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

["Ard Biesheuvel" <ardb@kernel.org>]

On Wed, 22 Apr 2026, at 23:09, Lothar Rubusch wrote:
> 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);
> 

Please combine this with the buffer size fix in the previous patch.

> -		/* 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;
> +

Again, this is either redundant or wrong.

>  	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