On Fri, 24 Jan 2025 17:01:23 +0100
Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxx> wrote:
> Checking the binary representation of two structs (of the same type)
> for equality doesn't have the same semantic as comparing all members for
> equality. The former might find a difference where the latter doesn't in
> the presence of padding or when ambiguous types like float or bool are
> involved. (Floats typically have different representations for single
> values, like -0.0 vs +0.0, or 0.5 * 2² vs 0.25 * 2³. The type bool has
> at least 8 bits and the raw values 1 and 2 (probably) both evaluate to
> true, but memcmp finds a difference.)
>
> When searching for a channel that already has the configuration we need,
> the comparison by member is the one that is needed.
>
> Convert the comparison accordingly to compare the members one after
> another. Also add a BUILD_BUG guard to (somewhat) ensure that when
> struct ad7124_channel_config::config_props is expanded, the comparison
> is adapted, too.
>
> Fixes: 7b8d045e497a ("iio: adc: ad7124: allow more than 8 channels")
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxx>
Hi Uwe,
Fully agree the change makes sense. Though the floating point example
is unlikely to bite us in kernel!
Did you see an actual failure to match? If not I'd be tempted
to take this for next cycle rather than as a fix to rush in.
I'd be surprised if current code doesn't happen to work.
Thanks,
Jonathan
> ---
> drivers/iio/adc/ad7124.c | 29 ++++++++++++++++++++++++++---
> 1 file changed, 26 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/iio/adc/ad7124.c b/drivers/iio/adc/ad7124.c
> index 6ae27cdd3250..5eb8ced416ba 100644
> --- a/drivers/iio/adc/ad7124.c
> +++ b/drivers/iio/adc/ad7124.c
> @@ -338,15 +338,38 @@ static struct ad7124_channel_config *ad7124_find_similar_live_cfg(struct ad7124_
> struct ad7124_channel_config *cfg)
> {
> struct ad7124_channel_config *cfg_aux;
> - ptrdiff_t cmp_size;
> int i;
>
> - cmp_size = sizeof_field(struct ad7124_channel_config, config_props);
> + /*
> + * This is just to make sure that the comparison is adapted after
> + * struct ad7124_channel_config was changed.
> + */
> + BUILD_BUG_ON(sizeof_field(struct ad7124_channel_config, config_props) !=
> + sizeof(struct {
> + enum ad7124_ref_sel refsel;
> + bool bipolar;
> + bool buf_positive;
> + bool buf_negative;
> + unsigned int vref_mv;
> + unsigned int pga_bits;
> + unsigned int odr;
> + unsigned int odr_sel_bits;
> + unsigned int filter_type;
> + }));
> +
> for (i = 0; i < st->num_channels; i++) {
> cfg_aux = &st->channels[i].cfg;
>
> if (cfg_aux->live &&
> - !memcmp(&cfg->config_props, &cfg_aux->config_props, cmp_size))
> + cfg->refsel == cfg_aux->refsel &&
> + cfg->bipolar == cfg_aux->bipolar &&
> + cfg->buf_positive == cfg_aux->buf_positive &&
> + cfg->buf_negative == cfg_aux->buf_negative &&
> + cfg->vref_mv == cfg_aux->vref_mv &&
> + cfg->pga_bits == cfg_aux->pga_bits &&
> + cfg->odr == cfg_aux->odr &&
> + cfg->odr_sel_bits == cfg_aux->odr_sel_bits &&
> + cfg->filter_type == cfg_aux->filter_type)
> return cfg_aux;
> }
>
>
> base-commit: 5ffa57f6eecefababb8cbe327222ef171943b183
