[PATCH qemu.git v2 9/9] hw/timer/imx_epit: fix compare timer handling

[~axelheider <axelheider@git.sr.ht>]

From: Axel Heider <axel.heider@hensoldt.net>

- fix #1263
- rework compare time handling
  - The compare timer has to run even if CR.OCIEN is not set,
    as SR.OCIF must be updated.
  - The compare timer fires exactly once when the
    compare value is less than the current value, but the
    reload values is less than the compare value.
  - The compare timer will never fire if the reload value is
    less than the compare value. Disable it in this case.

Signed-off-by: Axel Heider <axel.heider@hensoldt.net>
---
 hw/timer/imx_epit.c | 188 +++++++++++++++++++++++++++++---------------
 1 file changed, 123 insertions(+), 65 deletions(-)

diff --git a/hw/timer/imx_epit.c b/hw/timer/imx_epit.c
index 77bd2b0a2b..cb2880cabc 100644
--- a/hw/timer/imx_epit.c
+++ b/hw/timer/imx_epit.c
@@ -6,6 +6,7 @@
  * Originally written by Hans Jiang
  * Updated by Peter Chubb
  * Updated by Jean-Christophe Dubois <jcd@tribudubois.net>
+ * Updated by Axel Heider
  *
  * This code is licensed under GPL version 2 or later.  See
  * the COPYING file in the top-level directory.
@@ -110,33 +111,84 @@ static uint64_t imx_epit_read(void *opaque, hwaddr offset, unsigned size)
     return reg_value;
 }
 
-/* Must be called from ptimer_transaction_begin/commit block for s->timer_cmp */
-static void imx_epit_reload_compare_timer(IMXEPITState *s)
+/*
+ * Must be called from a ptimer_transaction_begin/commit block for
+ * s->timer_cmp, but outside of a transaction block of s->timer_reload,
+ * so the proper counter value is read.
+ */
+static void imx_epit_update_compare_timer(IMXEPITState *s)
 {
-    if ((s->cr & (CR_EN | CR_OCIEN)) == (CR_EN | CR_OCIEN))  {
-        /* if the compare feature is on and timers are running */
-        uint32_t tmp = ptimer_get_count(s->timer_reload);
-        uint64_t next;
-        if (tmp > s->cmp) {
-            /* It'll fire in this round of the timer */
-            next = tmp - s->cmp;
-        } else { /* catch it next time around */
-            next = tmp - s->cmp + ((s->cr & CR_RLD) ? EPIT_TIMER_MAX : s->lr);
+    uint64_t counter = 0;
+    bool is_oneshot = false;
+    /* The compare timer only has to run if the timer peripheral is active
+     * and there is an input clock, Otherwise it can be switched off.
+     */
+    bool is_active = (s->cr & CR_EN) && imx_epit_get_freq(s);
+    if (is_active)
+    {
+        /*
+         * Calculate next timeout for compare timer. Reading the reload
+         * counter returns proper results only if pending transactions
+         * on it are committed here. Otherwise stale values are be read.
+         */
+        counter = ptimer_get_count(s->timer_reload);
+        uint64_t limit = ptimer_get_limit(s->timer_cmp);
+        /* The compare timer is a periodic timer if the limit is at least
+         * the compare value. Otherwise it may fire at most once in the
+         * current round.
+         */
+        bool is_oneshot = (limit >= s->cmp);
+        if (counter >= s->cmp) {
+            /* The compare timer fires in the current round. */
+            counter -= s->cmp;
+        } else if (!is_oneshot) {
+            /*
+             * The compare timer fires after a reload, as it below the
+             * compare value already in this round. Note that the counter
+             * value calculated below can be above the 32-bit limit, which
+             * is legal here because the compare timer is an internal
+             * helper ptimer only.
+             */
+            counter += limit - s->cmp;
+        } else {
+            /*
+             * The compare timer wont fire in this round, and the limit is
+             * set to a value below the compare value. This practically means
+             * it will never fire, so it can be switched off.
+             */
+            is_active = false;
         }
-        ptimer_set_count(s->timer_cmp, next);
     }
+
+    /*
+     * Set the compare timer and let it run, or stop it. This is agnostic
+     * of CR.OCIEN bit, as this only matters for interrupt generation. The
+     * compare timer needs to run in any case, as the SR.OCIF bit must be
+     * updated even if no interrupt in generated.
+     * Note that the timer might already be stopped or be running with
+     * counter values. However, finding out when an update is needed and
+     * when not is not trivial. It's much easier applying the setting again,
+     * as this does not harm either and the overhead is negligible.
+     */
+    if (is_active) {
+        ptimer_set_count(s->timer_cmp, counter);
+        ptimer_run(s->timer_cmp, is_oneshot ? 1 : 0);
+    } else {
+        ptimer_stop(s->timer_cmp);
+    }
+
 }
 
