linux-kernel - Re: [PATCH 1/3] iio: adc: ti-ads8344: properly byte swap value

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Date:   Thu, 16 Apr 2020 08:29:45 +0200
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Jonathan Cameron <jic23@...nel.org>
Cc:     Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Gregory CLEMENT <gregory.clement@...tlin.com>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] iio: adc: ti-ads8344: properly byte swap value

On 4/15/20 11:22 PM, Alexandre Belloni wrote:
> The first received byte is the MSB, followed by the LSB so the value needs
> to be byte swapped.
>
> Also, the ADC actually has a delay of one clock on the SPI bus. Read three
> bytes to get the last bit.
>
> Fixes: 8dd2d7c0fed7 ("iio: adc: Add driver for the TI ADS8344 A/DC chips")
> Signed-off-by: Alexandre Belloni <alexandre.belloni@...tlin.com>
> ---
>   drivers/iio/adc/ti-ads8344.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/iio/adc/ti-ads8344.c b/drivers/iio/adc/ti-ads8344.c
> index 9a460807d46d..6da50ea35217 100644
> --- a/drivers/iio/adc/ti-ads8344.c
> +++ b/drivers/iio/adc/ti-ads8344.c
> @@ -29,7 +29,6 @@ struct ads8344 {
>   	struct mutex lock;
>   
>   	u8 tx_buf ____cacheline_aligned;
> -	u16 rx_buf;
>   };
>   
>   #define ADS8344_VOLTAGE_CHANNEL(chan, si)				\
> @@ -76,6 +75,7 @@ static int ads8344_adc_conversion(struct ads8344 *adc, int channel,
>   {
>   	struct spi_device *spi = adc->spi;
>   	int ret;
> +	u8 buf[3];
Same as with spi_write(), spi_read() transfer buffers should not be on 
the stack in case the driver tries to use it for DMA.
>   
>   	adc->tx_buf = ADS8344_START;
>   	if (!differential)
> @@ -89,11 +89,11 @@ static int ads8344_adc_conversion(struct ads8344 *adc, int channel,
>   
>   	udelay(9);
>   
> -	ret = spi_read(spi, &adc->rx_buf, 2);
> +	ret = spi_read(spi, buf, sizeof(buf));
>   	if (ret)
>   		return ret;
>   
> -	return adc->rx_buf;
> +	return buf[0] << 9 | buf[1] << 1 | buf[2] >> 7;
>   }
>   
>   static int ads8344_read_raw(struct iio_dev *iio,