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

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Message-ID: <20250412112852.19eacac3@jic23-huawei>
Date: Sat, 12 Apr 2025 11:28:52 +0100
From: Jonathan Cameron <jic23@...nel.org>
To: Marcelo Schmitt <marcelo.schmitt1@...il.com>
Cc: David Lechner <dlechner@...libre.com>, Marcelo Schmitt
 <marcelo.schmitt@...log.com>, linux-iio@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.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 Thu, 10 Apr 2025 12:08:27 -0300
Marcelo Schmitt <marcelo.schmitt1@...il.com> wrote:

> 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.
If it is an unnecessary over read in some cases, probably sensible to either
reduce the length or add a comment on why we don't care.

Jonathan

> 
> Thanks,
> Marcelo