Re: [RFC v2 01/11] iio: introduce iio backend device

[Nuno Sá]

Hi Olivier,

Sorry for the delay...

On Mon, 2023-09-18 at 17:52 +0200, Olivier MOYSAN wrote:
> Hi Nuno
> 
> On 9/11/23 11:39, Nuno Sá wrote:
> > On Tue, 2023-09-05 at 12:06 +0200, Olivier MOYSAN wrote:
> > > Hi Nuno,
> > > 
> > > On 9/1/23 10:01, Nuno Sá wrote:
> > > > Hi Olivier,
> > > > 
> > > > On Thu, 2023-08-31 at 18:14 +0200, Olivier MOYSAN wrote:
> > > > > Hi Nuno,
> > > > > 
> > > > > On 7/28/23 10:42, Nuno Sá wrote:
> > > > > > Hi Olivier,
> > > > > > 
> > > > > > On Thu, 2023-07-27 at 17:03 +0200, Olivier Moysan wrote:
> > > > > > > Add a new device type in IIO framework.
> > > > > > > This backend device does not compute channel attributes and does
> > > > > > > not
> > > > > > > expose
> > > > > > > them through sysfs, as done typically in iio-rescale frontend
> > > > > > > device.
> > > > > > > Instead, it allows to report information applying to channel
> > > > > > > attributes through callbacks. These backend devices can be
> > > > > > > cascaded
> > > > > > > to represent chained components.
> > > > > > > An IIO device configured as a consumer of a backend device can
> > > > > > > compute
> > > > > > > the channel attributes of the whole chain.
> > > > > > > 
> > > > > > > Signed-off-by: Olivier Moysan <olivier.moysan@xxxxxxxxxxx>
> > > > > > > ---
> > > > > > >     drivers/iio/Makefile               |   1 +
> > > > > > >     drivers/iio/industrialio-backend.c | 107
> > > > > > > +++++++++++++++++++++++++++++
> > > > > > >     include/linux/iio/backend.h        |  56 +++++++++++++++
> > > > > > >     3 files changed, 164 insertions(+)
> > > > > > >     create mode 100644 drivers/iio/industrialio-backend.c
> > > > > > >     create mode 100644 include/linux/iio/backend.h
> > > > > > > 
> > > > > > > diff --git a/drivers/iio/Makefile b/drivers/iio/Makefile
> > > > > > > index 9622347a1c1b..9b59c6ab1738 100644
> > > > > > > --- a/drivers/iio/Makefile
> > > > > > > +++ b/drivers/iio/Makefile
> > > > > > > @@ -5,6 +5,7 @@
> > > > > > >     
> > > > > > >     obj-$(CONFIG_IIO) += industrialio.o
> > > > > > >     industrialio-y := industrialio-core.o industrialio-event.o
> > > > > > > inkern.o
> > > > > > > +industrialio-$(CONFIG_IIO_BACKEND) += industrialio-backend.o
> > > > > > >     industrialio-$(CONFIG_IIO_BUFFER) += industrialio-buffer.o
> > > > > > >     industrialio-$(CONFIG_IIO_TRIGGER) += industrialio-trigger.o
> > > > > > >     
> > > > > > > diff --git a/drivers/iio/industrialio-backend.c
> > > > > > > b/drivers/iio/industrialio-
> > > > > > > backend.c
> > > > > > > new file mode 100644
> > > > > > > index 000000000000..7d0625889873
> > > > > > > --- /dev/null
> > > > > > > +++ b/drivers/iio/industrialio-backend.c
> > > > > > > @@ -0,0 +1,107 @@
> > > > > > > +// SPDX-License-Identifier: GPL-2.0
> > > > > > > +/* The industrial I/O core, backend handling functions
> > > > > > > + *
> > > > > > > + */
> > > > > > > +
> > > > > > > +#include <linux/kernel.h>
> > > > > > > +#include <linux/device.h>
> > > > > > > +#include <linux/property.h>
> > > > > > > +#include <linux/iio/iio.h>
> > > > > > > +#include <linux/iio/backend.h>
> > > > > > > +
> > > > > > > +static DEFINE_IDA(iio_backend_ida);
> > > > > > > +
> > > > > > > +#define to_iio_backend(_device) container_of((_device), struct
> > > > > > > iio_backend,
> > > > > > > dev)
> > > > > > > +
> > > > > > > +static void iio_backend_release(struct device *device)
> > > > > > > +{
> > > > > > > +       struct iio_backend *backend = to_iio_backend(device);
> > > > > > > +
> > > > > > > +       kfree(backend->name);
> > > > > > > +       kfree(backend);
> > > > > > > +}
> > > > > > > +
> > > > > > > +static const struct device_type iio_backend_type = {
> > > > > > > +       .release = iio_backend_release,
> > > > > > > +       .name = "iio_backend_device",
> > > > > > > +};
> > > > > > > +
> > > > > > > +struct iio_backend *iio_backend_alloc(struct device *parent)
> > > > > > > +{
> > > > > > > +       struct iio_backend *backend;
> > > > > > > +
> > > > > > > +       backend = devm_kzalloc(parent, sizeof(*backend),
> > > > > > > GFP_KERNEL);
> > > > > > > 
> > > > > > 
> > > > > > No error checking.
> > > > > > 
> > > > > > I guess a lot of cleanings are still missing but the important thing
> > > > > > I
> > > > > > wanted to
> > > > > > notice is that the above pattern is not ok.
> > > > > > Your 'struct iio_backend *backend'' embeds a 'stuct device' which is
> > > > > > a
> > > > > > refcounted object. Nevertheless, you're binding the lifetime of your
> > > > > > object to
> > > > > > the parent device and that is wrong. The reason is that as soon as
> > > > > > your
> > > > > > parent
> > > > > > device get's released or just unbinded from it's driver, all the
> > > > > > devres
> > > > > > stuff
