Message-ID: <20200516172445.7f0d96cb@archlinux>
Date:   Sat, 16 May 2020 17:24:45 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     "Ardelean, Alexandru" <alexandru.Ardelean@...log.com>
Cc:     "lars@...afoo.de" <lars@...afoo.de>,
        "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 Sat, 16 May 2020 13:08:46 +0000
"Ardelean, Alexandru" <alexandru.Ardelean@...log.com> wrote:

> On Tue, 2020-05-12 at 06:26 +0000, Ardelean, Alexandru wrote:
> > [External]
> > 
> > On Mon, 2020-05-11 at 21:56 +0200, Lars-Peter Clausen wrote:  
> > > [External]
> > > 
> > > 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@...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'?  
> > > 
> > > 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/.  
> > 
> > One thought about this, was that we keep the chardev for the IIO device for
> > this.
> > i.e.  /dev/iio:deviceX and /dev/iio/deviceX/buffer0 open the same buffer.
> > This means that for a device with 4 buffers, you get 5 chardevs.
> > This also seems a bit much/excessive. Maybe also in terms of source-code.
> > It would at least mean not moving the event-only chardev to 'industrialio-
> > event.c', OR move it, and have the same chardev in 3 places ['industrialio-
> > event.c', 'industrialio-buffer.c' & 'industrialio-buffer.c'
> > 
> > Maybe this sort-of makes sense to have for a few years/kernel-revisions until
> > things clean-up.
> > 
> > I guess at this point, the maintainer should have the final say about this.  
> 
> Another 'compromise' idea, is that we make this '/dev/iio/deviceX/bufferY' thing
> a feature for new devices, and leave '/dev/iio:deviceX' devices [for buffers] a
> thing for current devices.
> It would mean adding a 'new' iio_device_attach_buffer(); no idea on a name [for
> this yet].

Definitely a no to that.  If we make this transition it needs to be
automatic and subsystem wide.  At some point we could have a kconfig option
to disable the legacy interface subsystem wise as a precursor to eventually
dropping it.  

> 
> Over time, people can convert existing drivers to the new IIO-buffer format, if
> they want to. That also gives them a bit better control over symlinking
> '/dev/iio:deviceX' -> '/dev/iio/deviceX/bufferY' [or symlinking in reverse if
> they want to].
> 
> That may create confusion I guess during a transition period.
> And it would [ideally] have a mechanism [preferably at build/compile time] to
> notify users to use the new IIO buffer mechanism [vs the old one] when adding
> new drivers.
> Otherwise, there's the risk of people copying the old IIO buffer mechanism.
> This can be brought-up at review, but ¯\_(ツ)_/¯ ; it can be annoying.

If we can't do this in a transparent fashion we need to rethink.
The existing interface 'has' to remain and do something sensible. Realistically
we need to keep it in place for 3-5 years at least.

I'm not yet convinced the complexity is worthwhile.  We 'could' fallback to
the same trick used for events and use an ioctl to access all buffers
other than the first one...  Then we retain one chardev per iio device
and still get the flexibility we need to have multiple buffers.
In some ways it is tidier, even if a bit less intuitive...
If we can't build the symlinks we were all kind of assuming we could
we may need to rethink the overall path.

Anyhow, you are doing great work exploring the options!

Thanks,

Jonathan


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