linux-kernel - Re: [PATCH v1 05/15] iio: adc: ad7768-1: set MOSI idle state to high

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Message-ID: <Z4GXikxVw6mHIYHc@debian-BULLSEYE-live-builder-AMD64>
Date: Fri, 10 Jan 2025 18:56:26 -0300
From: Marcelo Schmitt <marcelo.schmitt1@...il.com>
To: David Lechner <dlechner@...libre.com>
Cc: Jonathan Santos <Jonathan.Santos@...log.com>, linux-iio@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	lars@...afoo.de, Michael.Hennerich@...log.com, jic23@...nel.org,
	robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org
Subject: Re: [PATCH v1 05/15] iio: adc: ad7768-1: set MOSI idle state to high

On 01/07, David Lechner wrote:
> On 1/7/25 9:25 AM, Jonathan Santos wrote:
> > All supported parts require that the MOSI line stays high
> > while in idle.
> > 
> > Configure SPI controller to set MOSI idle state to high.
> > 
> > Fixes: a5f8c7da3dbe ("iio: adc: Add AD7768-1 ADC basic support")
> > Signed-off-by: Jonathan Santos <Jonathan.Santos@...log.com>
> > ---
...
> > @@ -574,6 +574,15 @@ static int ad7768_probe(struct spi_device *spi)
> >  		return -ENOMEM;
> >  
> >  	st = iio_priv(indio_dev);
> > +	/*
> > +	 * The ADC SDI line must be kept high when
> > +	 * data is not being clocked out of the controller.
> > +	 * Request the SPI controller to make MOSI idle high.
> > +	 */
> > +	spi->mode |= SPI_MOSI_IDLE_HIGH;
> > +	ret = spi_setup(spi);
> > +	if (ret < 0)
> > +		return ret;
> >  	st->spi = spi;
> >  
> >  	st->vref = devm_regulator_get(&spi->dev, "vref");
> 
> Very few SPI controllers currently have the SPI_MOSI_IDLE_HIGH capability flag
> set in Linux right now (whether they actually support it or not), so this could
> break existing users.

Good point. Maybe only dev_warn() if SPI_MOSI_IDLE_HIGH is not supported?

> 
...
> 
> If we ever do implement a data read of more than 64 bits without toggling CS,
> then we could just set the TX data to be all 0xFF and have the same effect
> without requiring the SPI controller to support SPI_MOSI_IDLE_HIGH.

One point of having SPI_MOSI_IDLE_HIGH is that the controller may bring MOSI low
between data words of a transfer. I think all transfer words are going to be
either 16 or 24 with the new patches setting bits_per_word in all transfers but
that might still not be enough if eventually the controller is unable to support
those word sizes. Plus you would have the complication of filling the tx_buf for
all transfers.

For the part that instigated the development of SPI_MOSI_IDLE_HIGH, the MOSI line
also had to be high in between transfers. The diagrams at AD7768-1 datasheet
page 51 suggest the same would be needed for this chip too.