linux-kernel - Re: [PATCH 4/9] drivers/iio: Sign extend without triggering implementation-defined behavior

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Date:   Wed, 30 Oct 2019 19:43:25 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-iio@...r.kernel.org
Subject: Re: [PATCH 4/9] drivers/iio: Sign extend without triggering
 implementation-defined behavior

On Mon, 28 Oct 2019 13:06:55 -0700
Bart Van Assche <bvanassche@....org> wrote:

> From the C standard: "The result of E1 >> E2 is E1 right-shifted E2 bit
> positions. If E1 has an unsigned type or if E1 has a signed type and a
> nonnegative value, the value of the result is the integral part of the
> quotient of E1 / 2E2 . If E1 has a signed type and a negative value, the
> resulting value is implementation-defined."
> 
> Hence use sign_extend_24_to_32() instead of "<< 8 >> 8".

+CC linux-iio

> 
> Cc: Jonathan Cameron <jic23@...nel.org>
> Cc: Hartmut Knaack <knaack.h@....de>
> Cc: Lars-Peter Clausen <lars@...afoo.de>
> Cc: Peter Meerwald-Stadler <pmeerw@...erw.net>
> Signed-off-by: Bart Van Assche <bvanassche@....org>
> ---
>  drivers/iio/common/st_sensors/st_sensors_core.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/drivers/iio/common/st_sensors/st_sensors_core.c b/drivers/iio/common/st_sensors/st_sensors_core.c
> index 4a3064fb6cd9..94a9cec69cd7 100644
> --- a/drivers/iio/common/st_sensors/st_sensors_core.c
> +++ b/drivers/iio/common/st_sensors/st_sensors_core.c
> @@ -21,11 +21,6 @@
>  
>  #include "st_sensors_core.h"
>  
> -static inline u32 st_sensors_get_unaligned_le24(const u8 *p)
> -{
> -	return (s32)((p[0] | p[1] << 8 | p[2] << 16) << 8) >> 8;
> -}
> -
>  int st_sensors_write_data_with_mask(struct iio_dev *indio_dev,
>  				    u8 reg_addr, u8 mask, u8 data)
>  {
> @@ -556,7 +551,7 @@ static int st_sensors_read_axis_data(struct iio_dev *indio_dev,
>  	else if (byte_for_channel == 2)
>  		*data = (s16)get_unaligned_le16(outdata);
>  	else if (byte_for_channel == 3)
> -		*data = (s32)st_sensors_get_unaligned_le24(outdata);
> +		*data = get_unaligned_signed_le24(outdata);
>  
>  st_sensors_free_memory:
>  	kfree(outdata);