Re: [PATCH v2] iio: adc: ti-ads7950: inconsistency with spi msg

[Jonathan Cameron <jic23@kernel.org>]

On Fri, 25 Jan 2019 10:20:22 -0800
justinpopo6@gmail.com wrote:

> From: Justin Chen <justinpopo6@gmail.com>
> 
> To read a channel we require 3 cycles to send, process, and receive
> the data. The transfer buffer for the third transaction is left blank.
> This leaves it up to the SPI driver to decide what to do.
Interesting.  I think that means you may have a bug in your SPI driver.
The pointer in question is always left set to NULL in the adc
driver (not explicitly but it will be because ultimately comes from
a kzalloc).

Documentation for an SPI message makes it clear that NULL is
allowed.  From include/linux/spi/spi.h

"* Those segments always read the same number of bits as they
 * write; but one or the other is easily ignored by passing a null buffer
 * pointer. "

Hmm. It does say 'ignored'.  My assumption has always been that
means it would be filled with zeros, but maybe I'm wrong.

Mark?

> 
> In one particular case, if the tx buffer is not set the spi driver
> sets it to 0xff. This puts the ADC in a alarm programming state,
> therefore the following read to a channel becomes erroneous.
> 
> Instead of leaving us to the mercy of the SPI driver, we send the
> ADC cmd on the third transaction to prevent inconsistent behavior.
> 
> Fixes: 902c4b2446d4 ("iio: adc: New driver for TI ADS7950 chips")
> Signed-off-by: Justin Chen <justinpopo6@gmail.com>
> ---
>  drivers/iio/adc/ti-ads7950.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/iio/adc/ti-ads7950.c b/drivers/iio/adc/ti-ads7950.c
> index 0ad6359..1255d8b 100644
> --- a/drivers/iio/adc/ti-ads7950.c
> +++ b/drivers/iio/adc/ti-ads7950.c
> @@ -422,6 +422,7 @@ static int ti_ads7950_probe(struct spi_device *spi)
>  	st->scan_single_xfer[1].tx_buf = &st->single_tx;
>  	st->scan_single_xfer[1].len = 2;
>  	st->scan_single_xfer[1].cs_change = 1;
> +	st->scan_single_xfer[2].tx_buf = &st->single_tx;
>  	st->scan_single_xfer[2].rx_buf = &st->single_rx;
>  	st->scan_single_xfer[2].len = 2;
>