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This more or less reverts commits08be979(x86: Force HPET readback_cmp for all ATI chipsets) and30a564be(x86, hpet: Restrict read back to affected ATI chipsets) to the status of commit8da854c(x86, hpet: Erratum workaround for read after write of HPET comparator). The delta to commit8da854cis mostly comments and the change from WARN_ONCE to printk_once as we know the call path of this function already. This needs really in depth explanation: First of all the HPET design is a complete failure. Having a counter compare register which generates an interrupt on matching values forces the software to do at least one superfluous readback of the counter register. While it is nice in theory to program "absolute" time events it is practically useless because the timer runs at some absurd frequency which can never be matched to real world units. So we are forced to calculate a relative delta and this forces a readout of the actual counter value, adding the delta and programming the compare register. When the delta is small enough we run into the danger that we program a compare value which is already in the past. Due to the compare for equal nature of HPET we need to read back the counter value after writing the compare rehgister (btw. this is necessary for absolute timeouts as well) to make sure that we did not miss the timer event. We try to work around that by setting the minimum delta to a value which is larger than the theoretical time which elapses between the counter readout and the compare register write, but that's only true in theory. A NMI or SMI which hits between the readout and the write can easily push us beyond that limit. This would result in waiting for the next HPET timer interrupt until the 32bit wraparound of the counter happens which takes about 306 seconds. So we designed the next event function to look like: match = read_cnt() + delta; write_compare_ref(match); return read_cnt() < match ? 0 : -ETIME; At some point we got into trouble with certain ATI chipsets. Even the above "safe" procedure failed. The reason was that the write to the compare register was delayed probably for performance reasons. The theory was that they wanted to avoid the synchronization of the write with the HPET clock, which is understandable. So the write does not hit the compare register directly instead it goes to some intermediate register which is copied to the real compare register in sync with the HPET clock. That opens another window for hitting the dreaded "wait for a wraparound" problem. To work around that "optimization" we added a read back of the compare register which either enforced the update of the just written value or just delayed the readout of the counter enough to avoid the issue. We unfortunately never got any affirmative info from ATI/AMD about this. One thing is sure, that we nuked the performance "optimization" that way completely and I'm pretty sure that the result is worse than before some HW folks came up with those. Just for paranoia reasons I added a check whether the read back compare register value was the same as the value we wrote right before. That paranoia check triggered a couple of years after it was added on an Intel ICH9 chipset. Venki added a workaround (commit8da854c) which was reading the compare register twice when the first check failed. We considered this to be a penalty in general and restricted the readback (thus the wasted CPU cycles) to the known to be affected ATI chipsets. This turned out to be a utterly wrong decision. 2.6.35 testers experienced massive problems and finally one of them bisected it down to commit30a564bewhich spured some further investigation. Finally we got confirmation that the write to the compare register can be delayed by up to two HPET clock cycles which explains the problems nicely. All we can do about this is to go back to Venki's initial workaround in a slightly modified version. Just for the record I need to say, that all of this could have been avoided if hardware designers and of course the HPET committee would have thought about the consequences for a split second. It's out of my comprehension why designing a working timer is so hard. There are two ways to achieve it: 1) Use a counter wrap around aware compare_reg <= counter_reg implementation instead of the easy compare_reg == counter_reg Downsides: - It needs more silicon. - It needs a readout of the counter to apply a relative timeout. This is necessary as the counter does not run in any useful (and adjustable) frequency and there is no guarantee that the counter which is used for timer events is the same which is used for reading the actual time (and therefor for calculating the delta) Upsides: - None 2) Use a simple down counter for relative timer events Downsides: - Absolute timeouts are not possible, which is not a problem at all in the context of an OS and the expected max. latencies/jitter (also see Downsides of #1) Upsides: - It needs less or equal silicon. - It works ALWAYS - It is way faster than a compare register based solution (One write versus one write plus at least one and up to four reads) I would not be so grumpy about all of this, if I would not have been ignored for many years when pointing out these flaws to various hardware folks. I really hate timers (at least those which seem to be designed by janitors). Though finally we got a reasonable explanation plus a solution and I want to thank all the folks involved in chasing it down and providing valuable input to this. Bisected-by: Nix <nix@esperi.org.uk> Reported-by: Artur Skawina <art.08.09@gmail.com> Reported-by: Damien Wyart <damien.wyart@free.fr> Reported-by: John Drescher <drescherjm@gmail.com> Cc: Venkatesh Pallipadi <venki@google.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Arjan van de Ven <arjan@linux.intel.com> Cc: Andreas Herrmann <andreas.herrmann3@amd.com> Cc: Borislav Petkov <borislav.petkov@amd.com> Cc: stable@kernel.org Acked-by: Suresh Siddha <suresh.b.siddha@intel.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
117 lines
3.3 KiB
C
117 lines
3.3 KiB
C
#ifndef _ASM_X86_HPET_H
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#define _ASM_X86_HPET_H
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#include <linux/msi.h>
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#ifdef CONFIG_HPET_TIMER
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#define HPET_MMAP_SIZE 1024
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#define HPET_ID 0x000
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#define HPET_PERIOD 0x004
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#define HPET_CFG 0x010
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#define HPET_STATUS 0x020
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#define HPET_COUNTER 0x0f0
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#define HPET_Tn_CFG(n) (0x100 + 0x20 * n)
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#define HPET_Tn_CMP(n) (0x108 + 0x20 * n)
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#define HPET_Tn_ROUTE(n) (0x110 + 0x20 * n)
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#define HPET_T0_CFG 0x100
