Date:   Mon, 11 May 2020 14:56:31 +0000
From:   "Ardelean, Alexandru" <alexandru.Ardelean@...log.com>
To:     "jic23@...nel.org" <jic23@...nel.org>,
        "lars@...afoo.de" <lars@...afoo.de>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>
Subject: Re: [RFC PATCH 00/14] iio: buffer: add support for multiple buffers

On Mon, 2020-05-11 at 15:58 +0200, Lars-Peter Clausen wrote:
> [External]
> 
> On 5/11/20 3:24 PM, Ardelean, Alexandru wrote:
> > On Mon, 2020-05-11 at 13:03 +0000, Ardelean, Alexandru wrote:
> > > [External]
> > > 
> > > On Mon, 2020-05-11 at 12:37 +0200, Lars-Peter Clausen wrote:
> > > > [External]
> > > > 
> > > > On 5/11/20 12:33 PM, Ardelean, Alexandru wrote:
> > > > > On Sun, 2020-05-10 at 11:09 +0100, Jonathan Cameron wrote:
> > > > > > [External]
> > > > > > 
> > > > > > On Sat, 9 May 2020 10:52:14 +0200
> > > > > > Lars-Peter Clausen <lars@...afoo.de> wrote:
> > > > > > 
> > > > > > > On 5/8/20 3:53 PM, Alexandru Ardelean wrote:
> > > > > > > > [...]
> > > > > > > > What I don't like, is that iio:device3 has iio:buffer3:0 (to 3).
> > > > > > > > This is because the 'buffer->dev.parent = &indio_dev->dev'.
> > > > > > > > But I do feel this is correct.
> > > > > > > > So, now I don't know whether to leave it like that or symlink to
> > > > > > > > shorter
> > > > > > > > versions like 'iio:buffer3:Y' -> 'iio:device3/bufferY'.
> > > > > > > > The reason for naming the IIO buffer devices to 'iio:bufferX:Y'
> > > > > > > > is
> > > > > > > > mostly to make the names unique. It would have looked weird to
> > > > > > > > do
> > > > > > > > '/dev/buffer1' if I would have named the buffer devices
> > > > > > > > 'bufferX'.
> > > > > > > > 
> > > > > > > > So, now I'm thinking of whether all this is acceptable.
> > > > > > > > Or what is acceptable?
> > > > > > > > Should I symlink 'iio:device3/iio:buffer3:0' ->
> > > > > > > > 'iio:device3/buffer0'?
> > > > > > > > What else should I consider moving forward?
> > > > > > > > What means forward?
> > > > > > > > Where did I leave my beer?
> > > > > > > Looking at how the /dev/ devices are named I think we can provide
> > > > > > > a
> > > > > > > name
> > > > > > > that is different from the dev_name() of the device. Have a look
> > > > > > > at
> > > > > > > device_get_devnode() in drivers/base/core.c. We should be able to
> > > > > > > provide the name for the chardev through the devnode() callback.
> > > > > > > 
> > > > > > > While we are at this, do we want to move the new devices into an
> > > > > > > iio
> > > > > > > subfolder? So iio/buffer0:0 instead of iio:buffer0:0?
> > > > > > Possibly on the folder.  I can't for the life of me remember why I
> > > > > > decided
> > > > > > not to do that the first time around - I'll leave it at the
> > > > > > mysterious "it may turn out to be harder than you'd think..."
> > > > > > Hopefully not ;)
> > > > > I was also thinking about the /dev/iio subfolder while doing this.
> > > > > I can copy that from /dev/input
> > > > > They seem to do it already.
> > > > > I don't know how difficult it would be. But it looks like a good
> > > > > precedent.
> > > > All you have to do is return "iio/..." from the devnode() callback.
> > > I admit I did not look closely into drivers/input/input.c before
> > > mentioning
> > > this
> > > as as good precedent.
> > > 
> > > But, I looks like /dev/inpput is a class.
> > > While IIO devices are a bus_type devices.
> > > Should we start implementing an IIO class? or?
> > What I should have highlighted [before] with this, is that there is no
> > devnode()
> > callback for the bus_type [type].
> But there is one in device_type :)

Many thanks :)
That worked nicely.

I now have:

root@...log:~# ls /dev/iio/*
/dev/iio/iio:device0  /dev/iio/iio:device1

/dev/iio/device3:
buffer0  buffer1  buffer2  buffer3

/dev/iio/device4:
buffer0


It looks like I can shift these around as needed.
This is just an experiment.
I managed to move the iio devices under /dev/iio, though probably the IIO
devices will still be around as /dev/iio:deviceX for legacy reasons.

