Re: [PATCH v2 2/3] station: improve scan_freqs_order channel subsets

[James Prestwood <prestwoj@gmail.com>]

Hi Alexander,

On 8/29/25 12:43 AM, Alexander Ganslandt wrote:
> From: Alexander Ganslandt <alexanga@axis.com>
>
> Splits the scan frequencies into more subsets that have been ordered
> such that the more common frequencies appear first, and the more uncommon
> frequencies last. Non-DFS channels are also added to the earlier
> subsets to prioritize fast-scanning frequencies.
>
> This approach allows iwd to scan the frequencies with the statistically
> highest chance for BSSes first, resulting in shorter scan times until a
> good BSS is found. In future patches this will also be used when
> roaming.
> ---
>   src/station.c | 122 ++++++++++++++++++++++++++++++++++++++++++----------------
>   1 file changed, 89 insertions(+), 33 deletions(-)
>
> diff --git a/src/station.c b/src/station.c
> index f8069d89..c2e18fc9 100644
> --- a/src/station.c
> +++ b/src/station.c
> @@ -125,7 +125,7 @@ struct station {
>   	struct l_queue *roam_bss_list;
>   
>   	/* Frequencies split into subsets by priority */
> -	struct scan_freq_set *scan_freqs_order[3];
> +	struct scan_freq_set *scan_freqs_order[5];
>   	unsigned int dbus_scan_subset_idx;
>   
>   	uint32_t wiphy_watch;
> @@ -4500,8 +4500,7 @@ static bool station_dbus_scan_results(int err, struct l_queue *bss_list,
>   		return false;
>   	}
>   
> -	last_subset = next_idx >= L_ARRAY_SIZE(station->scan_freqs_order) ||
> -		station->scan_freqs_order[next_idx] == NULL;
> +	last_subset = next_idx >= L_ARRAY_SIZE(station->scan_freqs_order);
>   	station->dbus_scan_subset_idx = next_idx;
>   
>   	station_set_scan_results(station, bss_list, freqs, false);
> @@ -4516,6 +4515,15 @@ static bool station_dbus_scan_subset(struct station *station)
>   {
>   	unsigned int idx = station->dbus_scan_subset_idx;
>   
> +	/* Find the next non-empty subset */
> +	while (idx < L_ARRAY_SIZE(station->scan_freqs_order) &&
> +			scan_freq_set_isempty(station->scan_freqs_order[idx]))
> +		idx++;
> +	station->dbus_scan_subset_idx = idx;
> +
> +	if (idx >= L_ARRAY_SIZE(station->scan_freqs_order))
> +		return false;
> +
>   	station->dbus_scan_id = station_scan_trigger(station,
>   						station->scan_freqs_order[idx],
>   						station_dbus_scan_triggered,
> @@ -5061,40 +5069,91 @@ static void station_fill_scan_freq_subsets(struct station *station)
>   				wiphy_get_supported_freqs(station->wiphy);
>   	unsigned int subset_idx = 0;
>   
> -	/*
> -	 * Scan the 2.4GHz "social channels" first, 5GHz second, if supported,
> -	 * all other 2.4GHz channels last.  To be refined as needed.
> -	 */
> +	station->scan_freqs_order[subset_idx] = scan_freq_set_new();
> +
> +	/* Subset 0: 2.4GHz "social channels" and low 5GHz non-DFS channels */
>   	if (allowed_bands & BAND_FREQ_2_4_GHZ) {
> -		station->scan_freqs_order[subset_idx] = scan_freq_set_new();
> +		/* Channels 1, 6, 11 */
>   		scan_freq_set_add(station->scan_freqs_order[subset_idx], 2412);
>   		scan_freq_set_add(station->scan_freqs_order[subset_idx], 2437);
>   		scan_freq_set_add(station->scan_freqs_order[subset_idx], 2462);
> -		subset_idx++;
>   	}
>   
> -	/*
> -	 * TODO: It may might sense to split up 5 and 6ghz into separate subsets
> -	 *       since the channel set is so large.
> -	 */
> -	if (allowed_bands & (BAND_FREQ_5_GHZ | BAND_FREQ_6_GHZ)) {
> -		uint32_t mask = allowed_bands &
> -					(BAND_FREQ_5_GHZ | BAND_FREQ_6_GHZ);
> -		struct scan_freq_set *set = scan_freq_set_clone(supported,
> -								mask);
> -
> -		/* 5/6ghz didn't add any frequencies */
> -		if (scan_freq_set_isempty(set)) {
> -			scan_freq_set_free(set);
> -		} else
> -			station->scan_freqs_order[subset_idx++] = set;
