Re: [RFC PATCH 0/8] iio: dac: introducing ad3552r-axi

[Nuno Sá]

On Tue, 2024-09-03 at 20:39 +0100, Jonathan Cameron wrote:
> On Tue, 3 Sep 2024 11:17:24 -0500
> David Lechner <dlechner@xxxxxxxxxxxx> wrote:
> 
> > On 9/3/24 3:34 AM, Angelo Dureghello wrote:
> > > Hi Jonathan and all,
> > > 
> > > 
> > > On 31/08/24 1:38 PM, Jonathan Cameron wrote:  
> > > > On Thu, 29 Aug 2024 14:31:58 +0200
> > > > Angelo Dureghello <adureghello@xxxxxxxxxxxx> wrote:
> > > >  
> > > > > Hi, asking for comments for this patchset, that is mostly
> > > > > ready, at least feature-complete and functionally tested.
> > > > > 
> > > > > I am introducing ad3552r-axi variant, controlled from a fpga-based
> > > > > AXI IP, as a platform driver, using the DAC backend. The patchset is
> > > > > actually based on linux-iio, since some needed DAC backend features
> > > > > was already there on that repo only, still to be merged in mainline.
> > > > > 
> > > > > Comments i would like to ask are:
> > > > > 
> > > > > - i added some devicetree bindings inside current ad3552r yaml,
> > > > >    device is the same, so i wouldn't create a different yaml file.  
> > > > Agreed. If same device, it's usually better to keep it in one file.
> > > >  
> > > > > - if it's ok adding the bus-type property in the DAC backend:
> > > > >    actually, this platform driver uses a 4 lanes parallel bus, plus
> > > > >    a clock line, similar to a qspi. This to read an write registers
> > > > >    and as well to send samples at double data rate. Other DAC may
> > > > >    need "parallel" or "lvds" in the future.  
> > > > If it is for register read + write as well, sounds to me like you need
> > > > to treat this as a new bus type, possibly then combined with a
> > > > backend, or something similar to spi offload?
> > > > 
> > > > What bus does this currently sit on in your DT bindings?
> > > > (add an example)  
> > > 
> > > 
> > > &amba {
> > > 
> > >     ref_clk: clk@44B00000 {
> > >         compatible = "adi,axi-clkgen-2.00.a";
> > >         reg = <0x44B00000 0x10000>;
> > >         #clock-cells = <0>;
> > >         clocks = <&clkc 15>, <&clkc 15>;
> > >         clock-names = "s_axi_aclk", "clkin1";
> > >         clock-output-names = "ref_clk";
> > >     };
> > > 
> > >     dac_tx_dma: dma-controller@0x44a30000 {
> > >         compatible = "adi,axi-dmac-1.00.a";
> > >         reg = <0x44a30000 0x10000>;
> > >         #dma-cells = <1>;
> > >         interrupt-parent = <&intc>;
> > >         interrupts = <0 57 IRQ_TYPE_LEVEL_HIGH>;
> > >         clocks = <&clkc 15>;
> > > 
> > >         adi,channels {
> > >             #size-cells = <0>;
> > >             #address-cells = <1>;
> > > 
> > >             dma-channel@0 {
> > >                 reg = <0>;
> > >                 adi,source-bus-width = <32>;
> > >                 adi,source-bus-type = <0>;
> > >                 adi,destination-bus-width = <32>;
> > >                 adi,destination-bus-type = <1>;
> > >             };
> > >         };
> > >     };
> > > 
> > >     backend: controller@44a70000 {
> > >         compatible = "adi,axi-dac-9.1.b";
> > >         reg = <0x44a70000 0x1000>;
> > >         dmas = <&dac_tx_dma 0>;
> > >         dma-names = "tx";
> > >         #io-backend-cells = <0>;
> > >         clocks = <&ref_clk>;
> > >         bus-type = <1>;  /* IIO QSPI */
> > >     };
> > > 
> > >     axi-ad3552r {
> > >         compatible = "adi,ad3552r";
> > >         reset-gpios = <&gpio0 92 GPIO_ACTIVE_LOW>;
> > >         io-backends = <&backend>;
> > >         #address-cells = <1>;
> > >         #size-cells = <0>;
> > >         channel@0 {
> > >             reg = <0>;
> > >             adi,output-range-microvolt = <(-10000000) (10000000)>;
> > >         };
> > >     };  
> > 
> > Shouldn't the axi-ad3552r node be one level higher since it isn't
> > a memory-mapped device, but rather an external chip?
> Definitely not where it currently is..
> > 
> > But based on the other feedback we got in this series and some
> > #devicetree IRC chat here is an alternate binding suggestion we
> > could consider.
> > 
> > First, even though the FPGA IP block for use with AD3225R uses
> > the same register map as the AXI DAC IP block, some of the
> > registers behave differently, so it makes sense to have a
> > different compatible string rather than using the bus-type
> > property to tell the difference between the two IP blocks.
> > There are likely more differences than just the bus type.
> 
> I'd be amazed if they managed to keep things that similar
> given totally different buses.
> 

