Re: [PATCH v3 13/13] iio: core: Clarify the modes

[Jonathan Cameron <jic23@kernel.org>]

On Mon, 14 Feb 2022 09:53:08 +0100
Miquel Raynal <miquel.raynal@bootlin.com> wrote:

> Hi Jonathan,
> 
> jic23@kernel.org wrote on Sun, 13 Feb 2022 18:42:24 +0000:
> 
> > On Mon,  7 Feb 2022 15:38:40 +0100
> > Miquel Raynal <miquel.raynal@bootlin.com> wrote:
> >   
> > > As part of a previous discussion with Jonathan Cameron [1], it appeared
> > > necessary to clarify the meaning of each mode so that new developers
> > > could understand better what they should use or not use and when.
> > > 
> > > The idea of renaming these modes as been let aside because naming is a
> > > big deal and requires a lot of thinking. So for now let's focus on
> > > correctly explaining what each mode implies.
> > > 
> > > [1] https://lore.kernel.org/linux-iio/20210930165510.2295e6c4@jic23-huawei/
> > > 
> > > Suggested-by: Jonathan Cameron <jic23@kernel.org>
> > > Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>    
> > One trivial thing inline as a result of edits in v3.
> > 
> > Otherwise, I want to let this series sit a little longer and ideally get
> > some eyes on the st_sensors patches.  
> 
> Sure.

Denis, Linus, Lorenzo,

If any of you have time to take a look at patches 4-8 in this series or ideally
to run basic sanity tests with series in place that would be great.
https://patchwork.kernel.org/project/linux-iio/list/?series=611853

I don't have a convenient platform to test that driver on any more and the
changes are invasive enough to make me a little nervous about taking the
series without someone more familiar with that driver taking a look.

Thanks,

Jonathan

> 
> > 
> > Jonathan
> >   
> > > ---
> > >  include/linux/iio/iio.h | 49 ++++++++++++++++++++++++++++++++++++++++-
> > >  1 file changed, 48 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
> > > index 85cb924debd9..e383b0d96035 100644
> > > --- a/include/linux/iio/iio.h
> > > +++ b/include/linux/iio/iio.h
> > > @@ -315,7 +315,54 @@ static inline bool iio_channel_has_available(const struct iio_chan_spec *chan,
> > >  s64 iio_get_time_ns(const struct iio_dev *indio_dev);
> > >  unsigned int iio_get_time_res(const struct iio_dev *indio_dev);
> > >  
> > > -/* Device operating modes */
> > > +/**
> > > + * Device operating modes
> > > + * @INDIO_DIRECT_MODE: There is an access to either:
> > > + * a) The last single value available for devices that do not provide
> > > + *    on-demand reads.
> > > + * b) A new value after performing an on-demand read otherwise.    
> > 
> >   
> > > + * On most devices, this is a single-shot read. On some devices with data
> > > + * streams without an 'on-demand' function, this might also be the 'last value'
> > > + * feature.    
> > 
> > This block duplicates what you now have as a/b above. I can drop it whilst
> > applying if nothing else comes up.  
> 
> We can get rid of it indeed. Let's see what ST people have in mind
> regarding the st_sensors patches.
> 
> > >  Above all, this mode internally means that we are not in any of the
> > > + * other modes, and sysfs reads should work.
> > > + * Device drivers should inform the core if they support this mode.
> > > + * @INDIO_BUFFER_TRIGGERED: Common mode when dealing with kfifo buffers.
> > > + * It indicates that an explicit trigger is required. This requests the core to
> > > + * attach a poll function when enabling the buffer, which is indicated by the
> > > + * _TRIGGERED suffix.
> > > + * The core will ensure this mode is set when registering a triggered buffer
> > > + * with iio_triggered_buffer_setup().
> > > + * @INDIO_BUFFER_SOFTWARE: Another kfifo buffer mode, but not event triggered.
> > > + * No poll function can be attached because there is no triggered infrastructure
> > > + * we can use to cause capture. There is a kfifo that the driver will fill, but
> > > + * not "only one scan at a time". Typically, hardware will have a buffer that
> > > + * can hold multiple scans. Software may read one or more scans at a single time
> > > + * and push the available data to a Kfifo. This means the core will not attach
> > > + * any poll function when enabling the buffer.
> > > + * The core will ensure this mode is set when registering a simple kfifo buffer
> > > + * with devm_iio_kfifo_buffer_setup().
> > > + * @INDIO_BUFFER_HARDWARE: For specific hardware, if unsure do not use this mode.
> > > + * Same as above but this time the buffer is not a kfifo where we have direct
> > > + * access to the data. Instead, the consumer driver must access the data through
> > > + * non software visible channels (or DMA when there is no demux possible in
> > > + * software)
> > > + * The core will ensure this mode is set when registering a dmaengine buffer
> > > + * with devm_iio_dmaengine_buffer_setup().
> > > + * @INDIO_EVENT_TRIGGERED: Very unusual mode.
> > > + * Triggers usually refer to an external event which will start data capture.
> > > + * Here it is kind of the opposite as, a particular state of the data might
> > > + * produce an event which can be considered as an event. We don't necessarily
> > > + * have access to the data itself, but to the event produced. For example, this
> > > + * can be a threshold detector. The internal path of this mode is very close to
> > > + * the INDIO_BUFFER_TRIGGERED mode.
> > > + * The core will ensure this mode is set when registering a triggered event.
> > > + * @INDIO_HARDWARE_TRIGGERED: Very unusual mode.
> > > + * Here, triggers can result in data capture and can be routed to multiple
> > > + * hardware components, which make them close to regular triggers in the way
> > > + * they must be managed by the core, but without the entire interrupts/poll
> > > + * functions burden. Interrupts are irrelevant as the data flow is hardware
> > > + * mediated and distributed.
> > > + */
> > >  #define INDIO_DIRECT_MODE		0x01
> > >  #define INDIO_BUFFER_TRIGGERED		0x02
> > >  #define INDIO_BUFFER_SOFTWARE		0x04    
> >   
> 
> 
> Thanks,
> Miquèl