[v2,2/2] input: joystick: Fix buffer data parsing

Don't try to access buffer data of a channel by its scan index. Instead,
calculate its offset in the buffer.

This is necessary, as the scan index of a channel does not represent its
position in a buffer - the buffer will contain data for enabled channels
only, affecting data offsets and alignment.

While at it, also fix minor style issue in probe.

Reported-by: Chris Morgan <macromorgan@hotmail.com>
Closes: https://lore.kernel.org/linux-input/20220408212857.9583-1-macroalpha82@gmail.com/
Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Tested-by: Paul Cercueil <paul@crapouillou.net>
---

v2: - provide new implementation for calculating channel offsets 
    - cache the resulting offsets
    - fix minor style issue in probe
    - drop the "Fixes" tag

 drivers/input/joystick/adc-joystick.c | 102 +++++++++++++++++++++++---
 1 file changed, 90 insertions(+), 12 deletions(-)

On Mon, May 22, 2023 at 1:59 AM Artur Rojek <contact@artur-rojek.eu> wrote:
>
> Don't try to access buffer data of a channel by its scan index. Instead,
> calculate its offset in the buffer.
>
> This is necessary, as the scan index of a channel does not represent its
> position in a buffer - the buffer will contain data for enabled channels
> only, affecting data offsets and alignment.
>
> While at it, also fix minor style issue in probe.

a minor
the probe

> Reported-by: Chris Morgan <macromorgan@hotmail.com>
> Closes: https://lore.kernel.org/linux-input/20220408212857.9583-1-macroalpha82@gmail.com/

What is this tag? Anything documented? Otherwise use BugLink or Link.

...

>         struct adc_joystick_axis *axes;
>         struct iio_channel *chans;
> +       int *offsets;

Why not unsigned? I.o.w. is there any meaning for negative values?

>         int num_chans;
>         bool polled;

...

> +               off = joy->offsets[i];

> +

Move this blank line to be before the previous line.

> +               if (off < 0)
> +                       return -EINVAL;

...

>                 case 1:
> -                       val = ((const u8 *)data)[idx];
> +                       val = *chan_data;

Might be also

   get_unaligned((const u8 *)chan_data);

And with all this (see below) the chan_data actually can be declared
as const void *.

>                         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);
>                         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;

This probably needs to be

   get_unaligned((const u16 *)chan_data);

for the sake of consistency with the above.

>                         break;
>                 default:
>                         return -EINVAL;

...

> +static int adc_joystick_si_cmp(const void *a, const void *b, const void *priv)
> +{
> +       const struct iio_channel *chans = priv;
> +
> +       return chans[*(int *)a].channel->scan_index -
> +              chans[*(int *)b].channel->scan_index;

Discarding const?

> +}

...

> +       offsets = kmalloc_array(count, sizeof(int), GFP_KERNEL);

sizeof(*offsets)

> +       if (!offsets)
> +               return ERR_PTR(-ENOMEM);

...

> +       si_order = kmalloc_array(count, sizeof(int), GFP_KERNEL);

sizeof(*si_order)

> +       if (!si_order) {
> +               kfree(offsets);
> +               return ERR_PTR(-ENOMEM);
> +       }

...

> +       /* Channels in buffer are ordered by scan index. Sort to match that. */

the buffer

> +       sort_r(si_order, count, sizeof(int), adc_joystick_si_cmp, NULL, chans);

sizeof(*si_order) ?

sizeof(int) is a bit odd, the above will tell better without even
knowing the sort_r() parameters what it is about.

...

> +       for (i = 0; i < count; ++i) {
> +               idx = si_order[i];
> +               ch = chans[idx].channel;
> +               si = ch->scan_index;
> +
> +               if (si < 0 || !test_bit(si, indio_dev->active_scan_mask)) {
> +                       offsets[idx] = -1;
> +                       continue;
> +               }
> +
> +               /* Channels sharing scan indices also share the samples. */
> +               if (idx > 0 && si == chans[idx - 1].channel->scan_index) {
> +                       offsets[idx] = offsets[idx - 1];
> +                       continue;
> +               }
> +
> +               offsets[idx] = offset;

> +               length = ch->scan_type.storagebits / 8;

BITS_PER_BYTE ?

> +               if (ch->scan_type.repeat > 1)
> +                       length *= ch->scan_type.repeat;

> +               /* Account for channel alignment. */
> +               if (offset % length)
> +                       offset += length - (offset % length);

Would one of ALIGN() / rounddown / etc work here?

> +               offset += length;
> +       }

...

> +       joy->offsets = adc_joystick_get_chan_offsets(joy->chans,
> +                                                    joy->num_chans);
> +       if (IS_ERR(joy->offsets)) {
> +               dev_err(devp, "Unable to allocate channel offsets\n");

> +               return PTR_ERR(joy->offsets);
> +       }
> +
> +       return 0;

return PTR_ERR_OR_ZERO() ?

...

>                 error = devm_add_action_or_reset(dev, adc_joystick_cleanup,
>                                                  joy->buffer);
> -               if (error)  {
> +               if (error) {
>                         dev_err(dev, "Unable to add action\n");
>                         return error;
>                 }

Unrelated change.