[PATCH v3 3/3] crypto: atmel-sha204a - fix non-blocking read logic

[Lothar Rubusch <l.rubusch@gmail.com>]

The non-blocking path was (also) failing to provide valid entropy
due to improper buffer management and a lack of hardware execution
time.

Ensure cmd.msecs (30ms) and cmd.rxsize (35ms) are initialized before
enqueuing the background work. Fix the data offset to skip the
1-byte hardware count header when copying bits to the caller. Correctly
return 0 (busy) to the hwrng core while hardware execution is in
progress, preventing zero-filled buffers, which was the situation
before.

With this fix applied, tests will look similar to this:
$ socat -u OPEN:/dev/hwrng,nonblock - | head -c 32 | hexdump -C
00000000  23 cc 42 3c 90 b1 38 fc  54 37 35 4b 09 c5 e1 0d  |#.B<..8.T75K....|
2026/03/23 14:30:18 socat[858] E read(5, 0x55be363000, 8192): Resource temporarily unavailable
00000010  73 3b af d9 02 70 76 bd  2d 59 4b 12 01 ac ae 2b  |s;...pv.-YK....+|
00000020

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 | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/drivers/crypto/atmel-sha204a.c b/drivers/crypto/atmel-sha204a.c
index f7dc00d0f4cd..04cbf80c1411 100644
--- a/drivers/crypto/atmel-sha204a.c
+++ b/drivers/crypto/atmel-sha204a.c
@@ -33,7 +33,6 @@ static void atmel_sha204a_rng_done(struct atmel_i2c_work_data *work_data,
 				     "i2c transaction failed (%d)\n",
 				     status);
 		kfree(work_data);
-		rng->priv = 0;
 		atomic_dec(&i2c_priv->tfm_count);
 		return;
 	}
@@ -49,20 +48,19 @@ static int atmel_sha204a_rng_read_nonblocking(struct hwrng *rng, void *data,
 
 	i2c_priv = container_of(rng, struct atmel_i2c_client_priv, hwrng);
 
-	/* Verify if data available from last run */
 	if (rng->priv) {
 		work_data = (struct atmel_i2c_work_data *)rng->priv;
-		max = min(sizeof(work_data->cmd.data), max);
-		memcpy(data, &work_data->cmd.data, max);
+		max = min_t(size_t, ATMEL_RNG_BLOCK_SIZE, max);
+		memcpy(data, &work_data->cmd.data[1], max);
 
-		/* Now, free memory */
+		/* Free memory and clear the in-flight flag */
 		kfree(work_data);
 		rng->priv = 0;
 		atomic_dec(&i2c_priv->tfm_count);
 		return max;
 	}
 
-	/* When a request is still in-flight but not processed */
+	/* If a request is still in-flight, return 0 (busy) */
 	if (atomic_read(&i2c_priv->tfm_count) > 0)
 		return 0;
 
@@ -76,8 +74,14 @@ static int atmel_sha204a_rng_read_nonblocking(struct hwrng *rng, void *data,
 	work_data->client = i2c_priv->client;
 
 	atmel_i2c_init_random_cmd(&work_data->cmd);
+
+	/* Set the execution time for the RNG command (from datasheet) */
+	work_data->cmd.msecs = ATMEL_RNG_EXEC_TIME;
+	work_data->cmd.rxsize = RANDOM_RSP_SIZE;
+
 	atmel_i2c_enqueue(work_data, atmel_sha204a_rng_done, rng);
 
+	/* Return 0 to indicate 'busy', data will be ready on next call */
 	return 0;
 }
 
-- 
2.53.0