Re: IIO ring buffer

[Jonathan Cameron <jic23@cam.ac.uk>]

Hi All,

Just for reference as I'll be doing a proper announcement later.
We now have linux-iio@vger.kernel.org as a mailing list for the project.
Unless others have tracked it down it currently only has me as a member
though and I'm waiting for confirmation from marc.info of an archive.

> Hi Jonathan,
> Now users depend on iio ring events(IIO_EVENT_CODE_RING_50/75/100_FULL)
> to read data:
>                 read_size = fread(&dat, 1, sizeof(struct
> iio_event_data),
>                                   fp_ev);
>                 switch (dat.id) {
>                 case IIO_EVENT_CODE_RING_100_FULL:
>                         toread = RingLength;
>                         break;
>                 case IIO_EVENT_CODE_RING_75_FULL:
>                         toread = RingLength*3/4;
>                         break;
>                 case IIO_EVENT_CODE_RING_50_FULL:
>                         toread = RingLength/2;
>                         break;
>                 default:
>                         printf("Unexpecteded event code\n");
>                         continue;
>                 }   
>                 read_size = read(fp,
>                                  data,
>                                  toread*size_from_scanmode(NumVals,
> scan_ts));
>                 if (read_size == -EAGAIN) {
>                         printf("nothing available \n");
>                         continue;
>                 }   
> And iio ring access node doesn't support blocking io too.  It seems we
> lost to let users read data once ring is not empty. And some users maybe
> not care about iio ring events at all, but just want to read data like a
> input or audio driver. So how about adding the following support in iio
> ring:
> 1. add NOT EMPTY event in IIO event nodes
Not keen.  It might lead to a storm of events (at least it might in a 
cleverer ring buffer implementation or during a blocking read).  Actually
in this particular case it probably wouldn't do any harm.
> 2. add blocking io support in read function of IIO access nodes
That I agree would be a good idea.  If we support poll/select on the buffer access
chrdev then we will get the same effect per having a not empty event and cleaner
semantics for anyone not interested in the other events. Not to mention I expect
we will soon have alternative ring buffer implementations that don't supply any
events at all and hence don't event have the relevant chrdev.

As things are, you can quite happily read whenever you like.  Now you mention it,
that example code is somewhat missleading! The issue with
this ring buffer implementation is the handling of a true blocking read is complex
as at any given time you aren't guaranteed to get what you asked for even if it was
there when you started the read. It should be possible to work around that though.

It's possible this functionality might be better added to an alternative ring buffer
implementation. Be vary wary of that ring implementation in general! I am and I wrote it.
> If you agree with that, I can begin to add these and send you a patch.
> And a problem I don't know is what you and other people have changed to
> Greg's staging tree, and I am not sure what tree the patch should be
> againest. 
Nothing has changed in this region of the code.  In fact I think all that
has gone into Greg's tree is a clean up patch form Mark Brown making a few
functions static. Right now I'm still getting the max1363 driver into
a state where it will be possible to do the ABI changes.
> 
> For long term plan, is it possible for ring common level to handle more
> common work to avoid code repeating in different drivers?
I'm certainly happy for that to be the case if it becomes apparent which functionality
is shared.  I haven't seen any substantial cases as yet, but then I may well be missing
things so feel free to submit suggestions (or as ever the better option of patches).

Thanks,

Jonathan