Message-ID: <60e913e81ce67192ed0449fe7b718434fd360d97.camel@gmail.com>
Date:   Thu, 28 Sep 2023 09:15:49 +0200
From:   Nuno Sá <noname.nuno@...il.com>
To:     Olivier MOYSAN <olivier.moysan@...s.st.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>
Cc:     linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
        Fabrice GASNIER <fabrice.gasnier@...com>
Subject: Re: [RFC v2 01/11] iio: introduce iio backend device

Hi Olivier,

On Tue, 2023-09-26 at 18:44 +0200, Olivier MOYSAN wrote:
> Hi Nuno,
> 
> On 9/25/23 08:48, Nuno Sá wrote:
> > Hi Olivier,
> > 
> > On Fri, 2023-09-22 at 10:53 +0200, Nuno Sá wrote:
> > > 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@...s.st.com>
> > > > > > > > > > ---
> > > > > > > > > >      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)
> > > 
> > > 
> > 
> > Here it goes the 2 nodes of interest in my testing...
> > 
> > adc_ad9467: ad9467@0 {
> >          compatible = "adi,ad9467";
> >          reg = <0>;
> > 
> >          dmas = <&rx_dma 0>;
> >          dma-names = "rx";
> > 
> >          spi-max-frequency = <10000000>;
> >          adi,spi-3wire-enable;
> > 
> >          clocks = <&clk_ad9517 3>;
> >          clock-names = "adc-clk";
> > 
> >          converters = <&cf_ad9467_core_0>;
> > };
> > 
> > cf_ad9467_core_0: cf-ad9467-core-lpc@...00000 {
> >          compatible = "adi,axi-adc-10.0.a";
> >          reg = <0x44A00000 0x10000>;
> > 
> >          clocks = <&clkc 16>;
> > };
> > 
> > Naturally, converter-names only makes sense when you have more than one backend.
> > But
> > see that in 'cf_ad9467_core_0', you are free to place a regulator (as I have a
> > clock)
> > as long as you handle it in the backend driver.
> > 
> > - Nuno Sá
> 
> Thanks for the example. This helped me prototyping a dfsdm driver based 
> on the converter framework. Regarding device tree and driver update this 
> looks fine. I could integrate the API smartly in my frontend (dfsdm) and 
> backend (sd modulator).
> 
> My prototype executes up to probe. I have noticed however that init 
> (backend & frontend) ops are not called in my implementation. I can see 
> that init ops are called from bind ops. component_bind_all() calls 

Note that you need to call converter_frontend_add() from your frontend device (stm32-
dfsdm-adc) probe function and converter_add() from your backend's probes. And
ideally, this is the only thing you do at probe. Then, once all the elements are
probed, the complete aggregate device is initialised and the .bind()/.init() function
should be called.

And I want to reinforce the above, in the component API, things will only come up
when all the pieces (all the converters you specified in DT) are probed. The same is
true if one of the elements is unbound from it's driver - all the other elements in
the aggregate device will be torn down and converter_frontend_unbind() will be
called. This means it's an all or nothing solution... Let me know if this does not
work for you.

> converter bind ops, but component_bind_all() is called from converter 
> bind ops. So, I don't understand how initialization can proceed with 
> these circular calls. Maybe I missed something here.
> 

This one I'm not following... component_bind_all() should be called from
converter_frontend_bind() and this will call all converter_bind() you have (depends
on how many backends you have). After all backends are initialized, .frontend_init()
is called. In there, if you need (most likely you do) an handle to a converter you
then need to call converter_get(). So, component_bind_all() should not be called from
converter bind ops but from frontend_component_ops which are the
component_master_ops. If this is not happening, then we have an issue :)

> The change in the DT has an impact (But moderated) on legacy. Breaking 
> the legacy was unavoidable anyway.
> 
> DFSDM legacy binding (with two channels)
>        dfsdm_pdm1: filter@1 {
>          compatible = "st,stm32-dfsdm-adc";
>          st,adc-channels = <2 3>;
>          st,adc-channel-types = "SPI_R", "SPI_R";
>          ...
>          io-channels = <&sd_adc2 &sd_adc3>;
>        };
> 
> DFSDM binding with converter fw
>      dfsdm_pdm1: filter@1 {
>          compatible = "st,stm32-dfsdm-adc";
>          st,adc-channels = <2 3>;
>          st,adc-channel-types = "SPI_R", "SPI_R";
>          ...
>          converters = <&sd_adc2 &sd_adc3>;
>        };
> 
> I have also the aim to change DFSDM bindings to use IIO generic channels 
> bindings (bindings/iio/adc/adc.yaml).
> 
> Ideally the DFSDM bindings should looks like this:
>        dfsdm_pdm1: filter@1 {
>          compatible = "st,stm32-dfsdm-adc";
>          channel@2 {
>                  reg = <2>;
>                  st,adc-channel-types = "SPI_R";
>                  ...
>                  converters = <&sd_adc2>;
>          };
>          channel@3 {
>                  reg = <3>;
>                  st,adc-channel-types = "SPI_R";
>                  ...
>                  converters = <&sd_adc3>;
>          };
>        };
> 
> But it seems that current framework converter API cannot support this 
> topology.
> 

Indeed this won't work and I honestly it never crossed my mind ehehe,

> As a fallback solution the following binding may be adopted
>        dfsdm_pdm1: filter@1 {
>          compatible = "st,stm32-dfsdm-adc";
>          channel@2 {
>                  reg = <2>;
>                  st,adc-channel-types = "SPI_R";
>                  ...
>          };
>          channel@3 {
>                  reg = <3>;
>                  st,adc-channel-types = "SPI_R";
>                  ...
>          };
>          converters = <&sd_adc2 &sd_adc3>;
> 
> In this case the frontend driver needs a mean to map backend and 
> channels. It's not the smartest solution, yet. Especially since the use 
> of generic channel is quite common.

Yeah, I'm also not a fan of that... To support the above topology and from the top of
my head we could either:

1) Somehow split converter_frontend_add() to give more control to the caller to call
converter_frontend_add_matches() and in this case have another API that accepts a
fwnode.

2) Just extend converter_frontend_add_matches() so that we also look into child nodes
for 'converters'

Then, on the get() side, we would need something like converters_get_from_fwnode() to
get each handle. I would likely prefer to go with 2) because 1) already implies some
FW parsing during probe that I would like to avoid.

Anyways the above is already showing that maybe going with the component API for
something more generic might be a stretch and harder to scale for everyone needs.
With an OF lookup, the above topology would be easier to accomplish (though we would
always need a converters_get_from_fwnode() kind of function).

When you say: 

"In this case the frontend driver needs a mean to map backend and channels."

Could 'converter-names' be used for the above? Or would the above be trivial with an
OF lookup?

> Perhaps the converter_frontend_add() API needs to be extended to support 
> generic channel configuration. Maybe the IIO core should provide the 
> related helpers as well. (As far as I know this does not exists).
> So, still opened questions ..
> 

No sure what do you mean by the above?

> That said, I feel confident that the converter framework is a good 
> option for the DFSDM use case.
> 

Yeah, I'm also confident that we can get something that suits both our usecases
either with OF or component. I must say that I'm tempted to send a version of this
with an OF lookup just so we get a look on how it would look like and compare against
the component API.

- Nuno Sá
> > > 

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