+#define TPM_GETRANDOM_RESULT_SIZE 18
+static struct tpm_input_header tpm_getrandom_header = {
+ .tag = TPM_TAG_RQU_COMMAND,
+ .length = cpu_to_be32(14),
+ .ordinal = TPM_ORD_GET_RANDOM
+};
+
+/**
+ * tpm_get_random() - Get random bytes from the tpm's RNG
+ * @chip_num: A specific chip number for the request or TPM_ANY_NUM
+ * @out: destination buffer for the random bytes
+ * @max: the max number of bytes to write to @out
+ *
+ * Returns < 0 on error and the number of bytes read on success
+ */
+int tpm_get_random(u32 chip_num, u8 *out, size_t max)
+{
+ struct tpm_chip *chip;
+ struct tpm_cmd_t tpm_cmd;
+ u32 recd, num_bytes = min_t(u32, max, TPM_MAX_RNG_DATA);
+ int err, total = 0, retries = 5;
+ u8 *dest = out;
+
+ chip = tpm_chip_find_get(chip_num);
+ if (chip == NULL)
+ return -ENODEV;
+
+ if (!out || !num_bytes || max > TPM_MAX_RNG_DATA)
+ return -EINVAL;
+
+ do {
+ tpm_cmd.header.in = tpm_getrandom_header;
+ tpm_cmd.params.getrandom_in.num_bytes = cpu_to_be32(num_bytes);
+
+ err = transmit_cmd(chip, &tpm_cmd,
+ TPM_GETRANDOM_RESULT_SIZE + num_bytes,
+ "attempting get random");
+ if (err)
+ break;
+
+ recd = be32_to_cpu(tpm_cmd.params.getrandom_out.rng_data_len);
+ memcpy(dest, tpm_cmd.params.getrandom_out.rng_data, recd);
+
+ dest += recd;
+ total += recd;
+ num_bytes -= recd;
+ } while (retries-- && total < max);
+
+ return total ? total : -EIO;
+}
+EXPORT_SYMBOL_GPL(tpm_get_random);
+