dump(void)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 
via_l2_flush(int writeback)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 
via_nubus_init(void)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 
via_nubus_irq_startup(int irq)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 
via_nubus_irq_shutdown(int irq)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 
via1_irq(struct irq_desc * desc)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 
via2_irq(struct irq_desc * desc)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 
via_nubus_irq(struct irq_desc * desc)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 
via_register_interrupts(void)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 
via_irq_enable(int irq)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 
via_irq_disable(int irq)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 
via1_set_head(int head)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 
via2_scsi_drq_pending(void)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 
via_timer_handler(int irq,void * dev_id)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 
via_init_clock(void)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 
mac_read_clk(struct clocksource * cs)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 }
639