hpet: hpet driver periodic timer setup bug fixes

From: Nils Carlson <[email protected]>

The periodic interrupt from drivers/char/hpet.c does not work correctly, both when using the periodic capability of the hardware and while emulating the periodic interrupt (when hardware does not support periodic mode).

With timers capable of periodic interrupts, the comparator field is first set with the period value followed by set of hidden accumulator, which has the side effect of overwriting the comparator value. This results in wrong periodicity for the interrupts. For,
periodic interrupts to work, following steps are necessary, in that order.
* Set config with Tn_VAL_SET_CNF bit
* Write to hidden accumulator, the value written is the time when the first interrupt should be generated
* Write compartor with period interval for subsequent interrupts
(http://www.intel.com/hardwaredesign/hpetspec_1.pdf )

When emulating periodic timer with timers not capable of periodic interrupt, driver is adding the period to counter value instead of comparator value, which causes slow drift when using this emulation.

Also, driver seems to add hpetp->hp_delta both while setting up periodic interrupt and while emulating periodic interrupts with timers not capable of doing periodic interrupts. This hp_delta will result in slower than expected interrupt rate and should not be used while setting the interval.

Quoted 22 lines · Venkatesh

From: Nils Carlson <[email protected]>

The periodic interrupt from drivers/char/hpet.c does not work correctly, both when using the periodic capability of the hardware and while emulating the periodic interrupt (when hardware does not support periodic mode).

With timers capable of periodic interrupts, the comparator field is first set with the period value followed by set of hidden accumulator, which has the side effect of overwriting the comparator value. This results in wrong periodicity for the interrupts. For,
periodic interrupts to work, following steps are necessary, in that order.
* Set config with Tn_VAL_SET_CNF bit
* Write to hidden accumulator, the value written is the time when the first interrupt should be generated
* Write compartor with period interval for subsequent interrupts
(http://www.intel.com/hardwaredesign/hpetspec_1.pdf )

When emulating periodic timer with timers not capable of periodic interrupt, driver is adding the period to counter value instead of comparator value, which causes slow drift when using this emulation.

Quoted 6 lines

Also, driver seems to add hpetp->hp_delta both while setting up periodic interrupt and while emulating periodic interrupts with timers not capable of doing periodic interrupts. This hp_delta will result in slower than expected interrupt rate and should not be used while setting the interval.

Do you have any thoughts on which kernel versions this fix should be merged into, and when?

From: Nils Carlson <[email protected]>

The periodic interrupt from drivers/char/hpet.c does not work correctly, both when using the periodic capability of the hardware and while emulating the periodic interrupt (when hardware does not support periodic mode).

With timers capable of periodic interrupts, the comparator field is first set with the period value followed by set of hidden accumulator, which has the side effect of overwriting the comparator value. This results in wrong periodicity for the interrupts. For,
periodic interrupts to work, following steps are necessary, in that order.
* Set config with Tn_VAL_SET_CNF bit
* Write to hidden accumulator, the value written is the time when the first interrupt should be generated
* Write compartor with period interval for subsequent interrupts
(http://www.intel.com/hardwaredesign/hpetspec_1.pdf )

When emulating periodic timer with timers not capable of periodic interrupt, driver is adding the period to counter value instead of comparator value, which causes slow drift when using this emulation.

Also, driver seems to add hpetp->hp_delta both while setting up periodic interrupt and while emulating periodic interrupts with timers not capable of doing periodic interrupts. This hp_delta will result in slower than expected interrupt rate and should not be used while setting the interval.

Signed-off-by Venkatesh Pallipadi <[email protected]>

Signed-off-by Nils Carlson <[email protected]>

---
 drivers/char/hpet.c |   21 ++++++++++++---------

 1 files changed, 12 insertions(+), 9 deletions(-)

drivers/char/hpet.c +7 -9

index 4a9f349..70a770a 100644
@@ -166,9 +166,8 @@ static irqreturn_t hpet_interrupt(int irq, void *data)
                unsigned long m, t;
 
                t = devp->hd_ireqfreq;
-               m = read_counter(&devp->hd_hpet->hpet_mc);
-               write_counter(t + m + devp->hd_hpets->hp_delta,
-                             &devp->hd_timer->hpet_compare);
+               m = read_counter(&devp->hd_timer->hpet_compare);
+               write_counter(t + m, &devp->hd_timer->hpet_compare);
        }
 
        if (devp->hd_flags & HPET_SHARED_IRQ)

