[RFC PATCH v7 1/2] hw/misc/sifive_u_otp: Add write function and write-once protection

[Green Wan]

 - Add write operation to update fuse data bit when PWE bit is on.
 - Add array, fuse_wo, to store the 'written' status for all bits
   of OTP to block the write operation.

Signed-off-by: Green Wan <green.wan@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
---
 hw/misc/sifive_u_otp.c         | 30 +++++++++++++++++++++++++++++-
 include/hw/misc/sifive_u_otp.h |  3 +++
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/hw/misc/sifive_u_otp.c b/hw/misc/sifive_u_otp.c
index c2f3c8e129..565eec082f 100644
--- a/hw/misc/sifive_u_otp.c
+++ b/hw/misc/sifive_u_otp.c
@@ -25,6 +25,14 @@
 #include "qemu/module.h"
 #include "hw/misc/sifive_u_otp.h"
 
+#define WRITTEN_BIT_ON 0x1
+
+#define SET_FUSEARRAY_BIT(map, i, off, bit)    \
+    map[i] = bit ? (map[i] | bit << off) : (map[i] & ~(0x1 << off))
+
+#define GET_FUSEARRAY_BIT(map, i, off)    \
+    ((map[i] >> off) & 0x1)
+
 static uint64_t sifive_u_otp_read(void *opaque, hwaddr addr, unsigned int size)
 {
     SiFiveUOTPState *s = opaque;
@@ -123,7 +131,24 @@ static void sifive_u_otp_write(void *opaque, hwaddr addr,
         s->ptrim = val32;
         break;
     case SIFIVE_U_OTP_PWE:
-        s->pwe = val32;
+        s->pwe = val32 & SIFIVE_U_OTP_PWE_EN;
+
+        /* PWE is enabled. Ignore PAS=1 (no redundancy cell) */
+        if (s->pwe && !s->pas) {
+            if (GET_FUSEARRAY_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;
+            }
+
+            /* write bit data */
+            SET_FUSEARRAY_BIT(s->fuse, s->pa, s->paio, s->pdin);
+
+            /* update written bit */
+            SET_FUSEARRAY_BIT(s->fuse_wo, s->pa, s->paio, WRITTEN_BIT_ON);
+        }
+
         break;
     default:
         qemu_log_mask(LOG_GUEST_ERROR, "%s: bad write: addr=0x%" HWADDR_PRIx
@@ -165,6 +190,9 @@ 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/misc/sifive_u_otp.h b/include/hw/misc/sifive_u_otp.h
index 82c9176c8f..ebffbc1fa5 100644
--- a/include/hw/misc/sifive_u_otp.h
+++ b/include/hw/misc/sifive_u_otp.h
@@ -36,6 +36,8 @@
 #define SIFIVE_U_OTP_PTRIM      0x34
 #define SIFIVE_U_OTP_PWE        0x38
 
+#define SIFIVE_U_OTP_PWE_EN     (1 << 0)
+
 #define SIFIVE_U_OTP_PCE_EN     (1 << 0)
 
 #define SIFIVE_U_OTP_PDSTB_EN   (1 << 0)
@@ -75,6 +77,7 @@ 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;
 };
-- 
2.17.1