linux-kernel - Re: [PATCH v4 1/4] iio: adc: ad4000: Add support for SPI offload

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Message-ID: <97d0cc16-cdd4-449c-87f5-f62e0515de97@baylibre.com>
Date: Wed, 9 Apr 2025 12:22:30 -0500
From: David Lechner <dlechner@...libre.com>
To: Marcelo Schmitt <marcelo.schmitt@...log.com>, linux-iio@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: jic23@...nel.org, lars@...afoo.de, Michael.Hennerich@...log.com,
 corbet@....net, marcelo.schmitt1@...il.com
Subject: Re: [PATCH v4 1/4] iio: adc: ad4000: Add support for SPI offload

On 3/27/25 4:24 PM, Marcelo Schmitt wrote:
> FPGA HDL projects can include a PWM generator in addition to SPI-Engine.
> The PWM IP is used to trigger SPI-Engine offload modules that in turn set
> SPI-Engine to execute transfers to poll data from the ADC. That allows data
> to be read at the maximum sample rates. Also, it is possible to set a
> specific sample rate by setting the proper PWM duty cycle and related state
> parameters, thus allowing an adjustable ADC sample rate when a PWM (offload
> trigger) is used in combination with SPI-Engine.
> 
> Add support for SPI offload.
> 
> Signed-off-by: Marcelo Schmitt <marcelo.schmitt@...log.com>
> ---

I think I spotted a bug. With that fixed...

Reviewed-by: David Lechner <dlechner@...libre.com>

> @@ -690,6 +942,15 @@ static int ad4000_prepare_3wire_mode_message(struct ad4000_state *st,
>  
>  	xfers[1].rx_buf = &st->scan.data;
>  	xfers[1].len = BITS_TO_BYTES(chan->scan_type.storagebits);

Now this needs to be:

	xfer->len = chan->scan_type.realbits > 16 ? 4 : 2;

Otherwise when there is SPI offload, this could read 2 words because
storagebits is always == 32.

> +
> +	/*
> +	 * If the device is set up for SPI offloading, IIO channel scan_type is
> +	 * set to IIO_CPU. When that is the case, use larger SPI word sizes for
> +	 * single-shot reads too. Thus, sample data can be correctly handled in
> +	 * ad4000_single_conversion() according to scan_type endianness.
> +	 */
> +	if (chan->scan_type.endianness != IIO_BE)
> +		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;
>