@@ -504,21 +503,25 @@ static int hpet_ioctl_ieon(struct hpet_dev *devp)
        g = v | Tn_32MODE_CNF_MASK | Tn_INT_ENB_CNF_MASK;
 
        if (devp->hd_flags & HPET_PERIODIC) {
-               write_counter(t, &timer->hpet_compare);
                g |= Tn_TYPE_CNF_MASK;
-               v |= Tn_TYPE_CNF_MASK;
-               writeq(v, &timer->hpet_config);
-               v |= Tn_VAL_SET_CNF_MASK;
+               v |= Tn_TYPE_CNF_MASK | Tn_VAL_SET_CNF_MASK;
                writeq(v, &timer->hpet_config);
                local_irq_save(flags);
 
-               /* NOTE:  what we modify here is a hidden accumulator
+               /*
+                * NOTE: First we modify the hidden accumulator
                 * register supported by periodic-capable comparators.
                 * We never want to modify the (single) counter; that
-                * would affect all the comparators.
+                * would affect all the comparators. The value written
+                * is the counter value when the first interrupt is due.
                 */

Hi,

One comment on this patch.

Quoted 73 lines · Venkatesh

From: Nils Carlson <[email protected]>

The periodic interrupt from drivers/char/hpet.c does not work correctly, both when using the periodic capability of the hardware and while emulating the periodic interrupt (when hardware does not support periodic mode).

With timers capable of periodic interrupts, the comparator field is first set with the period value followed by set of hidden accumulator, which has the side effect of overwriting the comparator value. This results in wrong periodicity for the interrupts. For,
periodic interrupts to work, following steps are necessary, in that order.
* Set config with Tn_VAL_SET_CNF bit
* Write to hidden accumulator, the value written is the time when the first interrupt should be generated
* Write compartor with period interval for subsequent interrupts
(http://www.intel.com/hardwaredesign/hpetspec_1.pdf )

When emulating periodic timer with timers not capable of periodic interrupt, driver is adding the period to counter value instead of comparator value, which causes slow drift when using this emulation.

Also, driver seems to add hpetp->hp_delta both while setting up periodic interrupt and while emulating periodic interrupts with timers not capable of doing periodic interrupts. This hp_delta will result in slower than expected interrupt rate and should not be used while setting the interval.

Signed-off-by Venkatesh Pallipadi <[email protected]>

Signed-off-by Nils Carlson <[email protected]>

---
 drivers/char/hpet.c |   21 ++++++++++++---------
 1 files changed, 12 insertions(+), 9 deletions(-)

drivers/char/hpet.c +7 -9

index 4a9f349..70a770a 100644
@@ -166,9 +166,8 @@ static irqreturn_t hpet_interrupt(int irq, void *data)
               unsigned long m, t;
 
               t = devp->hd_ireqfreq;
-               m = read_counter(&devp->hd_hpet->hpet_mc);
-               write_counter(t + m + devp->hd_hpets->hp_delta,
-                             &devp->hd_timer->hpet_compare);
+               m = read_counter(&devp->hd_timer->hpet_compare);
+               write_counter(t + m, &devp->hd_timer->hpet_compare);
       }
 
       if (devp->hd_flags & HPET_SHARED_IRQ)

@@ -504,21 +503,25 @@ static int hpet_ioctl_ieon(struct hpet_dev *devp)
       g = v | Tn_32MODE_CNF_MASK | Tn_INT_ENB_CNF_MASK;
 
       if (devp->hd_flags & HPET_PERIODIC) {
-               write_counter(t, &timer->hpet_compare);
               g |= Tn_TYPE_CNF_MASK;
-               v |= Tn_TYPE_CNF_MASK;
-               writeq(v, &timer->hpet_config);
-               v |= Tn_VAL_SET_CNF_MASK;
+               v |= Tn_TYPE_CNF_MASK | Tn_VAL_SET_CNF_MASK;
               writeq(v, &timer->hpet_config);
               local_irq_save(flags);
 
-               /* NOTE:  what we modify here is a hidden accumulator
+               /*
+                * NOTE: First we modify the hidden accumulator
                * register supported by periodic-capable comparators.
                * We never want to modify the (single) counter; that
-                * would affect all the comparators.
+                * would affect all the comparators. The value written
+                * is the counter value when the first interrupt is due.
                */

drivers/char/hpet.c static int hpet_ioctl_ieon(struct hpet_dev *devp) · line 527

Quoted 10 lines

drivers/char/hpet.c +5 -0

               m = read_counter(&hpet->hpet_mc);
               write_counter(t + m + hpetp->hp_delta, &timer->hpet_compare);
+               /*
+                * Then we modify the comparator, indicating the period
+                * for subsequent interrupt.
+                */
+               write_counter(t, &timer->hpet_compare);
       } else {
               local_irq_save(flags);
               m = read_counter(&hpet->hpet_mc);

Shouldn't
write_counter(t + m + hpetp->hp_delta, &timer->hpet_compare); be deleted if we are doing
write_counter(t, &timer->hpet_compare);
?

Regards,