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Subject: Re: [RFC PATCH 00/14] iio: buffer: add support for multiple buffers
To:     "Ardelean, Alexandru" <alexandru.Ardelean@analog.com>,
        "jic23@kernel.org" <jic23@kernel.org>
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        "linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>
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From:   Lars-Peter Clausen <lars@metafoo.de>
Message-ID: <fb6cc3f4-b133-4ba4-a8fb-84349355ebc3@metafoo.de>
Date:   Mon, 11 May 2020 21:56:40 +0200
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On 5/11/20 4:56 PM, Ardelean, Alexandru wrote:
> 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@metafoo.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@analog:~# 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@analog:/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'?

I think that would be a bit excessive. Only for the legacy buffer we 
need to have a symlink.

> [...]
> 2. I know this is [still] stupid now; but any suggestions one how to symlink
> /dev/iio:device3 -> /dev/iio/device3/buffer0 ?
>
Does not seem to be possible. Userspace will have to take care of it. 
This means we need to keep legacy devices in /dev/ and only new buffers 
in /dev/iio/.