From: Arnd Bergmann <arnd@arndb.de>
To: Jonathan Cameron <jic23@cam.ac.uk>, Kay Sievers <kay.sievers@vrfy.org>
Subject: Re: IIO comments
Date: Wed, 16 Mar 2011 14:33:30 +0100
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Cc: linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org,
        Greg KH <greg@kroah.com>
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On Wednesday 16 March 2011, Jonathan Cameron wrote:
> Hi Arnd,
> > I've started looking at the current code base in staging,
> > this is basically a log of what I'm finding there that
> > may get improved:
> Thanks for taking a look!
> > 
> > * Your bus_type is not really a bus in the sense we normally
> > use it, because it does not have any driver/device matching.
> > You only register devices from the same files that drive
> > the hardware. This is normally done using a class, not
> > a bus.
> That's what I originally thought and indeed how it was
> initially done.  The responses from Greg KH and Kay Sievers
> to one of our ABI discussions convinced me otherwise.
> 
> http://lkml.org/lkml/2010/1/20/233 + the previous email in
> that thread.

(adding Kay to Cc here)

I see. However, I'd still disagree with Kay here. Given that
the common use of existing subsystems is to have "bus" for
things that are physically connected, and "class" for things
that are logical abstractions, I think we should continue
that way for new subsystems, even if there is little difference
technically.

There is little point discussing how things should have been
done in the past when we're stuck with them. The best we
can do now is make the code as consistent as possible.

> > * Since you specify the sysfs attributes globally in the
> > documentation, it would be nice if drivers did not have
> > to implement them as attribute show function, but as
> > a function that simply returns a value that is then
> > printed by common code. You can do this using a structure
> > of function pointers that each driver fills with the
> > relevant ones, like file_operations, replacing the
> > attribute groups.
> This may indeed work.  However it would be a fair bit
> more fiddly than that a simple function pointer structure
> as the sets of attributes supplied by different devices
> vary considerably.  

It's certainly more work for the subsystem, but the intention
is to simplify the individual drivers, to make things scale
better as many more drivers get added.

> So to confirm I understand you correctly we either have something like:
> /* access ops */
> 
> int (*get_accel_x)(struct iio_dev *devinfo)
> char *(*get_accel_offset)(struct iio_dev *devinfo); /* note these can't return a value as
> they may well be floating point */
> char *(*get_accel_gain)(struct iio_dev *devinfo)
> int (*get_accel_y)(struct iio_dev *devinfo)
> 
> Sticky cases that will make this fiddly.

You can't really return a char* because of the memory allocation,
but aside from that, the idea is right, yes.

Note that this forces all drivers to use the same unit
for data they return in the attributes, but I consider
this a good thing.

For attributes that really want to return a floating point
number, I think we should either define a fixed-point
form with a constant exponent, or some other representation
that is allowed in the kernel.

> * Unknown maximum number of some types of attributes.
>   - Could use a 
> 	int (*get_in)(struct iio_dev dev_info, int channel);
> 	int num_in;

Good point. So a driver may have multiple attributes
of the same type, which need to get enumerated, but should
all behave the same way, right?

Your suggestion seems good, but I'd still recommend making
it so that the core chooses the attribute names, not the
device driver. One possible way to lay this out is something
like:

/* global defines */
enum iio_channel_type {
	IIO_CHAN_TEMPERATURE,
	IIO_CHAN_ADC,
	IIO_CHAN_FOO,
	...
};

static const char *channel_names[] = {
	[IIO_CHAN_TEMPERATURE]	= "temperature%d",
	[IIO_CHAN_ADC]		= "adc%d",
	[IIO_CHAN_FOO]		= "foo%d",
	...
};

typedef unsigned long long iio_result_t;

struct iio_channel_defs {
	iio_result_t (*get)(struct iio_dev *dev_info, int channel);
	
	enum iio_channel_type channels[0];
};

/* example driver */
enum {
	FOO_CHAN_TEMP0,
	FOO_CHAN_TEMP1,
	FOO_CHAN_ADC,
};

struct iio_channel_defs foo_driver_channels = {
	.get = &foo_get_sensor,

	.channels = {
		/* this device has two temperature and one adc input */
		[FOO_CHAN_TEMP0] = IIO_CHAN_TEMPERATURE,
		[FOO_CHAN_TEMP1] = IIO_CHAN_TEMPERATURE,
		[FOO_CHAN_ADC]   = IIO_CHAN_ADC,
	},
};

iio_result_t foo_getsensor(struct iio_dev *dev_info, int channel)
{
	switch (channel) {
	case FOO_CHAN_TEMP0:
		return read_temp0(dev_info);
	case FOO_CHAN_TEMP1:
		return read_temp1(dev_info);
	case FOO_CHAN_ADC:
		return read_adc(dev_info);
	}
	return -EINVAL;
}

> * Whatever you pick also immediately becomes restrictive as the main
> class has to be updated to handle any new elements.

I believe that's a good thing, we want to limit the number
of different interfaces that people come up with. If two
drivers are similar enough, they should probably use the
same one.

It also makes it easier to verify that all attributes are
documented properly, because the documentation only has
to be matched with a single file, not with every single
driver.

It also forces out-of-tree modules to use the same interfaces
that other drivers are using. If someone has hardware for
something new, it will only have an interface to talk to
after it has been discussed and documented in mainline.

