Re: [PATCH 2/4] iio: ABI: add ABI file for the LTC2664 DAC

[David Lechner <dlechner@baylibre.com>]

On 4/13/24 10:25 AM, Jonathan Cameron wrote:
> On Fri, 12 Apr 2024 16:26:17 -0500
> David Lechner <dlechner@baylibre.com> wrote:
> 
>> On Thu, Apr 11, 2024 at 10:21 PM Kim Seer Paller
>> <kimseer.paller@analog.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.
> 
>>
>> 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.
> 
>>
>>>  * out_voltageY_raw_available
>>>  * out_voltageY_powerdown  
>>
>> Is _powerdown a standard attribute? I don't see it documented
>> anywhere. Perhaps you meant _en (via IIO_CHAN_INFO_ENABLE)?
> 
> It's there in Documentation/ABI/testing/sysfs-bus-iio
> Different form simple enable (which came much later as ABI) because
> it means entering a powerdown state in which a particular thing happens
> on the output pin.  It is defined alongside powerdown_mode which 
> defines what happens. (often a choice between different impedance / High Z etc)
> 
> 
>>
>>
>>>  * out_voltageY_scale
>>>  * out_voltageY_offset
>>>
>>> Co-developed-by: Michael Hennerich <michael.hennerich@analog.com>
>>> Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
>>> Signed-off-by: Kim Seer Paller <kimseer.paller@analog.com>
>>> ---
>>>  .../ABI/testing/sysfs-bus-iio-dac-ltc2664     | 30 +++++++++++++++++++
>>>  MAINTAINERS                                   |  1 +
>>>  2 files changed, 31 insertions(+)
>>>  create mode 100644 Documentation/ABI/testing/sysfs-bus-iio-dac-ltc2664
>>>
>>> diff --git a/Documentation/ABI/testing/sysfs-bus-iio-dac-ltc2664 b/Documentation/ABI/testing/sysfs-bus-iio-dac-ltc2664
>>> new file mode 100644
>>> index 000000000..4b656b7af
>>> --- /dev/null
>>> +++ b/Documentation/ABI/testing/sysfs-bus-iio-dac-ltc2664
>>> @@ -0,0 +1,30 @@
>>> +What:          /sys/bus/iio/devices/iio:deviceX/out_voltageY_toggle_en
>>> +KernelVersion: 5.18
>>> +Contact:       linux-iio@vger.kernel.org
>>> +Description:
>>> +               Toggle enable. Write 1 to enable toggle or 0 to disable it. This is
>>> +               useful when one wants to change the DAC output codes. The way it should
>>> +               be done is:
>>> +
>>> +               - disable toggle operation;
>>> +               - change out_voltageY_raw0 and out_voltageY_raw1;
>>> +               - enable toggle operation.
>>> +
>>> +What:          /sys/bus/iio/devices/iio:deviceX/out_voltageY_raw0
>>> +What:          /sys/bus/iio/devices/iio:deviceX/out_voltageY_raw1
>>> +KernelVersion: 5.18
>>> +Contact:       linux-iio@vger.kernel.org
>>> +Description:
>>> +               It has the same meaning as out_voltageY_raw. This attribute is
>>> +               specific to toggle enabled channels and refers to the DAC output
>>> +               code in INPUT_A (_raw0) and INPUT_B (_raw1). The same scale and offset
>>> +               as in out_voltageY_raw applies.
>>> +
>>> +What:          /sys/bus/iio/devices/iio:deviceX/out_voltageY_symbol
>>> +KernelVersion: 5.18
>>> +Contact:       linux-iio@vger.kernel.org
>>> +Description:
>>> +               Performs a SW toggle. This attribute is specific to toggle
>>> +               enabled channels and allows to toggle between out_voltageY_raw0
>>> +               and out_voltageY_raw1 through software. Writing 0 will select
>>> +               out_voltageY_raw0 while 1 selects out_voltageY_raw1.
>>> diff --git a/MAINTAINERS b/MAINTAINERS
>>> index bd8645f6e..9ed00b364 100644
>>> --- a/MAINTAINERS
>>> +++ b/MAINTAINERS
>>> @@ -12842,6 +12842,7 @@ M:      Kim Seer Paller <kimseer.paller@analog.com>
>>>  L:     linux-iio@vger.kernel.org
>>>  S:     Supported
>>>  W:     https://ez.analog.com/linux-software-drivers
>>> +F:     Documentation/ABI/testing/sysfs-bus-iio-dac-ltc2664
>>>  F:     Documentation/devicetree/bindings/iio/dac/adi,ltc2664.yaml
>>>
>>>  LTC2688 IIO DAC DRIVER
>>> --
>>> 2.34.1
>>>  
> 

Clearly I have a lot to learn on this one! Thanks for all of the info.