[RFC PATCH] target/mips: Allow building without Inter-Thread Communication hardware

["Philippe Mathieu-Daudé" <f4bug@amsat.org>]

The Inter-Thread Communication unit (TYPE_MIPS_ITU) is an optional
device that is only selected by a few machines. However it goes
deep into the translation code, as the MTC0/MTHC0 SAAR helpers
call itc_reconfigure().

When building with no machine selecting the ITU component (which
is implemented in hw/misc/mips_itu.c), we get the following link
failure:

  /usr/bin/ld: target_mips_cp0_helper.c.o: in function `helper_mtc0_saar':
  target/mips/cp0_helper.c:1118: undefined reference to `itc_reconfigure'
  /usr/bin/ld: target_mips_cp0_helper.c.o: in function `helper_mthc0_saar':
  target/mips/cp0_helper.c:1135: undefined reference to `itc_reconfigure'

Fix by adding a stub, built when the ITU isn't selected.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
RFC because too much Meson machinery to my taste.
But how to deal with such architectural devices else?

To reproduce:

$ echo CONFIG_JAZZ=y > default-configs/devices/mips64el-softmmu.mak
$ echo CONFIG_SEMIHOSTING=y >> default-configs/devices/mips64el-softmmu.mak
$ configure --without-default-devices
$ ninja qemu-system-mips64el
$ ./qemu-system-mips64el -M magnum -S
---
 target/mips/cp0_itu-stub.c | 15 +++++++++++++++
 target/mips/meson.build    |  3 +++
 2 files changed, 18 insertions(+)
 create mode 100644 target/mips/cp0_itu-stub.c

diff --git a/target/mips/cp0_itu-stub.c b/target/mips/cp0_itu-stub.c
new file mode 100644
index 00000000000..995b5a09ff8
--- /dev/null
+++ b/target/mips/cp0_itu-stub.c
@@ -0,0 +1,15 @@
+/*
+ * QEMU Inter-Thread Communication Unit emulation stubs
+ *
+ *  Copyright (c) 2021 Philippe Mathieu-Daudé
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ */
+#include "qemu/osdep.h"
+#include "cpu.h"
+#include "hw/misc/mips_itu.h"
+
+void itc_reconfigure(MIPSITUState *tag)
+{
+    /* nothing? */
+}
diff --git a/target/mips/meson.build b/target/mips/meson.build
index 3b131c4a7f6..a631688fae0 100644
--- a/target/mips/meson.build
+++ b/target/mips/meson.build
@@ -45,6 +45,9 @@
   'cp0_helper.c',
   'mips-semi.c',
 ))
+mips_softmmu_ss.add(when: 'CONFIG_MIPS_ITU', if_false: files(
+  'cp0_itu-stub.c',
+))
 
 mips_ss.add_all(when: 'CONFIG_TCG', if_true: [mips_tcg_ss])
 
-- 
2.26.3