hw/misc/mps2-scc: Support using CFG0 bit 0 for remapping

On some boards, SCC config register CFG0 bit 0 controls whether
parts of the board memory map are remapped. Support this with:
 * a device property scc-cfg0 so the board can specify the
   initial value of the CFG0 register
 * an outbound GPIO line which tracks bit 0 and which the board
   can wire up to provide the remapping

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210504120912.23094-3-peter.maydell@linaro.org

hw/misc/mps2-scc.c

@@ -23,6 +23,7 @@
 #include "qemu/bitops.h"
 #include "trace.h"
 #include "hw/sysbus.h"
+#include "hw/irq.h"
 #include "migration/vmstate.h"
 #include "hw/registerfields.h"
 #include "hw/misc/mps2-scc.h"
@@ -186,10 +187,13 @@ static void mps2_scc_write(void *opaque, hwaddr offset, uint64_t value,
     switch (offset) {
     case A_CFG0:
         /*
-         * TODO on some boards bit 0 controls RAM remapping;
-         * on others bit 1 is CPU_WAIT.
+         * On some boards bit 0 controls board-specific remapping;
+         * we always reflect bit 0 in the 'remap' GPIO output line,
+         * and let the board wire it up or not as it chooses.
+         * TODO on some boards bit 1 is CPU_WAIT.
          */
         s->cfg0 = value;
+        qemu_set_irq(s->remap, s->cfg0 & 1);
         break;
     case A_CFG1:
         s->cfg1 = value;
@@ -283,7 +287,7 @@ static void mps2_scc_reset(DeviceState *dev)
     int i;
 
     trace_mps2_scc_reset();
-    s->cfg0 = 0;
+    s->cfg0 = s->cfg0_reset;
     s->cfg1 = 0;
     s->cfg2 = 0;
     s->cfg5 = 0;
@@ -308,6 +312,7 @@ static void mps2_scc_init(Object *obj)
 
     memory_region_init_io(&s->iomem, obj, &mps2_scc_ops, s, "mps2-scc", 0x1000);
     sysbus_init_mmio(sbd, &s->iomem);
+    qdev_init_gpio_out_named(DEVICE(obj), &s->remap, "remap", 1);
 }
 
 static void mps2_scc_realize(DeviceState *dev, Error **errp)
@@ -353,6 +358,8 @@ static Property mps2_scc_properties[] = {
     DEFINE_PROP_UINT32("scc-cfg4", MPS2SCC, cfg4, 0),
     DEFINE_PROP_UINT32("scc-aid", MPS2SCC, aid, 0),
     DEFINE_PROP_UINT32("scc-id", MPS2SCC, id, 0),
+    /* Reset value for CFG0 register */
+    DEFINE_PROP_UINT32("scc-cfg0", MPS2SCC, cfg0_reset, 0),
     /*
      * These are the initial settings for the source clocks on the board.
      * In hardware they can be configured via a config file read by the

include/hw/misc/mps2-scc.h

@@ -18,8 +18,14 @@
  *  + QOM property "scc-cfg4": value of the read-only CFG4 register
  *  + QOM property "scc-aid": value of the read-only SCC_AID register
  *  + QOM property "scc-id": value of the read-only SCC_ID register
+ *  + QOM property "scc-cfg0": reset value of the CFG0 register
  *  + QOM property array "oscclk": reset values of the OSCCLK registers
  *    (which are accessed via the SYS_CFG channel provided by this device)
+ *  + named GPIO output "remap": this tracks the value of CFG0 register
+ *    bit 0. Boards where this bit controls memory remapping should
+ *    connect this GPIO line to a function performing that mapping.
+ *    Boards where bit 0 has no special function should leave the GPIO
+ *    output disconnected.
  */
 #ifndef MPS2_SCC_H
 #define MPS2_SCC_H
@@ -55,6 +61,9 @@ struct MPS2SCC {
     uint32_t num_oscclk;
     uint32_t *oscclk;
     uint32_t *oscclk_reset;
+    uint32_t cfg0_reset;
+
+    qemu_irq remap;
 };
 
 #endif