> +	if (allowed_bands & BAND_FREQ_5_GHZ) {
> +		/* Channels 32 - 48 */
> +		for (int i = 5160; i <= 5240; i+=20) {
> +			scan_freq_set_add(station->scan_freqs_order[subset_idx], i);
> +		}
No need for the braces around this loop and others below.
>   	}
>   
> -	/* Add remaining 2.4ghz channels to subset */
> +	scan_freq_set_constrain(station->scan_freqs_order[subset_idx], supported);
> +	station->scan_freqs_order[++subset_idx] = scan_freq_set_new();
> +
> +	/* Subset 1: Remaining common 2.4GHz channels and high 5GHz non-DFS channels */
>   	if (allowed_bands & BAND_FREQ_2_4_GHZ) {
> -		station->scan_freqs_order[subset_idx] = scan_freq_set_new();
> -		scan_freq_set_foreach(supported, station_add_2_4ghz_freq,
> -					station->scan_freqs_order[subset_idx]);
> +		/* Channels 2 - 10, except 6 */
> +		for (int i = 2417; i < 2462; i+=5) {
> +			if (i != 2437)
> +				scan_freq_set_add(station->scan_freqs_order[subset_idx], i);
> +		}
> +	}
> +
> +	if (allowed_bands & BAND_FREQ_5_GHZ) {
> +		/* Channels 149 - 177 */
> +		for (int i = 5745; i <= 5885; i+=20) {
> +			scan_freq_set_add(station->scan_freqs_order[subset_idx], i);
> +		}
> +	}
> +
> +	scan_freq_set_constrain(station->scan_freqs_order[subset_idx], supported);
> +	station->scan_freqs_order[++subset_idx] = scan_freq_set_new();
> +
> +	/* Subset 2: Uncommon 2.4GHz channels and 5GHz DFS channels */
> +	if (allowed_bands & BAND_FREQ_2_4_GHZ) {
> +		/* Channels 12 - 14 */
> +		scan_freq_set_add(station->scan_freqs_order[subset_idx], 2467);
> +		scan_freq_set_add(station->scan_freqs_order[subset_idx], 2472);
> +		scan_freq_set_add(station->scan_freqs_order[subset_idx], 2484);
> +	}
> +
> +	if (allowed_bands & BAND_FREQ_5_GHZ) {
> +		/* Channels 52 - 68 */
> +		for (int i = 5260; i <= 5340; i+=20) {
> +			scan_freq_set_add(station->scan_freqs_order[subset_idx], i);
> +		}
> +
> +		/* Channels 96 - 144 */
> +		for (int i = 5480; i <= 5720; i+=20) {
> +			scan_freq_set_add(station->scan_freqs_order[subset_idx], i);
> +		}
> +	}
> +
> +	scan_freq_set_constrain(station->scan_freqs_order[subset_idx], supported);
> +	station->scan_freqs_order[++subset_idx] = scan_freq_set_new();
> +
> +	/* Subset 3: 6GHz channels */
> +	if (allowed_bands & BAND_FREQ_6_GHZ) {
> +		struct scan_freq_set *set = scan_freq_set_clone(supported, BAND_FREQ_6_GHZ);
> +
> +		if (scan_freq_set_isempty(set))
> +			scan_freq_set_free(set);
> +		else
> +			scan_freq_set_merge(station->scan_freqs_order[subset_idx], set);
> +	}
> +
> +	scan_freq_set_constrain(station->scan_freqs_order[subset_idx], supported);
> +	station->scan_freqs_order[++subset_idx] = scan_freq_set_clone(supported, allowed_bands);
> +
> +	/* All channels that are both supported and allowed should be in the subsets,
> +	 * if this is not the case then some new channel has been added that we are
> +	 * not tracking, put it in the last subset to make sure it's scanned */
> +	for (int i = 0; i < 4; i++) {
> +		scan_freq_set_subtract(station->scan_freqs_order[subset_idx],
> +				station->scan_freqs_order[i]);
> +	}
> +
> +	if (!scan_freq_set_isempty(station->scan_freqs_order[subset_idx])) {
> +		l_warn("Final subset is not empty");
>   	}
>   
>   	/*
> @@ -5273,11 +5332,8 @@ static void station_free(struct station *station)
>   
>   	l_queue_destroy(station->anqp_pending, remove_anqp);
>   
> -	scan_freq_set_free(station->scan_freqs_order[0]);
> -	scan_freq_set_free(station->scan_freqs_order[1]);
> -
> -	if (station->scan_freqs_order[2])
> -		scan_freq_set_free(station->scan_freqs_order[2]);
> +	for (uint8_t i = 0; i < L_ARRAY_SIZE(station->scan_freqs_order); i++)
> +		scan_freq_set_free(station->scan_freqs_order[i]);
>   
>   	wiphy_state_watch_remove(station->wiphy, station->wiphy_watch);
>   
>