aspeed: Implement write-1-{set, clear} for AST2500 strapping

The AST2500 SoC family changes the runtime behaviour of the hardware
strapping register (SCU70) to write-1-set/write-1-clear, with
write-1-clear implemented on the "read-only" SoC revision register
(SCU7C). For the the AST2400, the hardware strapping is
runtime-configured with read-modify-write semantics.
Signed-off-by: NAndrew Jeffery <andrew@aj.id.au>
Reviewed-by: NJoel Stanley <joel@jms.id.au>
Message-id: 20180709143524.17480-1-andrew@aj.id.au
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/misc/aspeed_scu.c

@@ -247,11 +247,26 @@ static void aspeed_scu_write(void *opaque, hwaddr offset, uint64_t data,
         s->regs[reg] = data;
         aspeed_scu_set_apb_freq(s);
         break;
-
+    case HW_STRAP1:
+        if (ASPEED_IS_AST2500(s->regs[SILICON_REV])) {
+            s->regs[HW_STRAP1] |= data;
+            return;
+        }
+        /* Jump to assignment below */
+        break;
+    case SILICON_REV:
+        if (ASPEED_IS_AST2500(s->regs[SILICON_REV])) {
+            s->regs[HW_STRAP1] &= ~data;
+        } else {
+            qemu_log_mask(LOG_GUEST_ERROR,
+                          "%s: Write to read-only offset 0x%" HWADDR_PRIx "\n",
+                          __func__, offset);
+        }
+        /* Avoid assignment below, we've handled everything */
+        return;
     case FREQ_CNTR_EVAL:
     case VGA_SCRATCH1 ... VGA_SCRATCH8:
     case RNG_DATA:
-    case SILICON_REV:
     case FREE_CNTR4:
     case FREE_CNTR4_EXT:
         qemu_log_mask(LOG_GUEST_ERROR,

include/hw/misc/aspeed_scu.h

@@ -41,6 +41,8 @@ typedef struct AspeedSCUState {
 #define AST2500_A0_SILICON_REV   0x04000303U
 #define AST2500_A1_SILICON_REV   0x04010303U
 
+#define ASPEED_IS_AST2500(si_rev)     ((((si_rev) >> 24) & 0xff) == 0x04)
+
 extern bool is_supported_silicon_rev(uint32_t silicon_rev);
 
 #define ASPEED_SCU_PROT_KEY      0x1688A8A8