 static void imx_epit_write_cr(IMXEPITState *s, uint32_t value)
 {
     uint32_t freq = 0;
-    uint32_t oldcr = s->cr;
+    bool set_limit = false;
+    bool set_counter = false;
 
     /* SWR bit is never persisted, it clears itself once reset is done */
+    uint32_t old_cr = s->cr;
     s->cr = (value & ~CR_SWR) & 0x03ffffff;
-
-    ptimer_transaction_begin(s->timer_cmp);
-    ptimer_transaction_begin(s->timer_reload);
+    uint32_t toggled_cr = old_cr ^ s->cr;
 
     if (value & CR_SWR) {
         /* Soft reset doesn't touch some bits; only a hard reset clears them */
@@ -149,49 +201,52 @@ static void imx_epit_write_cr(IMXEPITState *s, uint32_t value)
         /* turn interrupt off since SR and the OCIEN bit got cleared */
         qemu_irq_lower(s->irq);
         /* reset timer limits, set timer values to these limits */
-        ptimer_set_limit(s->timer_cmp, EPIT_TIMER_MAX, 1);
-        ptimer_set_limit(s->timer_reload, EPIT_TIMER_MAX, 1);
+        set_limit = true;
+        set_counter = true;
     } else {
-        freq = imx_epit_get_freq(s);
-        if (freq) {
-            ptimer_set_freq(s->timer_reload, freq);
-            ptimer_set_freq(s->timer_cmp, freq);
-        }
+        /* re-initialize the limits if CR.RLD has changed */
+        set_limit = toggled_cr & CR_RLD;
+        /* set the counter if the timer got just enabled and CR.ENMOD is set */
+        set_counter = ((toggled_cr & s->cr) & CR_EN) && (s->cr & CR_ENMOD);
+    }
+
+    ptimer_transaction_begin(s->timer_cmp);
+    ptimer_transaction_begin(s->timer_reload);
+
+    freq = imx_epit_get_freq(s);
+    if (freq) {
+        ptimer_set_freq(s->timer_reload, freq);
+        ptimer_set_freq(s->timer_cmp, freq);
     }
 
-    if (freq && (s->cr & CR_EN) && !(oldcr & CR_EN)) {
-        if (s->cr & CR_ENMOD) {
-            if (s->cr & CR_RLD) {
-                ptimer_set_limit(s->timer_reload, s->lr, 1);
-                ptimer_set_limit(s->timer_cmp, s->lr, 1);
-            } else {
-                ptimer_set_limit(s->timer_reload, EPIT_TIMER_MAX, 1);
-                ptimer_set_limit(s->timer_cmp, EPIT_TIMER_MAX, 1);
-            }
+    if (set_limit || set_counter) {
+        uint64_t limit = (s->cr & CR_RLD) ? s->lr : EPIT_TIMER_MAX;
+        if (set_limit) {
+            ptimer_set_limit(s->timer_cmp, limit, 0);
         }
+        ptimer_set_limit(s->timer_reload, limit, set_counter ? 1 : 0);
+    }
 
