The ATSHA204A returns a 35-byte packet consisting of a 1-byte count,
32 bytes of entropy, and a 2-byte CRC. The current blocking read
implementation was incorrectly copying data starting from the
count byte, leading to offset data and truncated entropy.

Additionally, the chip requires significant execution time to
generate random numbers, going by the datasheet. Reading the I2C bus
too early results in the chip NACK-ing or returning a partial buffer
followed by zeros.

Verification:
Tests before showed repeadetly reading only 8 bytes of entropy:
$ head -c 32 /dev/hwrng | hexdump -C
00000000  02 28 85 b3 47 40 f2 ee  00 00 00 00 00 00 00 00  |.(..G@..........|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000020

After this patch applied, the result will be as follows:
$ head -c 32 /dev/hwrng | hexdump -C
00000000  5a fc 3f 13 14 68 fe 06  68 0a bd 04 83 6e 09 69  |Z.?..h..h....n.i|
00000010  75 ff cf 87 10 84 3b c9  c1 df ae eb 45 53 4c c3  |u.....;.....ESL.|
00000020

Fix these issues by:
Increase cmd.msecs to 30ms to provide sufficient execution time. Then
set cmd.rxsize to RANDOM_RSP_SIZE (35 bytes) to capture the entire
hardware response. Eventually, correct the memcpy() offset to index 1 of
the data buffer to skip the count byte and retrieve exactly 32 bytes of
entropy.

Fixes: da001fb651b0 ("crypto: atmel-i2c - add support for SHA204A random number generator")
Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
---
 drivers/crypto/atmel-sha204a.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/crypto/atmel-sha204a.c b/drivers/crypto/atmel-sha204a.c
index 1baf4750d311..350ba8618c69 100644
--- a/drivers/crypto/atmel-sha204a.c
+++ b/drivers/crypto/atmel-sha204a.c
@@ -18,6 +18,9 @@
 #include <linux/workqueue.h>
 #include "atmel-i2c.h"
 
+#define ATMEL_RNG_BLOCK_SIZE 32
+#define ATMEL_RNG_EXEC_TIME 30
+
 static void atmel_sha204a_rng_done(struct atmel_i2c_work_data *work_data,
 				   void *areq, int status)
 {
@@ -91,13 +94,15 @@ static int atmel_sha204a_rng_read(struct hwrng *rng, void *data, size_t max,
 	i2c_priv = container_of(rng, struct atmel_i2c_client_priv, hwrng);
 
 	atmel_i2c_init_random_cmd(&cmd);
+	cmd.msecs = ATMEL_RNG_EXEC_TIME;
+	cmd.rxsize = RANDOM_RSP_SIZE;
 
 	ret = atmel_i2c_send_receive(i2c_priv->client, &cmd);
 	if (ret)
 		return ret;
 
-	max = min(sizeof(cmd.data), max);
-	memcpy(data, cmd.data, max);
+	max = min_t(size_t, ATMEL_RNG_BLOCK_SIZE, max);
+	memcpy(data, &cmd.data[1], max);
 
 	return max;
 }
-- 
2.39.5