linux-kernel - Re: [PATCH 4/4] input: joystick: Fix buffer data parsing

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Date:   Fri, 19 Aug 2022 18:53:39 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Artur Rojek <contact@...ur-rojek.eu>
Cc:     Paul Cercueil <paul@...pouillou.net>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Chris Morgan <macromorgan@...mail.com>,
        linux-mips@...r.kernel.org, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-input@...r.kernel.org
Subject: Re: [PATCH 4/4] input: joystick: Fix buffer data parsing

On Wed, 17 Aug 2022 12:56:43 +0200
Artur Rojek <contact@...ur-rojek.eu> wrote:

> Don't try to access buffer data of a channel by its scan index. Instead,
> use the newly introduced `iio_find_channel_offset_in_buffer` to get the
> correct data offset.
> 
> The scan index of a channel does not represent its position in a buffer,
> as the buffer will contain data for enabled channels only, affecting
> data offsets and alignment.
> 
> Fixes: 2c2b364fddd5 ("Input: joystick - add ADC attached joystick driver.")
> Reported-by: Chris Morgan <macromorgan@...mail.com>
> Tested-by: Paul Cercueil <paul@...pouillou.net>
> Signed-off-by: Artur Rojek <contact@...ur-rojek.eu>
> ---
>  drivers/input/joystick/adc-joystick.c | 26 +++++++++++++++++---------
>  1 file changed, 17 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/input/joystick/adc-joystick.c b/drivers/input/joystick/adc-joystick.c
> index c0deff5d4282..aed853ebe1d1 100644
> --- a/drivers/input/joystick/adc-joystick.c
> +++ b/drivers/input/joystick/adc-joystick.c
> @@ -6,6 +6,7 @@
>  #include <linux/ctype.h>
>  #include <linux/input.h>
>  #include <linux/iio/iio.h>
> +#include <linux/iio/buffer.h>
>  #include <linux/iio/consumer.h>
>  #include <linux/module.h>
>  #include <linux/platform_device.h>
> @@ -46,36 +47,43 @@ static void adc_joystick_poll(struct input_dev *input)
>  static int adc_joystick_handle(const void *data, void *private)
>  {
>  	struct adc_joystick *joy = private;
> +	struct iio_buffer *buffer;
>  	enum iio_endian endianness;
> -	int bytes, msb, val, idx, i;
> -	const u16 *data_u16;
> +	int bytes, msb, val, off;
> +	const u8 *chan_data;
> +	unsigned int i;
>  	bool sign;
>  
>  	bytes = joy->chans[0].channel->scan_type.storagebits >> 3;
>  
>  	for (i = 0; i < joy->num_chans; ++i) {
> -		idx = joy->chans[i].channel->scan_index;
>  		endianness = joy->chans[i].channel->scan_type.endianness;
>  		msb = joy->chans[i].channel->scan_type.realbits - 1;
>  		sign = tolower(joy->chans[i].channel->scan_type.sign) == 's';
> +		buffer = iio_channel_cb_get_iio_buffer(joy->buffer);
> +		off = iio_find_channel_offset_in_buffer(joy->chans[i].indio_dev,
> +							joy->chans[i].channel,
> +							buffer);

With this call replaced with one that instead uses

		off = iio_find_channel_offset_in_buffer(joy->chans, i);

which I'm fairly sure is enough via the info in chans[x]->channel to establish this offset.

All is good, though you should probably cache it as doing that maths every
time seems excessive.


> +		if (off < 0)
> +			return off;
> +
> +		chan_data = (const u8 *)data + off;
>  
>  		switch (bytes) {
>  		case 1:
> -			val = ((const u8 *)data)[idx];
> +			val = *chan_data;
>  			break;
>  		case 2:
> -			data_u16 = (const u16 *)data + idx;
> -
>  			/*
>  			 * Data is aligned to the sample size by IIO core.
>  			 * Call `get_unaligned_xe16` to hide type casting.
>  			 */
>  			if (endianness == IIO_BE)
> -				val = get_unaligned_be16(data_u16);
> +				val = get_unaligned_be16(chan_data);

I obviously missed this previously but these are aligned so we don't need the
unaligned form.

>  			else if (endianness == IIO_LE)
> -				val = get_unaligned_le16(data_u16);
> +				val = get_unaligned_le16(chan_data);
>  			else /* IIO_CPU */
> -				val = *data_u16;
> +				val = *(const u16 *)chan_data;
>  			break;
>  		default:
>  			return -EINVAL;