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

["Nuno Sá" <noname.nuno@gmail.com>]

On Sat, 2024-09-07 at 15:12 +0100, Jonathan Cameron wrote:
> On Thu, 05 Sep 2024 11:16:03 +0200
> Nuno Sá <noname.nuno@gmail.com> wrote:
> 
> > On Tue, 2024-09-03 at 20:39 +0100, Jonathan Cameron wrote:
> > > On Tue, 3 Sep 2024 11:17:24 -0500
> > > David Lechner <dlechner@baylibre.com> 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@baylibre.com> 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). 
> 
> That breaks down if the backend you happen to be using (maybe a new
> one hasn't been written yet) is missing the DDR feature but the front end
> device can run with or without it.
> Unless the hardware makes this discoverable you'll have the backend driver
> writing some enable bit that does nothing.
> 
> Maybe it's a case of using fallback compatibles - so define more specific
> ones but with a fallback to one that doesn't provide the fancy features
> and only covers thins all IPs support.
> 
> > 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.
> 
> If you can influence the IP designers, the usual solution to this is
> discoverability of features. So standard register that all IP carries that
> has flags for each feature that has ever been implemented.
> 

That get's messy. We do have some flags for some of the more generic features
(I'm using them in the backends when available). But we have (and will have) so
many variations of these designs that it get's hard to get it right all the
time. And for thing like this bus quirk a flag itself may be not enough to
distinguish between different implementations...

Last time I spoke with the designers, they are thinking about just adding a set
of custom registers that (always the same range I think) for these IPs and then
leave it up to the driver implementation to deal with the different
implementations of the registers. Not sure if it's the best approach but it
feels like they're getting tired of dealing with all the subtle changes between
the different devices these IPs connect too :)

On the IIO backend "world", frontends are the ones with the knowledge of what
these custom registers could implement and so, it's very doable for backends to
export a range of valid registers for frontends to "directly" (of course not
reading/writing directly :)) access. Feels a bit hacky but also a bit
reasonable... Anyways, all of the above is still just speculation so not sure if
it will happen at all. Just some ramblings from me :)


> If not, best option is each IP gets a compatible but we assume fallbacks
> are fine until they aren't.
> 

Yeah, Conor made some compelling arguments about using new compatibles. At
least, for the more complicated cases like this.

- Nuno Sá