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

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Message-ID: <Z_fe69bn90oVjPNk@debian-BULLSEYE-live-builder-AMD64>
Date: Thu, 10 Apr 2025 12:08:27 -0300
From: Marcelo Schmitt <marcelo.schmitt1@...il.com>
To: David Lechner <dlechner@...libre.com>
Cc: Marcelo Schmitt <marcelo.schmitt@...log.com>, linux-iio@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	jic23@...nel.org, lars@...afoo.de, Michael.Hennerich@...log.com,
	corbet@....net
Subject: Re: [PATCH v4 1/4] iio: adc: ad4000: Add support for SPI offload

On 04/09, David Lechner wrote:
> 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.

I've just tested both versions with ADAQ4003 and with AD7687.
There's no practical difference between the suggestion and what already is in
IIO testing. I don't mind sending a patch with the change, but don't think it
fixes anything.

Thanks,
Marcelo