[SeaBIOS] [PATCH] stacks: always call check_irqs() in yield()

[Volker Rümelin]

The comment above the yield() function suggests that yield()
allows interrupts for a short time. Currently this is only true
if seabios was built without CONFIG_THREADS or if yield() is
called from the main thread. Change the code to always call
check_irqs() in yield().

This fixes PS/2 keyboard initialization failures.

The code in src/hw/ps2port.c relies on yield() to briefly enable
interrupts. There is a comment above the yield() function in
__ps2_command(), where the author left a remark why the call to
yield() is actually needed.

Here is one of the call sequences leading to a PS/2 keyboard
initialization failure.

ps2_keyboard_setup()
  |
  ret = i8042_command(I8042_CMD_KBD_DISABLE, NULL);
  # This command will register an interrupt if the PS/2 device
  # controller raises interrupts for replies to a controller
  # command.
  |
  ret = ps2_kbd_command(ATKBD_CMD_RESET_BAT, param);
    |
    ps2_command(0, command, param);
      |
      ret = __ps2_command(aux, command, param);
        |
        // Flush any interrupts already pending.
        yield();
        # yield() doesn't flush interrupts if the main thread
        # hasn't reached wait_threads().
        |
        ret = ps2_sendbyte(aux, command, 1000);
        # Reset the PS/2 keyboard controller and wait for
        # PS2_RET_ACK.
        |
        ret = ps2_recvbyte(aux, 0, 4000);
          |
          for (;;) {
            |
            status = inb(PORT_PS2_STATUS);
            # I8042_STR_OBF isn't set because the keyboard self
            # test reply is still on wire.
            |
            yield();
            # After a few yield()s the keyboard interrupt fires
            # and clears the I8042_STR_OBF status bit. If the
            # keyboard self test reply arrives before the
            # interrupt fires the keyboard reply is lost and
            # ps2_recvbyte() returns after the timeout.
          }

Signed-off-by: Volker Rümelin 
---
 src/stacks.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/src/stacks.c b/src/stacks.c
index 2fe1bfb..70e8283 100644
--- a/src/stacks.c
+++ b/src/stacks.c
@@ -623,9 +623,25 @@ yield(void)
         return;
     }
     struct thread_info *cur = getCurThread();
-    if (cur == &MainThread)
-        // Permit irqs to fire
+    struct thread_info *mt = &MainThread;
+
+    // Permit irqs to fire
+    if (cur == mt) {
         check_irqs();
+    } else {
+        asm volatile(
+            "  pushl %%ebp\n"           // backup %ebp
+            "  movl %%esp, %%esi\n"
+            "  movl (%%eax), %%esp\n"   // switch to MainThread stack
+            "  pushl %%esi\n"           // backup current thread stack pointer
+            "  calll %1\n"              // check_irqs();
+            "  popl %%esi\n"
+            "  movl %%esi, %%esp\n"     // switch back to current thread stack
+            "  popl %%ebp\n"            // restore %ebp
+            : "+a"(mt)
+            : "m"(check_irqs)
+            : "ebx", "ecx", "edx", "esi", "edi", "cc", "memory");
+    }

// Switch to the next thread
     switch_next(cur);