linux-kernel - Re: [PATCH 06/11] staging: iio: adt7316: fix the dac write calculation

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Date:   Sun, 16 Dec 2018 11:47:55 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Jeremy Fertic <jeremyfertic@...il.com>
Cc:     Lars-Peter Clausen <lars@...afoo.de>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-iio@...r.kernel.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/11] staging: iio: adt7316: fix the dac write
 calculation

On Tue, 11 Dec 2018 17:54:58 -0700
Jeremy Fertic <jeremyfertic@...il.com> wrote:

> The lsb calculation is not masking the correct bits from the user input.
> Subtract 1 from (1 << offset) to correctly set up the mask to be applied
> to user input.
> 
> The lsb register stores its value starting at the bit 7 position.
> adt7316_store_DAC() currently assumes the value is at the other end of the
> register. Shift the lsb value before storing it in a new variable lsb_reg,
> and write this variable to the lsb register.
> 
> Signed-off-by: Jeremy Fertic <jeremyfertic@...il.com>
Looks right to me.

Wow this driver had some impressively wrong maths in it ;)
Glad you are fixing this up.

I'll pick up in v2.

Thanks,

Jonathan

> ---
>  drivers/staging/iio/addac/adt7316.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/staging/iio/addac/adt7316.c b/drivers/staging/iio/addac/adt7316.c
> index b7d12d003ddc..77ef3c209b67 100644
> --- a/drivers/staging/iio/addac/adt7316.c
> +++ b/drivers/staging/iio/addac/adt7316.c
> @@ -1442,7 +1442,7 @@ static ssize_t adt7316_show_DAC(struct adt7316_chip_info *chip,
>  static ssize_t adt7316_store_DAC(struct adt7316_chip_info *chip,
>  				 int channel, const char *buf, size_t len)
>  {
> -	u8 msb, lsb, offset;
> +	u8 msb, lsb, lsb_reg, offset;
>  	u16 data;
>  	int ret;
>  
> @@ -1460,9 +1460,13 @@ static ssize_t adt7316_store_DAC(struct adt7316_chip_info *chip,
>  		return -EINVAL;
>  
>  	if (chip->dac_bits > 8) {
> -		lsb = data & (1 << offset);
> +		lsb = data & ((1 << offset) - 1);
> +		if (chip->dac_bits == 12)
> +			lsb_reg = lsb << ADT7316_DA_12_BIT_LSB_SHIFT;
> +		else
> +			lsb_reg = lsb << ADT7316_DA_10_BIT_LSB_SHIFT;
>  		ret = chip->bus.write(chip->bus.client,
> -			ADT7316_DA_DATA_BASE + channel * 2, lsb);
> +			ADT7316_DA_DATA_BASE + channel * 2, lsb_reg);
>  		if (ret)
>  			return -EIO;
>  	}