Re: [PATCH v1 1/4] iio: adc: ad4000: Add support for SPI offload

[David Lechner <dlechner@baylibre.com>]

On 3/17/25 10:31 AM, Marcelo Schmitt wrote:


> ...
>>> +/*
>>> + * This executes a data sample transfer when using SPI offloading for when the
>>> + * device connections are in "3-wire" mode, selected when the adi,sdi-pin device
>>> + * tree property is set to "high". In this connection mode, the ADC SDI pin is
>>> + * connected to VIO and ADC CNV pin is connected to a SPI controller CS (it
>>> + * can't be connected to a GPIO).
>>> + *
>>> + * In order to achieve the maximum sample rate, we only do one transfer per
>>> + * SPI offload trigger. This has the effect that the first sample data is not
>>> + * valid because it is reading the previous conversion result. We also use
>>
>> Say what happens to that invalid sample.  Is it dropped or provided to userspace
>> as if it were valid?  (I hope dropped!)
> 
> TL;DR: The invalid sample goes into the buffer as a valid one.
> 
> In AD4000 '3-wire' mode, data capture has a latency (delay) of one sample.
> 
> The ADC begins sampling data N at CNV rising edge
>           |   +-- CNV (usually SPI CS) is brought low to begin reading the data
>           |   |                                +-- Data N + 1 that will be read
>           |   |                                |   on the next transfer starts 
>           v   v                                v   being sampled at end of transfer N.
>            ___                                  ____            
> CNV  _____/   \________________________________/    \_____
>                     _     _             _
> SCLK ______________/ \___/ \_ ...   ___/ \_______________
>                    ___   ___           ___
> SDO  _____________/___\_/___\ ...   __/___\_______________
>                     ^
>                     |
>              Data from conversion N is output from here on
> 
> A better drawing can be found in datasheet page 29, Figure 57.
> https://www.analog.com/media/en/technical-documentation/data-sheets/ADAQ4003.pdf
> 
> In sum, we're always reading a conversion that started at the end of the
> previous SPI transfer or, in other words, the data comes out with a latency
> (delay) of one read.
> 
> Datasheet somehow mentions that by saying
> 	When turbo mode is enabled, the conversion result read on SDO corresponds to
> 	the result of the previous conversion.
> 
> I think I can do a dummy SPI transfer on buffer preenable so at least the
> first data is not invalid. Would that be better?

Not really. There will be a relatively long delay between that conversion
trigger and when the sample is read. So the data might be slightly less stale
in that case, but still not particularly useful, e.g. if you are doing any
kind of signal processing that expects equal time between all samples.

On similar chips, like ad7944, we just documented that the first sample does
not contain valid data and needs to be discarded.

> 
>>
>>> + * bits_per_word to ensure the minimum of SCLK cycles are used. And a delay is
>>> + * added to make sure we meet the minimum quiet time before releasing the CS
>>> + * line. Plus the CS change delay is set to ensure that we meet the minimum
>>> + * conversion time before asserting CS again.
>>> + *
>>> + * This timing is only valid if turbo mode is disabled (reading during acquisition).
>>> + */
>>> +static int ad4000_prepare_offload_message(struct ad4000_state *st,
>>> +					  const struct iio_chan_spec *chan)
>>> +
>>
> ...
>>> +		xfers[1].bits_per_word = chan->scan_type.realbits;
>>>  	xfers[1].delay.value = st->time_spec->t_quiet2_ns;
>>>  	xfers[1].delay.unit = SPI_DELAY_UNIT_NSECS;
>>>  
>>> @@ -719,7 +1007,9 @@ static int ad4000_prepare_4wire_mode_message(struct ad4000_state *st,
>>>  	xfers[0].delay.unit = SPI_DELAY_UNIT_NSECS;
>>>  
>>>  	xfers[1].rx_buf = &st->scan.data;
>>> -	xfers[1].len = BITS_TO_BYTES(chan->scan_type.storagebits);
>>> +	xfers[1].len = chan->scan_type.realbits > 16 ? 4 : 2;
>>> +	if (chan->scan_type.endianness != IIO_BE)
>>
>> This is odd enough to require a comment.  Why is endianness relevant?
> 
> When using SPI offloading (at least with PULSAR-ADC HDL project [1]), ADC data
> is read by SPI controller and pushed to DMA memory in CPU endianness. I don't
> know exactly where data gets rearranged in the data path (whether SPI-Engine,
> the DMA controller, or something else rearranges ADC data into CPU endianess).
> But I know, from testing with these ADCs and HDL project, that data is correct
> when read in CPU endianness because it converts back to expected mV values.
> When IIO buffers were set to IIO_BE and SPI offloading is used, data just looked
> weird and didn't convert to expected values in mV.
> 
> [1]: https://analogdevicesinc.github.io/hdl/projects/pulsar_adc/index.html
> 
> Other IIO drivers also set IIO_CPU buffer endianness when using offload support,
> e.g. ad7944, ad7380.

These drivers also use IIO_CPU for the non-SPI offload case though.

> 
> They only say buffer would use 32 storagebits when using SPI offload.
> https://lore.kernel.org/linux-iio/20250207-dlech-mainline-spi-engine-offload-2-v8-10-e48a489be48c@baylibre.com/
> https://lore.kernel.org/linux-iio/20250220-wip-bl-spi-offload-ad7380-v1-1-838aa873e62a@baylibre.com/#t
> 
> I also didn't expect to find out HDL support for 16-bit data width was removed.
> We used to have a build parameter for 16-bit precision ADCs.
> https://github.com/analogdevicesinc/hdl/commit/b2dc91b30dae891b6319d88e083f26e726f43ba0#diff-1117c2618353232e5f22aa6a12e8ae976757fa897b3425f470a12123cae26535L13

A while back the HDL engineers mentioned to us that they wanted to standardize
on 32-bit data words everywhere. While not the most efficient use of memory,
having fewer options does make things simpler across the entire software stack.

> 
> Would something like 'because SPI offloading leads to data being pushed to
> memory in CPU endianness' be a reasonable comment?

Another way to say it is that SPI offload reads data in complete words and not
in separate 8-bit xfers (bits_per_word = realbits vs. bits_per_word = 8).