Message-ID: <79734a9f54e320cd09aab83015f1c0e6d93e700c.camel@gmail.com>
Date: Mon, 15 Apr 2024 14:45:52 +0200
From: Nuno Sá <noname.nuno@...il.com>
To: Jonathan Cameron <jic23@...nel.org>, David Lechner
 <dlechner@...libre.com>
Cc: Kim Seer Paller <kimseer.paller@...log.com>, linux-iio@...r.kernel.org, 
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, Lars-Peter
 Clausen <lars@...afoo.de>, Rob Herring <robh@...nel.org>, Krzysztof
 Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley
 <conor+dt@...nel.org>,  Liam Girdwood <lgirdwood@...il.com>, Mark Brown
 <broonie@...nel.org>, Michael Hennerich <michael.hennerich@...log.com>
Subject: Re: [PATCH 2/4] iio: ABI: add ABI file for the LTC2664 DAC

On Sat, 2024-04-13 at 16:25 +0100, Jonathan Cameron wrote:
> On Fri, 12 Apr 2024 16:26:17 -0500
> David Lechner <dlechner@...libre.com> wrote:
> 
> > On Thu, Apr 11, 2024 at 10:21 PM Kim Seer Paller
> > <kimseer.paller@...log.com> wrote:
> > > 
> > > Define the sysfs interface for toggle capable channels.
> > > 
> > > Toggle enabled channels will have:
> > > 
> > >  * out_voltageY_toggle_en  
> The big missing thing in this ABI is a reference to existing precedence.
> You aren't actually defining anything new, it just hasn't yet been generalized
> beyond 1 device (unless you include PSK / FSK DDS drivers that are 'still'
> after
> 13+ years in staging!)
> 
> This patch needs to be generalizing that documentation from the ltc2688.
> 
> Probably in sysfs-bus-iio-dac
> 
> > 
> > It looks like there are 3 toggle modes.
> > 
> > Two involve the notion of "enabled" outputs that I assume this attribute is
> > for:
> > 
> > 1. Toggling all enabled pins at the same time using a software trigger
> > (global toggle bit)
> > 2. Toggling all enabled pins at the same time using a hardware trigger
> > (TGP pin) and toggling pins
> > 
> 
> This is presumably the tricky one as that hardware toggle may not be in
> control of the host CPU.
> 
> > The third mode though looks like it uses the same toggle select
> > register for selecting A or B for each channel instead of enabling or
> > disabling each channel.
> > 
> > 3. Toggling all pins to A or B based on the toggle select register. No
> > notion of enabled pins here.
> > 
> > I haven't looked at the driver implementation, but it sounds like
> > out_voltageY_toggle_en and out_voltageY_symbol would be writing to the
> > same register in conflicting ways. So maybe we need yet another custom
> > attribute to select the currently active toggle mode?
> 
> This one feels like it could be handled as a software optimisation over
> just changing the DAC value directly.

Things may be slightly different in these devices. But for ltc2688 and AFAIR,
the symbol attribute is about toggling between A and B through SW (not really
enabling the mode). That interface will only pop up if there's no HW (PWM for
example) toggle present.

> 
> > 
> > In any case, it would be helpful if the documentation below explained
> > a bit better the intention and conditions required to use each
> > attribute (or add a .rst documentation file for these chips to explain
> > how to use it in more detail since this is rather complex feature).
> > 
> > >  * out_voltageY_raw0
> > >  * out_voltageY_raw1  
> > 
> > I guess there is no enum iio_modifier that fits this. It seems like we
> > could still have out_voltageY_raw for register A so that users that
> > don't need to do any toggling can use standard ABI. And maybe
> > out_voltageY_raw_toggled for register B (question for Jonathan)?
> 
> There is precedence for doing it like this (ltc2688)
> Note that we should only see these attribute for changes specifically
> configured for 'hardware' triggered toggling.
> 
> Note that we cannot have duplicate documentation so we need to create
> a common docs file covering this and existing ltc2688 ABI that overlaps.
> That may need some generalising to cover both parts.
> 
> > 
> > Or just have 8 channels instead of 4 where even channels are register
> > A and odd channels are register B?
> > 
> > >  * out_voltageY_symbol  
> > 
> > "symbol" is a confusing name. It sounds like this just supports
> > toggling one channel individually so _toggle_select would make more
> > sense to me. Are there plans for supporting toggling multiple channels
> > at the same time using a software trigger as well?
> 
> Again, precedence should have been called out.  It's not great ABI
> but it corresponds to earlier work on Frequency Shift Keying DDS devices
> (and I think Phase Shift Keying as well) in which this is call symbol.
> Hence the name.
> 
> > 
> > > 
> > > The common interface present in all channels is:
> > > 
> > >  * out_voltageY_raw (not present in toggle enabled channels)  
> > 
> > As mentioned above, I don't think we need to have to make a
> > distinction between toggle enabled channels and not enabled channels.
> 
> Was a while back but I think that last time this turned up we concluded
> we did need a different interface because the current 'toggle value'
> is not in our control.  Hence you are programming one channel that
> does different things - think of it as setting the Max and Min values
> for a generated waveform - perhaps the toggle pin is connected to a PWM
> for example.
> 

Yeah and for the ltc2688 we also had the dither mode that has it's own unique
set of interfaces. Trying to deal with it all at runtime could be more confusing
than beneficial I guess.

- Nuno Sá
> 

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