hw/i2c: pmbus add support for block receive

PMBus devices can send and receive variable length data using the
block read and write format, with the first byte in the payload
denoting the length.

This is mostly used for strings and on-device logs. Devices can
respond to a block read with an empty string.

Reviewed-by: Hao Wu <wuhaotsh@google.com>
Acked-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Titus Rwantare <titusr@google.com>
Message-ID: <20231023-staging-pmbus-v3-v4-1-07a8cb7cd20a@google.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

hw/i2c/pmbus_device.c

@@ -102,7 +102,6 @@ void pmbus_send_string(PMBusDevice *pmdev, const char *data)
     }
 
     size_t len = strlen(data);
-    g_assert(len > 0);
     g_assert(len + pmdev->out_buf_len < SMBUS_DATA_MAX_LEN);
     pmdev->out_buf[len + pmdev->out_buf_len] = len;
 
@@ -112,6 +111,35 @@ void pmbus_send_string(PMBusDevice *pmdev, const char *data)
     pmdev->out_buf_len += len + 1;
 }
 
+uint8_t pmbus_receive_block(PMBusDevice *pmdev, uint8_t *dest, size_t len)
+{
+    /* dest may contain data from previous writes */
+    memset(dest, 0, len);
+
+    /* Exclude command code from return value */
+    pmdev->in_buf++;
+    pmdev->in_buf_len--;
+
+    /* The byte after the command code denotes the length */
+    uint8_t sent_len = pmdev->in_buf[0];
+
+    if (sent_len != pmdev->in_buf_len - 1) {
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "%s: length mismatch. Expected %d bytes, got %d bytes\n",
+                      __func__, sent_len, pmdev->in_buf_len - 1);
+    }
+
+    /* exclude length byte */
+    pmdev->in_buf++;
+    pmdev->in_buf_len--;
+
+    if (pmdev->in_buf_len < len) {
+        len = pmdev->in_buf_len;
+    }
+    memcpy(dest, pmdev->in_buf, len);
+    return len;
+}
+
 
 static uint64_t pmbus_receive_uint(PMBusDevice *pmdev)
 {

include/hw/i2c/pmbus_device.h

@@ -501,6 +501,13 @@ void pmbus_send64(PMBusDevice *state, uint64_t data);
  */
 void pmbus_send_string(PMBusDevice *state, const char *data);
 
+/**
+ * @brief Receive data sent with Block Write.
+ * @param dest - memory with enough capacity to receive the write
+ * @param len - the capacity of dest
+ */
+uint8_t pmbus_receive_block(PMBusDevice *pmdev, uint8_t *dest, size_t len);
+
 /**
  * @brief Receive data over PMBus
  * These methods help track how much data is being received over PMBus