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

[Jonathan Cameron <jic23@kernel.org>]

On Fri, 10 Jan 2025 18:56:26 -0300
Marcelo Schmitt <marcelo.schmitt1@gmail.com> wrote:

> 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@analog.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. 

Can we make the use of SPI_MOSI_IDLE_HIGH only apply if controller doesn't support
what is required to do the transfers in one go?

> Plus you would have the complication of filling the tx_buf for
> all transfers.

Wrap that up in a regmap, or read and write functions and that should be easy enough.

> 
> 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.

Whilst the datasheet indeed draws lines for that, i doubt it notices except on
clock transitions and between transfers the clock won't do anything.
If we confirm that the device does notice, then I don't mind limiting the controllers
to those with that can ensure it doesn't get set wrong.

Jonathan