kernel crypto hmac sha256 API call code

static int hmac_sha256(char *plaintext, unsigned int plain_text_size, char *key, unsigned int key_size, uint8_t *result)
{
    struct scatterlist sg;
    struct crypto_hash *tfm;
    struct hash_desc desc;
    int ret;

    if (!result) {
        printk(KERN_ERR "param err\n");
        return -EINVAL;
    } 

    tfm = crypto_alloc_hash("hmac(sha256)", 0, CRYPTO_ALG_ASYNC);
    if (IS_ERR(tfm)) {
        printk(KERN_ERR "crypto_alloc_ahash failed: err %ld", PTR_ERR(tfm));
        return -EINVAL;
     } 

     desc.tfm = tfm;
     desc.flags = 0;
     sg_set_buf(&sg, plaintext, plain_text_size);
     ret = crypto_hash_setkey(tfm, key, key_size);
     if (ret) {
         printk(KERN_ERR "crypto_ahash_setkey failed: err %d", ret);
         goto out;
     } 

     ret = crypto_hash_digest(&desc, &sg, plain_text_size, result);
     if(ret) {
         printk(KERN_ERR "digest() failed ret = %d\n", ret);
         goto out;
     }
     printk(KERN_DEBUG, "crypto hash digest size %d\n", crypto_hash_digestsize(tfm));

out:
    crypto_free_hash(tfm);
    return -EINVAL;
}