Message-ID: <642d61b23c58d9b846e42badb2f2d97691c92144.camel@gmail.com>
Date: Thu, 05 Sep 2024 11:50:45 +0200
From: Nuno Sá <noname.nuno@...il.com>
To: Conor Dooley <conor@...nel.org>, Angelo Dureghello
	 <adureghello@...libre.com>
Cc: Lars-Peter Clausen <lars@...afoo.de>, Michael Hennerich
 <Michael.Hennerich@...log.com>, Nuno Sá
 <nuno.sa@...log.com>,  Jonathan Cameron <jic23@...nel.org>, Rob Herring
 <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Olivier Moysan <olivier.moysan@...s.st.com>,
 linux-iio@...r.kernel.org,  devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, dlechner@...libre.com
Subject: Re: [PATCH RFC 4/8] dt-bindings: iio: dac: add adi axi-dac bus
 property

On Fri, 2024-08-30 at 16:33 +0100, Conor Dooley wrote:
> On Fri, Aug 30, 2024 at 10:19:49AM +0200, Angelo Dureghello wrote:
> > Hi Conor,
> > 
> > On 29/08/24 5:46 PM, Conor Dooley wrote:
> > > On Thu, Aug 29, 2024 at 02:32:02PM +0200, Angelo Dureghello wrote:
> > > > From: Angelo Dureghello <adureghello@...libre.com>
> > > > 
> > > > Add bus property.
> > > RFC it may be, but you do need to explain what this bus-type actually
> > > describes for commenting on the suitability of the method to be
> > > meaningful.
> > 
> > thanks for the feedbacks,
> > 
> > a "bus" is intended as a generic interface connected to the target,
> > may be used from a custom IP (fpga) to communicate with the target
> > device (by read/write(reg and value)) using a special custom interface.
> > 
> > The bus could also be physically the same of some well-known existing
> > interfaces (as parallel, lvds or other uncommon interfaces), but using
> > an uncommon/custom protocol over it.
> > 
> > In concrete, actually bus-type is added to the backend since the
> > ad3552r DAC chip can be connected (for maximum speed) by a 5 lanes DDR
> > parallel bus (interface that i named QSPI, but it's not exactly a QSPI
> > as a protocol), so it's a device-specific interface.
> > 
> > With additions in this patchset, other frontends, of course not only
> > DACs, will be able to add specific busses and read/wrtie to the bus
> > as needed.
> > 
> > > > Signed-off-by: Angelo Dureghello <adureghello@...libre.com>
> > > > ---
> > > >   Documentation/devicetree/bindings/iio/dac/adi,axi-dac.yaml | 9
> > > > +++++++++
> > > >   1 file changed, 9 insertions(+)
> > > > 
> > > > diff --git a/Documentation/devicetree/bindings/iio/dac/adi,axi-dac.yaml
> > > > b/Documentation/devicetree/bindings/iio/dac/adi,axi-dac.yaml
> > > > index a55e9bfc66d7..a7ce72e1cd81 100644
> > > > --- a/Documentation/devicetree/bindings/iio/dac/adi,axi-dac.yaml
> > > > +++ b/Documentation/devicetree/bindings/iio/dac/adi,axi-dac.yaml
> > > > @@ -38,6 +38,15 @@ properties:
> > > >     clocks:
> > > >       maxItems: 1
> > > You mentioned about new compatible strings, does the one currently
> > > listed in this binding support both bus types?
> 
> You didn't answer this, and there's insufficient explanation of the
> "hardware" in this RFC, but I found this which is supposedly the
> backend:
> https://github.com/analogdevicesinc/hdl/tree/main/library/axi_ad3552r
> adi,axi-dac.yaml has a single compatible, and that compatible has
> nothing to do with "axi_ad3552r" as it is "adi,axi-dac-9.1.b". I would
> expect either justification for reuse of the compatible, or a brand new
> compatible for this backend, even if the driver can mostly be reused.
> 

Hi Conor,

So most of these designs have some changes (even if minimal) in the register map
and the idea (mine actually) with this backend stuff was to keep the backend
driver (axi-dac/adc) with the generic compatible since all the (different)
functionality is basically defined by the frontend they connect too and that
functionality is modeled by IIO backend ops. For some more
significant/fundamental differences in the IP like this bus controller kind of
thing, we would add have proper FW properties. The main idea was kind of using
the frontend + generic backend combo so no need for new compatibles for every
new design.

It's still early days (at least upstream) for these IP cores and the backend
code so if you say that we should have new compatibles for every new design that
has some differences in the register map (even if minimal), I'm of course fine
with it. I've done it like this because I was (am) kind of afraid for things to
get complicated fairly quickly both in the bindings and driver (well maybe not
in the driver). OTOH, it can simplify things a lot as it's way easier to
identify different implementations of the IP directly in the driver so we have
way more flexibility.

> Could you please link to whatever ADI wiki has detailed information on
> how this stuff works so that I can look at it to better understand the
> axes of configuration here?
> 
> > > 
> > > Making the bus type decision based on compatible only really makes sense
> > > if they're different versions of the IP, but not if they're different
> > > configuration options for a given version.
> > > 
> > > > +  bus-type:
> > 
> > DAC IP on fpga actually respects same structure and register set, except
> > for a named "custom" register that may use specific bitfields depending
> > on the application of the IP.
> 
> To paraphrase:
> "The register map is the same, except for the bit that is different".
> If ADI is shipping several different configurations of this IP for
> different DACs, I'd be expecting different compatibles for each backend
> to be honest.

Yes, pretty much we have a generic core with most of the designs being based on
it but with some slight differences. At least for the new ones, almost all of
them have slight deviations from the generic/base core.

> If each DAC specific backend was to have a unique compatible, would the
> type of bus used be determinable from it? Doesn't have to work for all
> devices from now until the heath death of the universe, but at least for
> the devices that you're currently aware of?
> 

My original idea was to have a bus controller boolean for this core at least for
now that we only have one bus type (so we could assume qspi in the driver). If
the time comes we need to add support for something else, then we would need
another property to identify the type.

> > > If, as you mentioned, there are multiple bus types, a non-flag property
> > > does make sense. However, I am really not keen on these "forced" numerical
> > > properties at all, I'd much rather see strings used here.
> 
> > > > +    maxItems: 1
> > > > +    description: |
> > > > +      Configure bus type:
> > > > +        - 0: none
> > > > +        - 1: qspi
> 
> Also, re-reading the cover letter, it says "this platform driver uses a 4
> lanes parallel bus, plus a clock line, similar to a qspi."
> I don't think we should call this "qspi" if it is not actually qspi,
> that's just confusing.
> 

Just by looking at the datasheet it feels like typical qspi to be honest. And,
fwiw, even if not really qspi, this is how the datasheet names the interface.

- Nuno Sá
> > 

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