Hi Jonathan, Thanks for your inputs... > From: Jonathan Cameron <jic23@xxxxxxxxxx> > Sent: Friday, November 12, 2021 5:15 PM > To: Sa, Nuno <Nuno.Sa@xxxxxxxxxx> > Cc: linux-iio@xxxxxxxxxxxxxxx > Subject: Re: [RFC PATCH 0/1] LTC2688 support > > [External] > > On Thu, 11 Nov 2021 12:00:42 +0100 > Nuno Sá <nuno.sa@xxxxxxxxxx> wrote: > > Hi Nuno, > > > The reason why this is a RFC is because I'm still waiting for proper HW > > to test this thing. The reason why I'm sending this already is because > > there's some non usual features which might require extra ABI. > Hence, > > while waiting I thought we could already speed up the process in > regards > > with the ABI. > > Wise move as this is an unusual beast :) > > > > > I still pushed the driver but the intent here is not really to review it. > > Naturally, if someone already wants to give some feedback, that's > very > > much appreciated :) > > > > > Now, there are three main interfaces depending on the channel > mode: > > 1) default (no new ABI) > > 2) toggle mode > > 3) dither mode > > > > The channel mode will be a devicetree property as it does not really > > make much sense to change between modes at runtime even more > because the > > piece of HW we might want to control with a channel might be > different > > depending on the selected mode (I'm fairly sure on this between > toggle > > and other modes but not so sure between dither and default mode). > > I agree on toggle vs dither definitely being different, but normal you > could implement either as software toggle, or dither with a 0 > magnitude > sine wave. So might not be worth implementing default mode at all. > No harm in doing so though if there are advantages to having it. My feeling is that we could probably have dither as the "default mode". More on this below... > > > > toggle mode special ABI: > > > > * Toggle operation enables fast switching of a DAC output between > two > > different DAC codes without any SPI transaction. Two codes are set > in > > input_a and input_b and then the output switches according to an > input > > signal (input_a -> clk high; input_b -> clk low). > > > > out_voltageY_input_register > > - selects between input_a and input_b. After selecting one register, > we > > can write the dac code in out_voltageY_raw. > > out_voltageY_toggle_en > > - Disable/Enable toggle mode. The reason why I think this one is > > important is because if we wanna change the 2 codes, we should > first > > disable this, set the codes and only then enable the mode back... > > As I'm writing this, It kind of came to me that we can probably > > achieve this with out_voltageY_powerdown attr (maybe takes a bit > more > > time to see the outputs but...) > > Hmm. These corner cases always take a bit of figuring out. What you > have > here is a bit 'device specific' for my liking. > > So there is precedence for ABI in this area, on the frequency devices > but only > for devices we still haven't moved out of staging. For those we > needed a means > to define selectable phases for PSK - where the selection was either > software or, > much like here, a selection pin. > > out_altvotage0_phase0 etc > > so I guess the equivalent here would be > out_voltageY_raw0 > out_voltageY_raw1 > and the selection would be via something like > out_voltageY_symbol (0 or 1 in this case). - note this is only > relevant if you have the software toggle. Any enable needed for > setting > can be done as part of the write sequence for the raw0 / raw1 and > should > ideally not be exposed to userspace (controls that require manual > sequencing > tend to be hard to use / document). Yeah, I thought about that. I was even thinking in having something like *_amplitude for dither mode. In some cases, where we might be left in some non obvious state (eg: moved the select register to input b and then we failed to set the code;), I prefer to leave things as flexible as possible for userspace. But I agree it adds up more complexity and in this case, I can just always go to 'input_a' when writing 'raw0'... > However, I'm not 100% sure it really maps to this case. What do you > think? I think it can work. Though out_voltageY_symbol probably needs to be shared by type to be coherent with what we might have with TGPx. Note the sw_toggle register has a bit mask of the channels we want to toggle which means we can toggle multiple channels at the same time. It works the same with TGPx if you map multiple channels to the same pin. There's also the question on how to handle this if a TGPx is provided? I guess it will just override the pin... But most likely having them both at the same time will lead to non desired results (unless we have a way to gate/ungate the clocks)... > I'm not sure if whether a channel is in toggle mode is a circuit thing or > not.. > (and hence DT or userspace control?) The only reason I can think off to have it as DT is that toggle mode seems to be for more specific use cases so I guess the HW we want to control ( and connect to a toggle enabled channel) will be different. I'm also not seeing an use case for ping ponging between the modes mostly because of the above... > Can see that even in a case where you did want to use external > controls to > pick the values, you might also want to override from software... > Given there is a software toggle I guess we can use that as override. > Actually that