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#define HPET_T0_CMP 0x108
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#define HPET_T0_ROUTE 0x110
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#define HPET_T1_CFG 0x120
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#define HPET_T1_CMP 0x128
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#define HPET_T1_ROUTE 0x130
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#define HPET_T2_CFG 0x140
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#define HPET_T2_CMP 0x148
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#define HPET_T2_ROUTE 0x150
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#define HPET_ID_REV 0x000000ff
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#define HPET_ID_NUMBER 0x00001f00
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#define HPET_ID_64BIT 0x00002000
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#define HPET_ID_LEGSUP 0x00008000
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#define HPET_ID_VENDOR 0xffff0000
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#define HPET_ID_NUMBER_SHIFT 8
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#define HPET_ID_VENDOR_SHIFT 16
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#define HPET_ID_VENDOR_8086 0x8086
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#define HPET_CFG_ENABLE 0x001
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#define HPET_CFG_LEGACY 0x002
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#define HPET_LEGACY_8254 2
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#define HPET_LEGACY_RTC 8
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#define HPET_TN_LEVEL 0x0002
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#define HPET_TN_ENABLE 0x0004
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#define HPET_TN_PERIODIC 0x0008
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#define HPET_TN_PERIODIC_CAP 0x0010
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#define HPET_TN_64BIT_CAP 0x0020
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#define HPET_TN_SETVAL 0x0040
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#define HPET_TN_32BIT 0x0100
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#define HPET_TN_ROUTE 0x3e00
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#define HPET_TN_FSB 0x4000
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#define HPET_TN_FSB_CAP 0x8000
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#define HPET_TN_ROUTE_SHIFT 9
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/* Max HPET Period is 10^8 femto sec as in HPET spec */
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#define HPET_MAX_PERIOD 100000000UL
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/*
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* Min HPET period is 10^5 femto sec just for safety. If it is less than this,
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* then 32 bit HPET counter wrapsaround in less than 0.5 sec.
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*/
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#define HPET_MIN_PERIOD 100000UL
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/* hpet memory map physical address */
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extern unsigned long hpet_address;
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extern unsigned long force_hpet_address;
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extern u8 hpet_blockid;
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extern int hpet_force_user;
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extern u8 hpet_msi_disable;
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extern int is_hpet_enabled(void);
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extern int hpet_enable(void);
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extern void hpet_disable(void);
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extern unsigned int hpet_readl(unsigned int a);
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extern void force_hpet_resume(void);
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extern void hpet_msi_unmask(unsigned int irq);
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extern void hpet_msi_mask(unsigned int irq);
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extern void hpet_msi_write(unsigned int irq, struct msi_msg *msg);
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extern void hpet_msi_read(unsigned int irq, struct msi_msg *msg);
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#ifdef CONFIG_PCI_MSI
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extern int arch_setup_hpet_msi(unsigned int irq, unsigned int id);
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#else
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static inline int arch_setup_hpet_msi(unsigned int irq, unsigned int id)
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{
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return -EINVAL;
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}
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#endif
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#ifdef CONFIG_HPET_EMULATE_RTC
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#include <linux/interrupt.h>
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typedef irqreturn_t (*rtc_irq_handler)(int interrupt, void *cookie);
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extern int hpet_mask_rtc_irq_bit(unsigned long bit_mask);
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extern int hpet_set_rtc_irq_bit(unsigned long bit_mask);
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extern int hpet_set_alarm_time(unsigned char hrs, unsigned char min,
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unsigned char sec);
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extern int hpet_set_periodic_freq(unsigned long freq);
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extern int hpet_rtc_dropped_irq(void);
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extern int hpet_rtc_timer_init(void);
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extern irqreturn_t hpet_rtc_interrupt(int irq, void *dev_id);
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extern int hpet_register_irq_handler(rtc_irq_handler handler);
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extern void hpet_unregister_irq_handler(rtc_irq_handler handler);
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#endif /* CONFIG_HPET_EMULATE_RTC */
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#else /* CONFIG_HPET_TIMER */
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static inline int hpet_enable(void) { return 0; }
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static inline int is_hpet_enabled(void) { return 0; }
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#define hpet_readl(a) 0
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#endif
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#endif /* _ASM_X86_HPET_H */
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