-        imx_epit_reload_compare_timer(s);
+    /*
+     * If there is an input clock and the peripheral is enabled, then ensure
+     * the wall clock timer is ticking. Otherwise stop the timers. The compare
+     * timer will be updated later.
+     */
+    if (freq && (s->cr & CR_EN)) {
         ptimer_run(s->timer_reload, 0);
-        if (s->cr & CR_OCIEN) {
-            ptimer_run(s->timer_cmp, 0);
-        } else {
-            ptimer_stop(s->timer_cmp);
-        }
-    } else if (!(s->cr & CR_EN)) {
-        /* stop both timers */
-        ptimer_stop(s->timer_reload);
-        ptimer_stop(s->timer_cmp);
-    } else  if (s->cr & CR_OCIEN) {
-        if (!(oldcr & CR_OCIEN)) {
-            imx_epit_reload_compare_timer(s);
-            ptimer_run(s->timer_cmp, 0);
-        }
     } else {
+        ptimer_stop(s->timer_reload);
+        /* Stop the compare timer also. This just plays safe, the call to
+         * imx_epit_update_compare_timer() below should also so this. */
         ptimer_stop(s->timer_cmp);
     }
 
-    ptimer_transaction_commit(s->timer_cmp);
+    /* Commit the changes to s->timer_reload, so they can propagate. */
     ptimer_transaction_commit(s->timer_reload);
+
+    /* Update the compare timer based on the committed reload timer value. */
+    imx_epit_update_compare_timer(s);
+    ptimer_transaction_commit(s->timer_cmp);
 }
 
 static void imx_epit_write_sr(IMXEPITState *s, uint32_t value)
@@ -218,14 +273,10 @@ static void imx_epit_write_lr(IMXEPITState *s, uint32_t value)
         /* If IOVW bit is set then set the timer value */
         ptimer_set_count(s->timer_reload, s->lr);
     }
-    /*
-     * Commit the change to s->timer_reload, so it can propagate. Otherwise
-     * the timer interrupt may not fire properly. The commit must happen
-     * before calling imx_epit_reload_compare_timer(), which reads
-     * s->timer_reload internally again.
-     */
+    /* Commit the changes to s->timer_reload, so they can propagate. */
     ptimer_transaction_commit(s->timer_reload);
-    imx_epit_reload_compare_timer(s);
+    /* Update the compare timer based on the committed reload timer value. */
+    imx_epit_update_compare_timer(s);
     ptimer_transaction_commit(s->timer_cmp);
 }
 
@@ -233,8 +284,9 @@ static void imx_epit_write_cmp(IMXEPITState *s, uint32_t value)
 {
     s->cmp = value;
 
+    /* Update the compare timer based on the committed reload timer value. */
     ptimer_transaction_begin(s->timer_cmp);
-    imx_epit_reload_compare_timer(s);
+    imx_epit_update_compare_timer(s);
     ptimer_transaction_commit(s->timer_cmp);
 }
 
@@ -274,16 +326,22 @@ static void imx_epit_cmp(void *opaque)
 {
     IMXEPITState *s = IMX_EPIT(opaque);
 
-    /* Set the interrupt status flag to signaled. */
-    DPRINTF("sr was %d\n", s->sr);
-    s->sr = 1;
+    if (s->cr & CR_EN) {
+        /* Set the interrupt status flag to signaled. */
+        DPRINTF("sr was %d\n", s->sr);
+        s->sr = 1;
 
-    /*
-     * An actual interrupt is generated only if the peripheral is enabled
-     * and the interrupt generation is enabled.
-     */
-    if ((s->cr & (CR_EN | CR_OCIEN)) == (CR_EN | CR_OCIEN)) {
-        qemu_irq_raise(s->irq);
+        /* If CR,OCIEN is set, an actual interrupt is generated */
+        if (s->cr & CR_OCIEN) {
+            qemu_irq_raise(s->irq);
+        }
+    } else {
+        /*
+         * The cmp ptimer is not supposed to be running when the
+         * peripheral is not enabled. Ignore this. However, it's
+         * worth investigating why this happened.
+         */
+        DPRINTF("compare trigger when timer not enabled\n");
     }
 }
 
-- 
2.34.5