[PATCH v2 01/18] staging: comedi: comedi_buf: introduce comedi_buf_read_samples()

[Ian Abbott]

On 22/10/14 22:36, H Hartley Sweeten wrote:
> Introduce a generic method to read samples from the async buffer.
>
> The number of requested samples is clampled to the number of samples that
> would fill the async buffer. The size of each sample is determined using
> the bytes_per_sample() helper. The number of bytes need are then read
> from the async buffer using comedi_read_array_from_buffer().
>
> This will allow converting all the comedi drivers to use a common method
> to read data from the async buffer.
>
> Since comedi_read_array_from_buffer() sets the COMEDI_CB_BLOCK event after
> reading the data, those events can be removed from the drivers.
>
> In addition, comedi_inc_scan_progress() will automatically detect the end of
> scan and set the COMEDI_CB_EOS event. Those events can also be removed from
> the drivers.
>
> Signed-off-by: H Hartley Sweeten <hsweeten at visionengravers.com>
> Cc: Ian Abbott <abbotti at mev.co.uk>
> Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> ---
>   drivers/staging/comedi/comedi_buf.c | 27 +++++++++++++++++++++++++++
>   drivers/staging/comedi/comedidev.h  |  2 ++
>   2 files changed, 29 insertions(+)
>
> diff --git a/drivers/staging/comedi/comedi_buf.c b/drivers/staging/comedi/comedi_buf.c
> index c60a45ad..88a7cae 100644
> --- a/drivers/staging/comedi/comedi_buf.c
> +++ b/drivers/staging/comedi/comedi_buf.c
> @@ -575,3 +575,30 @@ unsigned int comedi_read_array_from_buffer(struct comedi_subdevice *s,
>   	return num_bytes;
>   }
>   EXPORT_SYMBOL_GPL(comedi_read_array_from_buffer);
> +
> +/**
> + * comedi_buf_read_samples - read sample data from comedi buffer
> + * @s: comedi_subdevice struct
> + * @data: destination
> + * @nsamples: maximum number of samples to read
> + *
> + * Reads up to nsamples from the comedi buffer associated with the subdevice,
> + * marks it as read and updates the acquisition scan progress.
> + *
> + * Returns the amount of data read in bytes.
> + */
> +unsigned int comedi_buf_read_samples(struct comedi_subdevice *s,
> +				     void *data, unsigned int nsamples)
> +{
> +	unsigned int max_samples;
> +	unsigned int nbytes;
> +
> +	max_samples = s->async->prealloc_bufsz / bytes_per_sample(s);
> +	if (nsamples > max_samples)
> +		nsamples = max_samples;
> +
> +	nbytes = nsamples * bytes_per_sample(s);
> +
> +	return comedi_read_array_from_buffer(s, data, nbytes);
> +}
> +EXPORT_SYMBOL_GPL(comedi_buf_read_samples);

There is a bit of a problem if the buffer contains a partial sample at 
the end (because the file read/write deals with bytes, but the drivers 
deal with with 2-or-4-byte samples).  In that case, 
comedi_read_array_from_buffer() could return a value that is not a 
multiple of the sample width.

For example, cb_pcidas_ao_load_fifo() in your patch 03 could read an odd 
number of bytes, round that down to a whole number of samples and ignore 
the last byte.  But that last byte comes back to haunt you on the next 
call because the next call to comedi_buf_read_samples() would start 
reading from the buffer in the middle of a sample.

A simple solution is to change the max_samples assignment to:

	max_samples = comedi_buf_read_n_available(s) / bytes_per_sample(s);

This works because comedi_read_array_from_buffer() should return the 
number of bytes you asked for if you've already checked they are there. 
  The only time it wouldn't do so is if the buffer is reset inbetween 
the calls, and that should only occur at end of acquisition or when the 
acquisition is cancelled.

Alternatively, it could be done in comedi_read_array_from_buffer(), but 
you're going to merge that function into this one anyway.

> diff --git a/drivers/staging/comedi/comedidev.h b/drivers/staging/comedi/comedidev.h
> index fb8ff84..ba4084b 100644
> --- a/drivers/staging/comedi/comedidev.h
> +++ b/drivers/staging/comedi/comedidev.h
> @@ -453,6 +453,8 @@ unsigned int comedi_write_array_to_buffer(struct comedi_subdevice *s,
>   					  unsigned int num_bytes);
>   unsigned int comedi_read_array_from_buffer(struct comedi_subdevice *s,
>   					   void *data, unsigned int num_bytes);
> +unsigned int comedi_buf_read_samples(struct comedi_subdevice *s,
> +				     void *data, unsigned int nsamples);
>
>   /* drivers.c - general comedi driver functions */
>
>


-- 
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti at mev.co.uk> )=-
-=(                          Web: http://www.mev.co.uk/  )=-