Re: [PATCH AUTOSEL 4.4 23/60] iio: buffer: Don't allow buffers without any channels enabled to be activated

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Wed, 17 Jun 2020 21:29:27 -0400
Sasha Levin <sashal@kernel.org> wrote:

> From: Lars-Peter Clausen <lars@metafoo.de>
> 
> [ Upstream commit b7329249ea5b08b2a1c2c3f24a2f4c495c4f14b8 ]
> 
> Before activating a buffer make sure that at least one channel is enabled.
> Activating a buffer with 0 channels enabled doesn't make too much sense and
> disallowing this case makes sure that individual driver don't have to add
> special case code to handle it.
> 
> Currently, without this patch enabling a buffer is possible and no error is
> produced. With this patch -EINVAL is returned.
> 
> An example of execution with this patch and some instrumented print-code:
>    root@analog:~# cd /sys/bus/iio/devices/iio\:device3/buffer
>    root@analog:/sys/bus/iio/devices/iio:device3/buffer# echo 1 > enable
>    0: iio_verify_update 748 indio_dev->masklength 2 *insert_buffer->scan_mask 00000000
>    1: iio_verify_update 753
>    2:__iio_update_buffers 1115 ret -22
>    3: iio_buffer_store_enable 1241 ret -22
>    -bash: echo: write error: Invalid argument
> 1, 2 & 3 are exit-error paths. 0 the first print in iio_verify_update()
> rergardless of error path.
> 
> Without this patch (and same instrumented print-code):
>    root@analog:~# cd /sys/bus/iio/devices/iio\:device3/buffer
>    root@analog:/sys/bus/iio/devices/iio:device3/buffer# echo 1 > enable
>    0: iio_verify_update 748 indio_dev->masklength 2 *insert_buffer->scan_mask 00000000
>    root@analog:/sys/bus/iio/devices/iio:device3/buffer#
> Buffer is enabled with no error.
> 
> Note from Jonathan: Probably not suitable for automatic application to stable.
> This has been there from the very start.  It tidies up an odd corner
> case but won't effect any 'real' users.
> 

As noted. I don't think it matters if we do apply this to stable.
It closes an interface oddity rather than an actual known bug.

> Fixes: 84b36ce5f79c0 ("staging:iio: Add support for multiple buffers")
> Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>  drivers/iio/industrialio-buffer.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
> index 864a61b05665..fea41b328ab9 100644
> --- a/drivers/iio/industrialio-buffer.c
> +++ b/drivers/iio/industrialio-buffer.c
> @@ -641,6 +641,13 @@ static int iio_verify_update(struct iio_dev *indio_dev,
>  	bool scan_timestamp;
>  	unsigned int modes;
>  
> +	if (insert_buffer &&
> +	    bitmap_empty(insert_buffer->scan_mask, indio_dev->masklength)) {
> +		dev_dbg(&indio_dev->dev,
> +			"At least one scan element must be enabled first\n");
> +		return -EINVAL;
> +	}
> +
>  	memset(config, 0, sizeof(*config));
>  
>  	/*