Re: [PATCH RFC 4/8] dt-bindings: iio: dac: add adi axi-dac bus property

["Nuno Sá" <noname.nuno@gmail.com>]

On Fri, 2024-09-06 at 10:04 +0100, Conor Dooley wrote:
> On Mon, Sep 02, 2024 at 11:32:37AM +0200, Angelo Dureghello wrote:
> > Hi Conor,
> > 
> > 
> > On 30/08/24 5:33 PM, 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@baylibre.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@baylibre.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.
> > > 
> > > 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?
> > 
> > https://analogdevicesinc.github.io/hdl/library/axi_ad3552r/index.html
> > 
> > that has same structure and register set of the generic ADI AXI-DAC IP:
> > https://wiki.analog.com/resources/fpga/docs/axi_dac_ip
> > 
> > 
> > > > > 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
> > 
> > i am still quite new to this fpga-based implementations, at least for how
> > such IPs are actually interfacing to the linux subsystem, so i may miss
> > some point.
> > 
> > About the "adi,axi-dac-9.1.b" compatible, the generic DAC IP register set
> > is mostly the same structure of this ad3552r IP (links above), except for
> > bitfields in the DAC_CUSTOM_CTRL register.
> > 
> > My choice for now was to add a bus-type property.
> > 
> > Not an HDL expert, but i think a different bus means, from an hardware point
> > of
> > view, a different IP in terms of internal fpga circuitry, even if not as a
> > register-set.
> 
> Depending on whether or not the unmodified driver can be used with this
> IP (so the QSPI bus stuff would need to be optional) then a fallback
> should be used given the degree of similarity. It, however, seems likely
> that is not the case, and without the QSPI bus there'd be no way to
> communicate with the device. Is there any reason to use this IP as a
> backend, without connecting the QSPI bus at all, leaving the ADC/DAC on
> a regular SPI bus?
> 

Somewhere in my replies, I'm doing the exact same question to myself. We probably
need to speak with the FPGA folks but I guess (hope) they had a good reason for this.

- Nuno Sá