[RFC PATCH v4 2/2] hw/riscv: sifive_u: Add write-once protection

[Green Wan]

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         | 21 +++++++++++++++++++++
 include/hw/riscv/sifive_u_otp.h |  1 +
 2 files changed, 22 insertions(+)

diff --git a/hw/riscv/sifive_u_otp.c b/hw/riscv/sifive_u_otp.c
index aab2220494..e9605b9ae9 100644
--- a/hw/riscv/sifive_u_otp.c
+++ b/hw/riscv/sifive_u_otp.c
@@ -27,6 +27,12 @@
 #include "sysemu/blockdev.h"
 #include "sysemu/block-backend.h"
 
+#define SET_WRITTEN_BIT(map, idx, bit)    \
+    (map[idx] |= (0x1 << bit))
+
+#define GET_WRITTEN_BIT(map, idx, bit)    \
+    ((map[idx] >> bit) & 0x1)
+
 static uint64_t sifive_u_otp_read(void *opaque, hwaddr addr, unsigned int size)
 {
     SiFiveUOTPState *s = opaque;
@@ -135,6 +141,18 @@ 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,
+                              "Error: write idx<%u>, bit<%u>\n",
+                              s->pa, s->paio);
+                break;
+            }
+
+            SET_WRITTEN_BIT(s->fuse_wo, s->pa, s->paio);
+        }
+
         /* write to backend */
         if (s->blk) {
             blk_pwrite(s->blk, s->pa * SIFIVE_U_OTP_FUSE_WORD, &val32,
@@ -215,6 +233,9 @@ 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 13d2552e43..49d40a6430 100644
--- a/include/hw/riscv/sifive_u_otp.h
+++ b/include/hw/riscv/sifive_u_otp.h
@@ -74,6 +74,7 @@ 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;
     BlockBackend *blk;
-- 
2.17.1