Re: [PATCH v2] iio: allow userspace to flush the hwfifo with non-blocking reads

[Jonathan Cameron <jic23@kernel.org>]

On 05/06/15 13:56, Octavian Purdila wrote:
> This patch changes the semantics of non-blocking reads so that a
> hardware fifo flush is triggered if the available data in the device
> buffer is less then the requested size.
> 
> This allows userspace to accurately generate hardware fifo flushes, by
> doing a non-blocking read with a size greater then the sum of the
> device buffer and hardware fifo size.
> 
> Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Hi Octavian,

I'm personally happy with this approach, but would like Lars to have
a chance to take a look as he's been getting his hands a lot dirtier in
this area than I have recently!

This seems a logical bit of API to me even without the desire to use it
to force a flush.  If we want whatever data is available now,
we want that data now, not when the fifo gets around to giving it to us!

Feel free to poke me if this sits here uncommented upon for a few more
weeks!

Jonathan
> ---
> 
> This is the second iteration of the patch that allows userspace to
> accurately generate flush events. The first RFC patch set was
> discussed here:
> 
> https://lkml.org/lkml/2015/4/29/202
> 
> 
>  drivers/iio/industrialio-buffer.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
> index df919f4..24085db 100644
> --- a/drivers/iio/industrialio-buffer.c
> +++ b/drivers/iio/industrialio-buffer.c
> @@ -71,8 +71,9 @@ static bool iio_buffer_ready(struct iio_dev *indio_dev, struct iio_buffer *buf,
>  
>  	if (avail >= to_wait) {
>  		/* force a flush for non-blocking reads */
> -		if (!to_wait && !avail && to_flush)
> -			iio_buffer_flush_hwfifo(indio_dev, buf, to_flush);
> +		if (!to_wait && avail < to_flush)
> +			iio_buffer_flush_hwfifo(indio_dev, buf,
> +						to_flush - avail);
>  		return true;
>  	}
>  
> @@ -100,8 +101,7 @@ ssize_t iio_buffer_read_first_n_outer(struct file *filp, char __user *buf,
>  	struct iio_dev *indio_dev = filp->private_data;
>  	struct iio_buffer *rb = indio_dev->buffer;
>  	size_t datum_size;
> -	size_t to_wait = 0;
> -	size_t to_read;
> +	size_t to_wait;
>  	int ret;
>  
>  	if (!indio_dev->info)
> @@ -119,14 +119,14 @@ ssize_t iio_buffer_read_first_n_outer(struct file *filp, char __user *buf,
>  	if (!datum_size)
>  		return 0;
>  
> -	to_read = min_t(size_t, n / datum_size, rb->watermark);
> -
> -	if (!(filp->f_flags & O_NONBLOCK))
> -		to_wait = to_read;
> +	if (filp->f_flags & O_NONBLOCK)
> +		to_wait = 0;
> +	else
> +		to_wait = min_t(size_t, n / datum_size, rb->watermark);
>  
>  	do {
>  		ret = wait_event_interruptible(rb->pollq,
> -			iio_buffer_ready(indio_dev, rb, to_wait, to_read));
> +		      iio_buffer_ready(indio_dev, rb, to_wait, n / datum_size));
>  		if (ret)
>  			return ret;
>  
>