[Qemu-devel] [PATCH v2] target/xtensa: fix timers test

[Max Filippov <jcmvbkbc@gmail.com>]

The value of CCOUNT special register is calculated as time elapsed
since CCOUNT == 0 multiplied by the core frequency. In icount mode time
increment between consecutive instructions that don't involve time
warps is constant, but unless the result of multiplication of this
constant by the core frequency is a whole number the CCOUNT increment
between these instructions may not be constant. E.g. with icount=7 each
instruction takes 128ns, with core clock of 10MHz CCOUNT values for
consecutive instructions are:

  502: (128 * 502 * 10000000) / 1000000000 = 642.56
  503: (128 * 503 * 10000000) / 1000000000 = 643.84
  504: (128 * 504 * 10000000) / 1000000000 = 645.12

I.e.the CCOUNT increments depend on the absolute time. This results in
varying CCOUNT differences for consecutive instructions in tests that
involve time warps and don't set CCOUNT explicitly.

Change frequency of the core used in tests so that clock cycle takes
exactly 64ns. Change icount power used in tests to 6, so that each
instruction takes exactly 1 clock cycle. With these changes CCOUNT
increments only depend on the number of executed instructions and that's
what timer tests expect, so they work correctly.

Longer story:
  http://lists.nongnu.org/archive/html/qemu-devel/2018-03/msg04326.html

Cc: Pavel Dovgaluk <Pavel.Dovgaluk@ispras.ru>
Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
---
Changes v1->v2:
- expand changelog;
- replace clock_freq_khz initializer constant with a more meaningful
  expression.

 target/xtensa/core-dc232b.c | 2 +-
 tests/tcg/xtensa/Makefile   | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/target/xtensa/core-dc232b.c b/target/xtensa/core-dc232b.c
index aa07018af4e7..7331eeea2fb6 100644
--- a/target/xtensa/core-dc232b.c
+++ b/target/xtensa/core-dc232b.c
@@ -47,7 +47,7 @@ static XtensaConfig dc232b __attribute__((unused)) = {
         }
     },
     .isa_internal = &xtensa_modules,
-    .clock_freq_khz = 10000,
+    .clock_freq_khz = (NANOSECONDS_PER_SECOND / 64) / 1000,
     DEFAULT_SECTIONS
 };
 
diff --git a/tests/tcg/xtensa/Makefile b/tests/tcg/xtensa/Makefile
index 2882c431e4a9..091518c05583 100644
--- a/tests/tcg/xtensa/Makefile
+++ b/tests/tcg/xtensa/Makefile
@@ -5,7 +5,7 @@ CROSS=xtensa-$(CORE)-elf-
 
 ifndef XT
 SIM = ../../../xtensa-softmmu/qemu-system-xtensa
-SIMFLAGS = -M sim -cpu $(CORE) -nographic -semihosting -icount 7 $(EXTFLAGS) -kernel
+SIMFLAGS = -M sim -cpu $(CORE) -nographic -semihosting -icount 6 $(EXTFLAGS) -kernel
 SIMDEBUG = -s -S
 else
 SIM = xt-run
-- 
2.11.0