> * Note we'll have function points for all the scan element related stuff
> as well.  Basically the set that would be needed is huge. 
> 
> At the end of the day I'm unconvinced this makes sense from the point
> of view of simplifying the driver code.  The gain is that it enforces the abi
> and may allow in kernel uses.
> Ultimately we lifted this approach from hwmon where it works well
> under similar circumstances.
> 
> Alternative is a small set such as:
> 
> /* access ops */
> int (*get_channel_value)(struct iio_dev *info, enum CHANNEL);
> char (*get_channel_offset)(struct iio_dev *info, enum CHANNEL);
> 
> and a list of which channels exist.  Then the driver side becomes a massive
> switch statement rather than the attribute tables.  Again, I'm not convinced
> that is actually cleaner or easier to understand. Also this approach pretty
> much means you end up with large numbers of equal attributes as you can't
> trivially group them.  e.g we have accel_scale to cover case where it's the
> same for all accelerometer axes.
 
The whole point would be to avoid duplication, so maybe something
a little smarter is needed, but the current code duplicates the
definition of attributes to every driver, and I'm sure we can
find a solution that requires less code.

In many cases, you should be able to replace a long switch statement
by a table driven approach inside of the driver.

> > * It's not clear to me why you need one additional device
> > and one chardev for each interrupt line of a device, my
> > feeling is that this could be simplified a lot if you find
> > a way to represent an iio_dev with a single chardev to
> > user space. This may have been an attempt to avoid ioctl()
> > calls, but I think in this case an ioctl interface would
> > be cleaner than a complex hierarchy of devices.
> > Another alternative would be for the device driver to
> > register multiple iio_devs in case it needs multiple
> > interfaces.
> In a sense this isn't quite as bad as it seems. The only cases we
> have so far (and it's more or less all that's out there) are those where
> the device has one interrupt line for 'events' - e.g threshold
> interrupts of one type or another, and one for one for data ready
> signals.  The data ready signals are handled as triggers and hence
> don't have a chardev (directly).

If you only have the two, you might be able to handle them both
in the poll function, where the regular case means POLLIN
or POLLRDNORM, while the threshold interrupt gets turned
into POLLPRI or POLLRDBAND.

I'm not sure if that covers all cases you are interested in,
but it sounds like a useful simplification. It also makes
it possible for user programs to only wait for one type of
event.

> The other uses of chrdevs are for accessing buffers.  Each buffer may have
> two of them.  The first is a dirty read chrdev.  The second is
> for out of band flow information to userspace.  Basically you
> control what events will come out of the flow control chrdev.
> Programs block on that and read from the data flow chrdev
> according to what comes down the line.  The messy events stuff
> is to handle event escalation.  Dealing with getting a 50% full
> event and then a 75% full event with no inherent knowledge of
> if you read anything in between is a pain.  We could specify
> only one level event exists, which would simplify the code
> somewhat.  Perhaps wise for an initial merge though I'm loath
> to not support functionality that is fairly common in hardware
> buffers.
> 
> Some buffer implementations don't provide flow control (kfifo_buf)
> so for them you just have a read chrdev (actually this isn't true
> right now - I need to fix this).  This chrdev is separate from
> the 'event' chrdevs purely because for speed reasons. You don't
> want to have what is coming down this line change except due
> to software interaction (and to do that you normally have to stop
> the buffer fill).

Doesn't epoll handle the event escalation with its edge triggered
mode? Basically you can get notified every time that there is
more to be read.

> > * Neither of your two file operations supports poll(), only
> > read. Having poll() support is essential if you want to
> > wait for multiple devices in user space. Maybe it's
> > possible to convert the code to use eventfd instead of
> > rolling your own event interface?
> I'll look into this one. Curiously I don't think it has
> ever been suggested before. (maybe I'm just forgetful)
> 
> We have had the suggestion of using inputs event interface and
> I am still considering that option. The issue
> there is that it is really far too heavyweight. So far we have
> no use cases for multiple readers and we never want to aggregate
> across devices.  Also input views events as carrying data. All
> actual readings in our case are going through the faster buffer
> route.

I'm slightly confused, mainly because I have no idea how the
buffer and event interfaces are actually being used. I would
have expected an interface as simple as:

* One iio device per sensor that can provide data, possibly
  many per hardware device
* One chardev for each iio device
* Use epoll to wait for data and/or out-of-band messages
* Use chrdev read to get events from the buffer
* Use sysfs to read from sensors that are not event based

What you have is obviously more complex than this, and you
probably have good reasons that I still need to understand.

> > Also, there is only one driver providing such a function,
> > so this is probably a good candidate for deferring to a later
> > stage, along with all the the entire ring file code that seems
> > rather pointless otherwise.
> Lots of drivers do it.  They are simply using one of the software
> provided buffers.  ring_sw or kfifo_buf (and yes they do have
> useless names as well so we'll clean that up for which ever
> ones we keep!)

Ok, I see. I did not realize that iio_rip_sw_rb was getting assigned
to ->rip_lots.

> > * iio-trig-sysfs should not be a platform driver
> This one got discussed before merging it and I agree.  We let it
> go then because there was a clear use case, a working driver and
> no one had the time to do a better job. Michael eventually talked Greg
> round to letting it in.
> 
> http://marc.info/?t=129665398600002&r=1&w=2

Ok. I still think that it is a really bad idea because the driver
is essentially a software feature that could be use on any machine,
but using a platform device ties it directly to a specific machine
that registers the device. It would make much more sense to
remove all the driver registration from there and just create the
iio device at module load time, or to have some interface that
can be used to dynamically create these triggers if the module
is loaded.

	Arnd
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