linux-kernel - Re: [RFC PATCH 00/14] iio: buffer: add support for multiple buffers

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Message-ID: <7d2c8174e345bf8e241d48de65066d2606143503.camel@analog.com>
Date:   Mon, 11 May 2020 13:24:47 +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 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].

> 
> 
> > > My concern regarding going to use stuff from core [like
> > > device_get_devnode()] is
> > > that it seems to bypass some layers of kernel.
> > > If I do 'git grep device_get_devnode', I get:
> > > 
> > > drivers/base/core.c:            name = device_get_devnode(dev, &mode,
> > > &uid,
> > > &gid, &tmp);
> > > drivers/base/core.c: * device_get_devnode - path of device node file
> > > drivers/base/core.c:const char *device_get_devnode(struct device *dev,
> > > drivers/base/devtmpfs.c:        req.name = device_get_devnode(dev,
> > > &req.mode,
> > > &req.uid, &req.gid, &tmp);
> > > drivers/base/devtmpfs.c:        req.name = device_get_devnode(dev, NULL,
> > > NULL,
> > > NULL, &tmp);
> > > include/linux/device.h:extern const char *device_get_devnode(struct device
> > > *dev,
> > > (END)
> > > 
> > > So, basically, most uses of device_get_devnode() are in core code, and I
> > > feel
> > > that this may be sanctioned somewhere by some core people, if I do it.
> > > I could be wrong, but if you disagree, I'll take your word for it.
> > You are not supposed to use the function itself, you should implement 
> > the devnode() callback for the IIO bus, which is then used by the core 
> > device_get_devnode() function.
> >