Re: [PATCH 3/4] iio: add helper function for reading channel offset in buffer

[Jonathan Cameron <jic23@kernel.org>]

On Wed, 17 Aug 2022 12:56:42 +0200
Artur Rojek <contact@artur-rojek.eu> wrote:

> This is useful for consumers that wish to parse raw buffer data.
> 
> Tested-by: Paul Cercueil <paul@crapouillou.net>
> Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
> ---
>  drivers/iio/industrialio-buffer.c | 28 ++++++++++++++++++++++++++++
>  include/linux/iio/buffer.h        |  4 ++++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
> index 228598b82a2f..cf23736610d9 100644
> --- a/drivers/iio/industrialio-buffer.c
> +++ b/drivers/iio/industrialio-buffer.c
> @@ -691,6 +691,34 @@ static unsigned int iio_storage_bytes_for_si(struct iio_dev *indio_dev,
>  	return bytes;
>  }
>  
> +int iio_find_channel_offset_in_buffer(struct iio_dev *indio_dev,
> +				      const struct iio_chan_spec *chan,
> +				      struct iio_buffer *buffer)
> +{
> +	int length, offset = 0;
> +	unsigned int si;
> +
> +	if (chan->scan_index < 0 ||
> +	    !test_bit(chan->scan_index, buffer->scan_mask)) {
> +		return -EINVAL;
> +	}
> +
> +	for (si = 0; si < chan->scan_index; ++si) {
> +		if (!test_bit(si, buffer->scan_mask))

You are walking channels that the consumer should not even know about
here.

I think you can establish the same easily enough from the channels it
does know about.  It would be fiddly if you had lots of channels (as
you might be best off sorting them) but for small numbers of channels
loop over the array to find lowest scan_index - compute offset due
to that then move on to next one etc until you reach the scan index
of the channel you want.


> +			continue;
> +
> +		length = iio_storage_bytes_for_si(indio_dev, si);
> +
> +		/* Account for channel alignment. */
> +		if (offset % length)
> +			offset += length - (offset % length);
> +		offset += length;
> +	}
> +
> +	return offset;
> +}
> +EXPORT_SYMBOL_GPL(iio_find_channel_offset_in_buffer);
> +
>  static unsigned int iio_storage_bytes_for_timestamp(struct iio_dev *indio_dev)
>  {
>  	struct iio_dev_opaque *iio_dev_opaque = to_iio_dev_opaque(indio_dev);
> diff --git a/include/linux/iio/buffer.h b/include/linux/iio/buffer.h
> index 418b1307d3f2..b1db74772e77 100644
> --- a/include/linux/iio/buffer.h
> +++ b/include/linux/iio/buffer.h
> @@ -16,6 +16,10 @@ enum iio_buffer_direction {
>  	IIO_BUFFER_DIRECTION_OUT,
>  };
>  
> +int iio_find_channel_offset_in_buffer(struct iio_dev *indio_dev,
> +				      const struct iio_chan_spec *chan,
> +				      struct iio_buffer *buffer);
> +
>  int iio_push_to_buffers(struct iio_dev *indio_dev, const void *data);
>  
>  int iio_pop_from_buffer(struct iio_buffer *buffer, void *data);