> > > > > > (including your 'struct iio_backend' object) will be released
> > > > > > independentof
> > > > > > your 'struct device' refcount value...
> > > > > > 
> > > > > > So, you might argue this won't ever be an issue in here but the
> > > > > > pattern
> > > > > > is still
> > > > > > wrong. There are some talks about this, the last one was given at
> > > > > > the
> > > > > > latest
> > > > > > EOSS:
> > > > > > 
> > > > > > https://www.youtube.com/watch?v=HCiJL7djGw8&list=PLbzoR-pLrL6pY8a8zSKRC6-AihFrruOkq&index=27&ab_channel=TheLinuxFoundation
> > > > > > 
> > > > > 
> > > > > This is a good point. Thanks for pointing it out. Sure, there are
> > > > > still
> > > > > many things to improve.
> > > > > 
> > > > > I have seen the comment from Jonathan on your "Add converter
> > > > > framework"
> > > > > serie. I had a quick look at the serie. It seems that we share the
> > > > > need
> > > > > to aggregate some IIO devices. But I need to read it more carefully to
> > > > > check if we can find some convergences here.
> > > > 
> > > > Yeah, In my case, the backend devices are typically FPGA soft cores and
> > > > the
> > > > aggregate
> > > > device might connect to multiple of these backends. That was one of the
> > > > reason why I
> > > > used the component API where the aggregate device is only configured
> > > > when
> > > > all the
> > > > devices are probed. Similarly, when one of them is unbind, the whole
> > > > thing
> > > > should be
> > > > torn down. Also, in my case, the frontend device needs to do a lot of
> > > > setup
> > > > on the
> > > > backend device so the whole thing works (so I do have/need a lot more
> > > > .ops).
> > > > 
> > > > Anyways, it does not matter much what the backend device is and from a
> > > > first
> > > > glance
> > > > and looking at the .ops you have, it seems that this could easily be
> > > > supported in the
> > > > framework I'm adding. The only things I'm seeing are:
> > > 
> > > Thanks for your feedback. Yes, my feeling is that the API I need for the
> > > dfsdm use case, can be covered by the API you propose. I'm not familiar
> > > with component API however, as I discovered it in your serie. It is not
> > > clear for me how this affects device tree description of the hardware.
> > 
> > Your aggregate device (that we can think of as a frontend device needs to
> > properly reference all the backends it needs - in your case I guess it's
> > just
> > one device). The dts properties I have for now are 'converters' and
> > 'converter-
> > names'. But one thing that starts to become clear to me is that I should
> > probably change the name for the framework. Maybe industrialio-aggregate.c
> > if we
> > keep the component API (and so the same frontend + backend naming) or just
> > industrialio-backend.c (as you have now) if we go with a typical OF lookup.
> > 
> 
> In my case I have a digital filter peripheral (frontend) linked to 
> several sigma delta converters (backends). So, here 'converters' 
> property may be relevant as well. But I agree that a more generic name 
> seems better for the long term.
> 
> My backend devices need to get a regulator phandle from the device tree.
> It seems that the component API does not offer services allowing to 
> retrieve DT properties for the sub-devices. Tell me if I'm wrong, but I 
> think this constraint require to change converter framework to a typical 
> OF lookup.
> 
> Could you please share the structure of your DT for your ad9476 based 
> example ? This will help me identify the gaps regarding my need.
> 

I might be missing something but there should be no limitation in the component
stuff for this. Note your frontend/backend devices are just normal device tree
nodes (meaning that they can have all the properties they want as a normal node)
and then in the correspondent drivers you handle all the properties. For now,
the only FW properties supported in the framework I sent are 'converters' and
'converter-name' which will be used to "create" the aggregate device. This
pretty much means that the complete thing should only come up when all the
devices you set in DT probe.

Of course we can move more properties into the framework if we start to see some
generic ones that are almost always present...

One thing that Jonathan already mentioned is that the component API works in a
away that you can have either 1->1 or 1->N (frontends->backends). So, if you
have setups where you have more than one frontend (basically M->N) we need to
make sure it still works. In theory (in the component API), I think you can have
one backend associated with more than one frontend so we should be able to still
get the M->N topology. Of course the "communications link" is always between
frontend -> backend.

I'll see if I send the devicetree over the weekend (don't have it in my current
machine)

- Nuno Sá

> > 
>