linux-kernel - Re: [PATCH] drivers: iio: adc: ltc2497: fix LSB shift

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Message-ID: <20230127174756.00007299@Huawei.com>
Date:   Fri, 27 Jan 2023 17:47:56 +0000
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Ian Ray <ian.ray@...com>
CC:     <lars@...afoo.de>, <Michael.Hennerich@...log.com>,
        <linux-kernel@...r.kernel.org>, <linux-iio@...r.kernel.org>
Subject: Re: [PATCH] drivers: iio: adc: ltc2497: fix LSB shift

On Fri, 27 Jan 2023 14:57:14 +0200
Ian Ray <ian.ray@...com> wrote:

> Correct the "sub_lsb" shift for both ltc2497 and ltc2499.

Hi Iain, Thanks for the patch.

> 
> An earlier version of the code shifted by 14 but this was a consequence
> of reading three bytes into a __be32 buffer and using be32_to_cpu(), so
> eight extra bits needed to be skipped.  Now we use get_unaligned_be24()
> and thus the additional skip is wrong.
> 
> Fixes 2187cfe ("drivers: iio: adc: ltc2497: LTC2499 support")

This isn't in standard Fixes tag format.  If nothing else comes
up I'll fix up whilst applying.

> Signed-off-by: Ian Ray <ian.ray@...com>
> ---
>  drivers/iio/adc/ltc2497.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iio/adc/ltc2497.c b/drivers/iio/adc/ltc2497.c
> index 17370c5..ec198c6 100644
> --- a/drivers/iio/adc/ltc2497.c
> +++ b/drivers/iio/adc/ltc2497.c
> @@ -28,7 +28,6 @@ struct ltc2497_driverdata {
>  	struct ltc2497core_driverdata common_ddata;
>  	struct i2c_client *client;
>  	u32 recv_size;
> -	u32 sub_lsb;
>  	/*
>  	 * DMA (thus cache coherency maintenance) may require the
>  	 * transfer buffers to live in their own cache lines.
> @@ -65,10 +64,10 @@ static int ltc2497_result_and_measure(struct ltc2497core_driverdata *ddata,
>  		 * equivalent to a sign extension.
>  		 */
>  		if (st->recv_size == 3) {
> -			*val = (get_unaligned_be24(st->data.d8) >> st->sub_lsb)
> +			*val = (get_unaligned_be24(st->data.d8) >> 6)
>  				- BIT(ddata->chip_info->resolution + 1);
>  		} else {
> -			*val = (be32_to_cpu(st->data.d32) >> st->sub_lsb)
> +			*val = (be32_to_cpu(st->data.d32) >> 6)
>  				- BIT(ddata->chip_info->resolution + 1);
>  		}
>  
> @@ -122,7 +121,6 @@ static int ltc2497_probe(struct i2c_client *client)
>  	st->common_ddata.chip_info = chip_info;
>  
>  	resolution = chip_info->resolution;
> -	st->sub_lsb = 31 - (resolution + 1);
>  	st->recv_size = BITS_TO_BYTES(resolution) + 1;
>  
>  	return ltc2497core_probe(dev, indio_dev);