Re: [PATCH] ath9k: spectral - simplify max_index calculation

[Nick Kossifidis <mickflemm@gmail.com>]

max_index is a 6bit signed integer in both cases (sorry for the 5bit
typo in the comments), so the current function handles it correctly
for both HT20 and dynamic HT20/40 modes (I've tested it extensively).
Also you don't handle the negative indices we get from the hardware
(you just remove the sign). Have you tested this and if you did on
which hardware did you do the test ?

2015-06-16 11:21 GMT+02:00 Zefir Kurtisi <zefir.kurtisi@neratec.com>:
> The max_index value provided in the spectral data set
> has to be interpreted differently for HT20 and HT40.
>
> The spectral module initially supported HT20 only
> and the spectral_max_index() function handled the
> conversion from signed to unsigned.
>
> In HT40, the max_index is an unsigned value and does
> not need to be fixed. When HT40 support was added,
> the function was exteded to handle the conversion
> for HT20 based on the provided num_bins parameter.
>
> This is misleading and complex. Instead this patch
> shifts the correction of the value to the caller,
> which in effect reduces it to a singel bit-operation
> for HT20 data.
>
> Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
> ---
>  drivers/net/wireless/ath/ath9k/common-spectral.c | 20 ++++++-------
>  drivers/net/wireless/ath/ath9k/common-spectral.h | 36 +++++++-----------------
>  2 files changed, 18 insertions(+), 38 deletions(-)
>
> diff --git a/drivers/net/wireless/ath/ath9k/common-spectral.c b/drivers/net/wireless/ath/ath9k/common-spectral.c
> index a876271..e4bae40 100644
> --- a/drivers/net/wireless/ath/ath9k/common-spectral.c
> +++ b/drivers/net/wireless/ath/ath9k/common-spectral.c
> @@ -59,8 +59,8 @@ ath_cmn_max_idx_verify_ht20_fft(u8 *sample_end, int bytes_read)
>
>         sample = sample_end - SPECTRAL_HT20_SAMPLE_LEN + 1;
>
> -       max_index = spectral_max_index(mag_info->all_bins,
> -                                      SPECTRAL_HT20_NUM_BINS);
> +       /* in ht20, this is a 6-bit signed number => shift it to 0 */
> +       max_index = spectral_max_index(mag_info->all_bins) ^ 0x20;
>         max_magnitude = spectral_max_magnitude(mag_info->all_bins);
>
>         max_exp = mag_info->max_exp & 0xf;
> @@ -100,12 +100,10 @@ ath_cmn_max_idx_verify_ht20_40_fft(u8 *sample_end, int bytes_read)
>         sample = sample_end - SPECTRAL_HT20_40_SAMPLE_LEN + 1;
>
>         lower_mag = spectral_max_magnitude(mag_info->lower_bins);
> -       lower_max_index = spectral_max_index(mag_info->lower_bins,
> -                                            SPECTRAL_HT20_40_NUM_BINS);
> +       lower_max_index = spectral_max_index(mag_info->lower_bins);
>
>         upper_mag = spectral_max_magnitude(mag_info->upper_bins);
> -       upper_max_index = spectral_max_index(mag_info->upper_bins,
> -                                            SPECTRAL_HT20_40_NUM_BINS);
> +       upper_max_index = spectral_max_index(mag_info->upper_bins);
>
>         max_exp = mag_info->max_exp & 0xf;
>
> @@ -169,8 +167,8 @@ ath_cmn_process_ht20_fft(struct ath_rx_status *rs,
>         magnitude = spectral_max_magnitude(mag_info->all_bins);
>         fft_sample_20.max_magnitude = __cpu_to_be16(magnitude);
>
> -       max_index = spectral_max_index(mag_info->all_bins,
> -                                       SPECTRAL_HT20_NUM_BINS);
> +       /* in ht20, this is a 6-bit signed number => shift it to 0 */
> +       max_index = spectral_max_index(mag_info->all_bins) ^ 0x20;
>         fft_sample_20.max_index = max_index;
>
>         bitmap_w = spectral_bitmap_weight(mag_info->all_bins);
> @@ -302,12 +300,10 @@ ath_cmn_process_ht20_40_fft(struct ath_rx_status *rs,
>         upper_mag = spectral_max_magnitude(mag_info->upper_bins);
>         fft_sample_40.upper_max_magnitude = __cpu_to_be16(upper_mag);
>
> -       lower_max_index = spectral_max_index(mag_info->lower_bins,
> -                                       SPECTRAL_HT20_40_NUM_BINS);
> +       lower_max_index = spectral_max_index(mag_info->lower_bins);
>         fft_sample_40.lower_max_index = lower_max_index;
>
> -       upper_max_index = spectral_max_index(mag_info->upper_bins,
> -                                       SPECTRAL_HT20_40_NUM_BINS);
> +       upper_max_index = spectral_max_index(mag_info->upper_bins);
>         fft_sample_40.upper_max_index = upper_max_index;
>
>         lower_bitmap_w = spectral_bitmap_weight(mag_info->lower_bins);
> diff --git a/drivers/net/wireless/ath/ath9k/common-spectral.h b/drivers/net/wireless/ath/ath9k/common-spectral.h
> index 998743b..4cc5a6c 100644
> --- a/drivers/net/wireless/ath/ath9k/common-spectral.h
> +++ b/drivers/net/wireless/ath/ath9k/common-spectral.h
> @@ -116,33 +116,17 @@ static inline u16 spectral_max_magnitude(u8 *bins)
>                (bins[2] & 0x03) << 10;
>  }
>
> -/* return the max magnitude from the all/upper/lower bins */
> -static inline u8 spectral_max_index(u8 *bins, int num_bins)
> +/* return the max magnitude from the all/upper/lower bins
> + *
> + * in HT20: 6-bit signed number of range -28 to +27
> + * in HT40: 6-bit unsigned number of range 0 to +63
> + *          (upper sub-channel index 0 is DC)
> + *
> + * Correct interpretation of the value has to be done at caller
> + */
> +static inline u8 spectral_max_index(u8 *bins)
>  {
> -       s8 m = (bins[2] & 0xfc) >> 2;
> -       u8 zero_idx = num_bins / 2;
> -
> -       /* It's a 5 bit signed int, remove its sign and use one's
> -        * complement interpretation to add the sign back to the 8
> -        * bit int
> -        */
> -       if (m & 0x20) {
> -               m &= ~0x20;
> -               m |= 0xe0;
> -       }
> -
> -       /* Bring the zero point to the beginning
> -        * instead of the middle so that we can use
> -        * it for array lookup and that we don't deal
> -        * with negative values later
> -        */
> -       m += zero_idx;
> -
> -       /* Sanity check to make sure index is within bounds */
> -       if (m < 0 || m > num_bins - 1)
> -               m = 0;
> -
> -       return m;
> +       return (bins[2] & 0xfc) >> 2;
>  }
>
>  /* return the bitmap weight from the all/upper/lower bins */
> --
> 1.9.1
>



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As you read this post global entropy rises. Have Fun ;-)
Nick