[RFC qemu legoater/aspeed-3.1 4/4] timer: aspeed: Provide back-pressure information for short periods

[Andrew Jeffery <andrew@aj.id.au>]

First up: This is not the way the hardware behaves.

However, it helps resolve real-world problems with short periods being
used under Linux. Commit 4451d3f59f2a ("clocksource/drivers/fttmr010:
Fix set_next_event handler") in Linux fixed the timer driver to
correctly schedule the next event for the Aspeed controller, and in
combination with 5daa8212c08e ("ARM: dts: aspeed: Describe random number
device") Linux will now set a timer with a period as low as 1us.

Configuring a qemu timer with such a short period results in spending
time handling the interrupt in the model rather than executing guest
code, leading to noticeable "sticky" behaviour in the guest.

The behaviour of Linux is correct with respect to the hardware, so we
need to improve our handling under emulation. The approach chosen is to
provide back-pressure information by calculating an acceptable minimum
number of ticks to be set on the model. Under Linux an additional read
is added in the timer configuration path to detect back-pressure, which
will never occur on hardware. However if back-pressure is observed, the
driver alerts the clock event subsystem, which then performs its own
next event dilation via a config option - d1748302f70b ("clockevents:
Make minimum delay adjustments configurable")

A minimum period of 5us was experimentally determined on a Lenovo
T480s, which I've increased to 20us for "safety".

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
---
 hw/misc/aspeed_scu.c            | 6 ++++++
 hw/timer/aspeed_timer.c         | 6 +++++-
 include/hw/timer/aspeed_timer.h | 1 +
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/hw/misc/aspeed_scu.c b/hw/misc/aspeed_scu.c
index 257f9a6c6b8a..0410776b456a 100644
--- a/hw/misc/aspeed_scu.c
+++ b/hw/misc/aspeed_scu.c
@@ -432,6 +432,12 @@ static void aspeed_scu_realize(DeviceState *dev, Error **errp)
                           TYPE_ASPEED_SCU, SCU_IO_REGION_SIZE);
 
     sysbus_init_mmio(sbd, &s->iomem);
+
+    /*
+     * Reset on realize to ensure the APB clock value is calculated in time for
+     * use by the timer model, which is reset before the SCU.
+     */
+    aspeed_scu_reset(dev);
 }
 
 static const VMStateDescription vmstate_aspeed_scu = {
diff --git a/hw/timer/aspeed_timer.c b/hw/timer/aspeed_timer.c
index 35b40a7c4010..0f3501ac5a5c 100644
--- a/hw/timer/aspeed_timer.c
+++ b/hw/timer/aspeed_timer.c
@@ -254,7 +254,7 @@ static void aspeed_timer_set_value(AspeedTimerCtrlState *s, int timer, int reg,
     switch (reg) {
     case TIMER_REG_RELOAD:
         old_reload = t->reload;
-        t->reload = value;
+        t->reload = value < t->min_ticks ? t->min_ticks : value;
 
         /* If the reload value was not previously set, or zero, and
          * the current value is valid, try to start the timer if it is
@@ -306,7 +306,11 @@ static void aspeed_timer_ctrl_enable(AspeedTimer *t, bool enable)
 
 static void aspeed_timer_ctrl_external_clock(AspeedTimer *t, bool enable)
 {
+    AspeedTimerCtrlState *s = timer_to_ctrl(t);
+    uint32_t rate = enable ? TIMER_CLOCK_EXT_HZ : s->scu->apb_freq;
+
     trace_aspeed_timer_ctrl_external_clock(t->id, enable);
+    t->min_ticks = muldiv64(20 * SCALE_US, rate, NANOSECONDS_PER_SECOND);
 }
 
 static void aspeed_timer_ctrl_overflow_interrupt(AspeedTimer *t, bool enable)
diff --git a/include/hw/timer/aspeed_timer.h b/include/hw/timer/aspeed_timer.h
index 1fb949e16710..10c851ebb6d7 100644
--- a/include/hw/timer/aspeed_timer.h
+++ b/include/hw/timer/aspeed_timer.h
@@ -41,6 +41,7 @@ typedef struct AspeedTimer {
      * interrupts, signalling with both the rising and falling edge.
      */
     int32_t level;
+    uint32_t min_ticks;
     uint32_t reload;
     uint32_t match[2];
     uint64_t start;
-- 
2.19.1