[RFC PATCH v2 2/2] hw/riscv: sifive_u: Add write-once protection.

[Green Wan <green.wan@sifive.com>]

Add array to store the 'written' status for all bit of OTP to block
the write operation to the same bit. Ignore the control register
offset from 0x0 to 0x38 of OTP memory mapping.

Signed-off-by: Green Wan <green.wan@sifive.com>
---
 hw/riscv/sifive_u_otp.c         | 20 ++++++++++++++++++++
 include/hw/riscv/sifive_u_otp.h |  1 +
 2 files changed, 21 insertions(+)

diff --git a/hw/riscv/sifive_u_otp.c b/hw/riscv/sifive_u_otp.c
index e359f30fdb..a793093d47 100644
--- a/hw/riscv/sifive_u_otp.c
+++ b/hw/riscv/sifive_u_otp.c
@@ -35,6 +35,11 @@
 
 #define TRACE_PREFIX            "FU540_OTP: "
 #define SIFIVE_FU540_OTP_SIZE   (SIFIVE_U_OTP_NUM_FUSES * 4)
+#define SET_WRITTEN_BIT(map, idx, bit)    \
+    (map[idx] |= (0x1 << bit))
+
+#define GET_WRITTEN_BIT(map, idx, bit)    \
+    ((map[idx] >> bit) & 0x1)
 
 static int32_t sifive_u_otp_backed_open(const char *filename, int32_t *fd,
                                         uint32_t **map)
@@ -195,6 +200,18 @@ static void sifive_u_otp_write(void *opaque, hwaddr addr,
         s->ptrim = val32;
         break;
     case SIFIVE_U_OTP_PWE:
+        /* Keep written state for data only and PWE is enabled. Ignore PAS=1 */
+        if ((s->pa > SIFIVE_U_OTP_PWE) && (val32 & 0x1) && !s->pas) {
+            if (GET_WRITTEN_BIT(s->fuse_wo, s->pa, s->paio)) {
+                qemu_log_mask(LOG_GUEST_ERROR,
+                              TRACE_PREFIX "Error: write idx<%u>, bit<%u>\n",
+                              s->pa, s->paio);
+                break;
+            } else {
+                SET_WRITTEN_BIT(s->fuse_wo, s->pa, s->paio);
+            }
+        }
+
         /* open and mmap OTP image file */
         if (0 == sifive_u_otp_backed_open(s->otp_file, &fd, &map)) {
             /* store value */
@@ -248,6 +265,9 @@ static void sifive_u_otp_reset(DeviceState *dev)
     /* Make a valid content of serial number */
     s->fuse[SIFIVE_U_OTP_SERIAL_ADDR] = s->serial;
     s->fuse[SIFIVE_U_OTP_SERIAL_ADDR + 1] = ~(s->serial);
+
+    /* Initialize write-once map */
+    memset(s->fuse_wo, 0x00, sizeof(s->fuse_wo));
 }
 
 static void sifive_u_otp_class_init(ObjectClass *klass, void *data)
diff --git a/include/hw/riscv/sifive_u_otp.h b/include/hw/riscv/sifive_u_otp.h
index f3d71ce82d..4c6ac2e75e 100644
--- a/include/hw/riscv/sifive_u_otp.h
+++ b/include/hw/riscv/sifive_u_otp.h
@@ -73,6 +73,7 @@ typedef struct SiFiveUOTPState {
     uint32_t ptrim;
     uint32_t pwe;
     uint32_t fuse[SIFIVE_U_OTP_NUM_FUSES];
+    uint32_t fuse_wo[SIFIVE_U_OTP_NUM_FUSES];
     /* config */
     uint32_t serial;
     char *otp_file;
-- 
2.17.1