Two things remain unresolved.
1. The name of the IIO buffer device.

root@...log:/sys/bus/iio/devices# ls iio\:device3/
buffer          in_voltage0_test_mode           name
events          in_voltage1_test_mode           of_node
iio:buffer:3:0  in_voltage_sampling_frequency   power
iio:buffer:3:1  in_voltage_scale                scan_elements
iio:buffer:3:2  in_voltage_scale_available      subsystem
iio:buffer:3:3  in_voltage_test_mode_available  uevent


Right now, each buffer device is named 'iio:buffer:X:Y'.
One suggesttion was  'iio:deviceX:bufferY'
I'm suspecting the latter is preferred as when you sort the folders, buffers
come right after the iio:deviceX folders in /sys/bus/iio/devices.

I don't feel it matters much the device name of the IIO buffer if we symlink it
to a shorter form.
 
I'm guessing, we symlink these devices to short-hand 'bufferY' folders in each
'iio:deviceX'?

So, you'd get something like:

drwxr-xr-x 8 root root    0 May 11 14:40 .
drwxr-xr-x 4 root root    0 May 11 14:35 ..
lrwxrwxrwx 1 root root    0 May 11 14:35 buffer -> iio:buffer:3:0
lrwxrwxrwx 4 root root    0 May 11 14:35 buffer0 -> iio:buffer:3:0
lrwxrwxrwx 4 root root    0 May 11 14:35 buffer1 -> iio:buffer:3:1
lrwxrwxrwx 4 root root    0 May 11 14:35 buffer2 -> iio:buffer:3:2
lrwxrwxrwx 4 root root    0 May 11 14:35 buffer3 -> iio:buffer:3:3
drwxrwxrwx 2 root root    0 May 11 14:35 events
drwxrwxrwx 4 root root    0 May 11 14:35 iio:buffer:3:0
drwxrwxrwx 4 root root    0 May 11 14:35 iio:buffer:3:1
drwxrwxrwx 4 root root    0 May 11 14:35 iio:buffer:3:2
drwxrwxrwx 4 root root    0 May 11 14:35 iio:buffer:3:3
-rw-rw-rw- 1 root root 4096 May 11 14:35 in_voltage0_test_mode
-rw-rw-rw- 1 root root 4096 May 11 14:35 in_voltage1_test_mode
-rw-rw-rw- 1 root root 4096 May 11 14:35 in_voltage_sampling_frequency
-rw-rw-rw- 1 root root 4096 May 11 14:35 in_voltage_scale
-rw-rw-rw- 1 root root 4096 May 11 14:35 in_voltage_scale_available
-rw-rw-rw- 1 root root 4096 May 11 14:35 in_voltage_test_mode_available
-rw-rw-rw- 1 root root 4096 May 11 14:35 name
lrwxrwxrwx 1 root root    0 May 11 14:35 of_node -> 
../../../../../firmware/devicetree/base/fpga-axi@...xi-ad9680-hpc@...10000
drwxrwxrwx 2 root root    0 May 11 14:35 power
lrwxrwxrwx 1 root root    0 May 11 14:35 scan_elements ->
iio:buffer:3:0/scan_elements
lrwxrwxrwx 1 root root    0 May 11 14:35 subsystem -> ../../../../../bus/iio
-rw-rw-rw- 1 root root 4096 May 11 14:35 uevent

1a. /sys/bus/iio/devices looks like this:
iio:buffer:3:0  (this would become iio:device3:buffer0 )
iio:buffer:3:1
iio:buffer
:3:2
iio:buffer:3:3
iio:buffer:4:0
iio:device0
iio:device1
iio:device2
iio:device3
iio:
device4

One minor issue here is that the buffers get listed in the /sys/bus/iio/devices
folder, because I'm adding them to the iio bus, to be able to get a chardev
[from the pre-allocated chardev region of IIO].

libiio gets a little confused, as it sees these buffers are IIO buffer capable
devices;

	iio:buffer:3:0: (buffer capable)
		2 channels found:
			voltage0:  (input, index: 0, format: le:S14/16>>0)
			voltage1:  (input, index: 1, format: le:S14/16>>0)
		5 device-specific attributes found:
				attr  0: data_available value: 0
				attr  1: enable value: 0
				attr  2: length value: 4096
				attr  3: length_align_bytes value: 8
				attr  4: watermark value: 2048
	iio:buffer:3:1: (buffer capable)

Hopefully, this is not a big problem, but let's see.

2. I know this is [still] stupid now; but any suggestions one how to symlink 
/dev/iio:device3 -> /dev/iio/device3/buffer0 ?

Regarding this one, I may try a few things, but any suggestion is welcome.



Thanks
Alex

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