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

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Date:   Mon, 22 Aug 2022 11:03:03 +0200
From:   Paul Cercueil <paul@...pouillou.net>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Artur Rojek <contact@...ur-rojek.eu>,
        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

Hi Jonathan,

Le ven., août 19 2022 at 18:53:39 +0100, Jonathan Cameron 
<jic23@...nel.org> a écrit :
> 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.

Yes, the comment above says that it's used to hide type casting.

Cheers,
-Paul

>>   			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;
>