Yeah, I was trying to avoid new compatibles as much as I can because things can
get pretty confusing (with lots of new compatibles and quirks) pretty quickly.
Typically yes, most designs have slight differences between them (with new
features and so on) but so far I was trying (thinking) to have those as a
generic new backend op (plus a matching binding property if needed). For this
particular case, I'm fairly sure we could get away with the bus controller
property and having different implementations depending on the bus being
implemented. For the other bits that might differ between designs (eg: DDR
support) is up to frontends to call it or not (depending on they having that
feature or not). Naturally we need that the IPs having DDR support to not have
the same thing supported in different registers but we do control that since
these are FPGA cores.

All the above said, I'm fine with new compatibles but we need to draw a line
when we add new ones. If the reasoning is the IP has some new bits or new
registers, then things can get very confusing (even more if we think about
fallback compatibles) as most of the new designs have some quirks (even if
minimal). So I would say to add new compatibles when things get different enough
that a sane/generic API is not doable.

> > 
> > Second, technically, the AXI DAC IP block can't be used as
> > a generic SPI controller, so it wouldn't make sense to put
> > it in drivers/spi.
> 
> I wonder if there is any precedence of restricted controllers
> for SPI?  (For i2c we have the smbus ones as a vaguely similar
> example). +CC Mark.
> 
> >  But, from wiring point of view, it could
> > still make sense to use SPI DT bindings since we have SPI
> > wiring. At the same time, the AXI DAC IP block is also
> > providing extra functionality in addition to the SPI bus
> > so it makes sense to keep the io-backend bindings for those
> > extra bits.
> > 
> >     backend: spi@44a70000 {
> >         compatible = "adi,axi-dac-ad3225r";
> >         reg = <0x44a70000 0x1000>;
> >         dmas = <&dac_tx_dma 0>;
> >         dma-names = "tx";
> >         #io-backend-cells = <0>;
> >         clocks = <&ref_clk>;
> > 
> >         #address-cells = <1>;
> >         #size-cells = <0>;
> > 
> >         dac@0 {
> >             compatible = "adi,ad3552r";
> >             reg = <0>;
> > 
> >             /* 
> >              * Not sure how right this is - attempting to say that
> >              * the QSPI select pin is hardwired high, so the 4 SPI I/O
> >              * pins on the DAC are always functioning as SDIO0/1/2/3
> >              * as opposed to the usual 2 SDI/SDO pins and 2 unused.
> >              */
> >             spi-3-wire;
> >             spi-tx-bus-width = <4>;
> >             spi-rx-bus-width = <4>;
> > 
> >             reset-gpios = <&gpio0 92 GPIO_ACTIVE_LOW>;
> >             io-backends = <&backend>;
> > 
> >             #address-cells = <1>;
> >             #size-cells = <0>;
> > 
> >             channel@0 {
> >                 reg = <0>;
> >                 adi,output-range-microvolt = <(-10000000) (10000000)>;
> >             };
> >         };
> >     };
> 
> That's definitely an improvement.  It's a little strange to have
> a reference back to the parent but I'm fine with that.
> 

Agreed...

- Nuno Sá