Re: [PATCH 1/3] iio: dht11: Improve reliability - be more tolerant about missing start bits

[Jonathan Cameron <jic23@kernel.org>]

On 30/12/15 14:26, Harald Geyer wrote:
> Instead of guessing where the data starts, we now just try to decode from
> every possible start position. This causes no additional overhead if we
> properly received the full preamble and only costs a few extra CPU cycles
> in the case where the preamble is corrupted. This is much more efficient
> than to return an error to userspace and start over again.
> 
> Signed-off-by: Harald Geyer <harald@ccbib.org>
Seems sensible, but ideally I'd like a tested by!  Sounds like
Jonathan Bell might be able to provide one.

Changes to this driver always worry me given how fiddly the hardware
interface is.

Jonathan
> ---
> Jonathan Bell:
> Can you test if this fixes the problems you reported on RPi?
> 
> (It does fix problems I noticed when I slowed down my test HW with
> enabling the debug logging introduced later in this series, so including
> this patch is a good thing even if it doesn't fix Jonathan's problems.)
> 
>  drivers/iio/humidity/dht11.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/iio/humidity/dht11.c b/drivers/iio/humidity/dht11.c
> index 1165b1c..1ca284a 100644
> --- a/drivers/iio/humidity/dht11.c
> +++ b/drivers/iio/humidity/dht11.c
> @@ -161,7 +161,7 @@ static int dht11_read_raw(struct iio_dev *iio_dev,
>  			int *val, int *val2, long m)
>  {
>  	struct dht11 *dht11 = iio_priv(iio_dev);
> -	int ret, timeres;
> +	int ret, timeres, offset;
>  
>  	mutex_lock(&dht11->lock);
>  	if (dht11->timestamp + DHT11_DATA_VALID_TIME < ktime_get_real_ns()) {
> @@ -208,11 +208,14 @@ static int dht11_read_raw(struct iio_dev *iio_dev,
>  		if (ret < 0)
>  			goto err;
>  
> -		ret = dht11_decode(dht11,
> -				   dht11->num_edges == DHT11_EDGES_PER_READ ?
> -					DHT11_EDGES_PREAMBLE :
> -					DHT11_EDGES_PREAMBLE - 2,
> -				timeres);
> +		offset = DHT11_EDGES_PREAMBLE +
> +				dht11->num_edges - DHT11_EDGES_PER_READ;
> +		for (; offset >= 0; --offset) {
> +			ret = dht11_decode(dht11, offset, timeres);
> +			if (!ret)
> +				break;
> +		}
> +
>  		if (ret)
>  			goto err;
>  	}
>