raises the question of what the point in having normal > mode is? > Can we just implement that as a software toggle toggle mode? One > less thing to > worry about if we can. I did thought about the sw_toggle thing (it's something that is only valid for channels in dither/toggle mode). My reasoning was that either 1) I did not supported it and made the TGPx selection mandatory (in case dither/toggle mode enabled) or 2) I did support it and hence the pins are not really mandatory. I went with 1) because, honestly, I'm not seeing the point of having these modes and use sw toggle (at least on a production system). However, if we want to get rid of the default mode and have it as the dither mode, I agree we need sw_toggle because If someone just wants to use the channel without any dithering, we can't have an hard requirement to provide a external TGPx. Moreover, if the default channel will be a dither capable one, we need to provide full functionality and hence, sw_toggle. As I stated before, I'm just not sure on how to handle things if a TGPx is also provided. Maybe they should be mutual exclusive? I mean, if someone tries to toggle a channel with a mapped TGPx we return some error code? > There is also the question of whether selection of which toggle pin is > used > should be a dt thing or a userspace control... Well, this definitely means some HW wiring to have the external signals and I'm not sure if there's any added valuable in being able to change the external signal at runtime? > > > > dither mode special ABI: > > > > * Dither operation adds a small sinusoidal wave to the digital DAC > > signal path. Dithering is a signal processing technique that involves > > the injection of ac noise to the signal path to reduce system > > nonlinearities. > > > > This is a complex feature to describe as (if I read it correctly) we have > a dither clocked from an external pin, or in theory from software. That > clock > frequency must match the dither. Realistically that means it is a clock > in our control or we have to match the period below to the frequency > of that > clock. Yeah, the frequency of the dither signal is fsig = fclk / N, where N can only be: [4 8 16 32 64]. So, we kind of just have these available options for the signal frequency and fclk is something we can control and know (assuming we have TGPx mapping which I'm bundling with a clk). The only quirk with having this with frequency rather than raw N is to handle the sw_toggle where we have no idea about fclk? We could also think of this attr as some kind of decimation... > > out_voltage0_dither_en > > - Same as in toggle mode. > > out_voltage0_dither_period > > out_voltage0_dither_phase > > - Period and phase of the signal. Only some values are valid so > there's > > also *_available files for these. I'm not sure if we can use the more > > generic IIO_CHAN_INFO_PHASE and IIO_CHAN_INFO_FREQUENCY > here as these > > parameters don't really apply to the channel output signal.. > > Possibly not helpful to do so, but you could describe the channel as an > out_altvoltage channel that happens to have a significant offset (the > DC > level) and phase, frequency etc as for a normal altvoltage channel. > That would hide the intention here though so perhaps not a good plan > even if it ensures we end up with standard ABI. I think altvoltage might not be optimal here because the phase and frequency are really not characteristics of the output signal of the channel. > > out_voltage0_input_register > > - Same as in toggle mode. However in this mode the code set in the > > input_b register has a special meaning. It's the amplitude of the > > dither signal. > Don't do that - provide a direct attribute representing the value of > register_b and when it is written implicitly switch to the right register. > Any ABI that requires a sequence of events is hard to use. I guess we can just use the same raw1 attr here? Even though, in dither mode this has special meaning (it is the amplitude)... > > > > > One special mention to the dac scale. In this part this is something > > that can be purely controlled by SW so that I'm allowing userspace to > > change it rather then having it in dts. The available scales are: > > > > * [0 5V] -> offset 0 > > * [0 10V] -> offset 0 > > * [-5 5V] -> offset -32678 > > * [-10 10V] -> offset -32768 > > * [-15 15V] -> offset -32768 > > > > With the above, we also need to have the offset configurable and > right > > now I'm going to some trouble to make sure the scale + offset is > > something valid. Honestly, I think I'm overdoing it because things can > > still go wrong with [0 10V] and [-5 5V] as the scales are the same > > here. Furthermore, there's no real arm that can be done to the HW. > Is > > just that the readings won't match with what someone might be > expecting. > > My plan is to just remove those checks and assume is up to > userspace to > > make it right and not have [-10 10V] scale with 0 offset for example. > > So this is something we've debated a few times in the past. > There is a fairly strong argument for output devices that the range is > a characteristic of the circuit. At the very least it makes sense to > restrict it in DT even if we allow safe forms of tweaking in the driver. > For an initial driver, I'd just have it in DT. > No complaints against that and makes things way simpler to handle. - Nuno Sá
