target/hppa: Add "diag 0x101" for console output support

For debugging purposes at the early stage of the bootup process,
the SeaBIOS-hppa firmware sometimes needs to output characters to the
serial console. Note that the serial console is the default output
method for parisc machines.

At this stage PCI busses and other devices haven't been initialized
yet. So, SeaBIOS-hppa will not be able to find the correct I/O ports
for the serial ports yet.

Instead, add an emulation for the "diag 0x101" opcode to assist here.
Without any other dependencies, SeaBIOS-hppa can then load the character
to be printed in register %r26 and issue the diag assembly instruction.

The qemu diag_console_output() helper function will then print
that character to the first serial port.

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

target/hppa/helper.h

@@ -103,4 +103,5 @@ DEF_HELPER_FLAGS_1(ptlbe, TCG_CALL_NO_RWG, void, env)
 DEF_HELPER_FLAGS_2(lpa, TCG_CALL_NO_WG, tl, env, tl)
 DEF_HELPER_FLAGS_1(change_prot_id, TCG_CALL_NO_RWG, void, env)
 DEF_HELPER_1(diag_btlb, void, env)
+DEF_HELPER_1(diag_console_output, void, env)
 #endif

target/hppa/sys_helper.c

@@ -23,6 +23,8 @@
 #include "exec/helper-proto.h"
 #include "qemu/timer.h"
 #include "sysemu/runstate.h"
+#include "sysemu/sysemu.h"
+#include "chardev/char-fe.h"
 
 void HELPER(write_interval_timer)(CPUHPPAState *env, target_ulong val)
 {
@@ -109,3 +111,37 @@ void HELPER(rfi_r)(CPUHPPAState *env)
     helper_getshadowregs(env);
     helper_rfi(env);
 }
+
+#ifndef CONFIG_USER_ONLY
+/*
+ * diag_console_output() is a helper function used during the initial bootup
+ * process of the SeaBIOS-hppa firmware.  During the bootup phase, addresses of
+ * serial ports on e.g. PCI busses are unknown and most other devices haven't
+ * been initialized and configured yet.  With help of a simple "diag" assembler
+ * instruction and an ASCII character code in register %r26 firmware can easily
+ * print debug output without any dependencies to the first serial port and use
+ * that as serial console.
+ */
+void HELPER(diag_console_output)(CPUHPPAState *env)
+{
+    CharBackend *serial_backend;
+    Chardev *serial_port;
+    unsigned char c;
+
+    /* find first serial port */
+    serial_port = serial_hd(0);
+    if (!serial_port) {
+        return;
+    }
+
+    /* get serial_backend for the serial port */
+    serial_backend = serial_port->be;
+    if (!serial_backend ||
+        !qemu_chr_fe_backend_connected(serial_backend)) {
+        return;
+    }
+
+    c = (unsigned char)env->gr[26];
+    qemu_chr_fe_write(serial_backend, &c, sizeof(c));
+}
+#endif

target/hppa/translate.c

@@ -4411,6 +4411,12 @@ static bool trans_diag(DisasContext *ctx, arg_diag *a)
         gen_helper_diag_btlb(tcg_env);
         return nullify_end(ctx);
     }
+    if (a->i == 0x101) {
+        /* print char in %r26 to first serial console, used by SeaBIOS-hppa */
+        nullify_over(ctx);
+        gen_helper_diag_console_output(tcg_env);
+        return nullify_end(ctx);
+    }
 #endif
     qemu_log_mask(LOG_UNIMP, "DIAG opcode 0x%04x ignored\n", a->i);
     return true;