hw/ssi/pl022: Correct wrong DMACR and ICR handling

In the PL022, register offset 0x20 is the ICR, a write-only
interrupt-clear register.  Register offset 0x24 is DMACR, the DMA
control register.  We were incorrectly implementing (a stub version
of) DMACR at 0x20, and not implementing anything at 0x24.  Fix this
bug.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20180820141116.9118-21-peter.maydell@linaro.org
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

hw/ssi/pl022.c

@@ -146,7 +146,7 @@ static uint64_t pl022_read(void *opaque, hwaddr offset,
         return s->is;
     case 0x1c: /* MIS */
         return s->im & s->is;
-    case 0x20: /* DMACR */
+    case 0x24: /* DMACR */
         /* Not implemented.  */
         return 0;
     default:
@@ -192,7 +192,15 @@ static void pl022_write(void *opaque, hwaddr offset,
         s->im = value;
         pl022_update(s);
         break;
-    case 0x20: /* DMACR */
+    case 0x20: /* ICR */
+        /*
+         * write-1-to-clear: bit 0 clears ROR, bit 1 clears RT;
+         * RX and TX interrupts cannot be cleared this way.
+         */
+        value &= PL022_INT_ROR | PL022_INT_RT;
+        s->is &= ~value;
+        break;
+    case 0x24: /* DMACR */
         if (value) {
             qemu_log_mask(LOG_UNIMP, "pl022: DMA not implemented\n");
         }