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Date: Tue, 17 Jan 2012 19:10:14 -0600
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Subject: Re: carl9170: How to control transmit power?
From: Harshal Chhaya <harshal@gmail.com>
To: Christian Lamparter <chunkeey@googlemail.com>
Cc: linux-wireless <linux-wireless@vger.kernel.org>
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Sender: linux-wireless-owner@vger.kernel.org

On Wed, Jan 11, 2012 at 5:41 PM, Christian Lamparter
<chunkeey@googlemail.com> wrote:
> On Thursday, January 12, 2012 12:07:54 AM Harshal Chhaya wrote:
>> Does the carl9170 driver/firmware allow real-time updates to the tx
>> power parameter?
>
> No. not yet.
>
>> On my AR9170-based AP, I can't set the tx power using either 'iw' or 'iwconfig'.
>>
>> 'iw' fails with an error message:
>>
>> root@OpenWrt:/etc/hostapd# iw wlan0 set txpower fixed 10
>> command failed: Operation not supported (-95)
>
> iw uses mBm (millibels referenced to 1 milliwatt) rather than the more familiar dBm.
> try: iw wlan0 set txpower fixed 1000
>
>> 'iwconfig' seems to update the information that's displayed but it
>> doesn't affect the actual tx power
[..]
>> Is there another way to set the tx power? I want to reduce the range
>> of my AP and am trying to lower the tx power.
>
> see attached patch. But I have to add that it won't change the
> txpower of frames which are generated by the MAC [like RTS/CTS and
> (B)ACKS].
>
> Best Regards and good night.
> ---
> diff --git a/drivers/net/wireless/ath/carl9170/tx.c b/drivers/net/wireless/ath/carl9170/tx.c
> index d19a9ee..771e1a9 100644
> --- a/drivers/net/wireless/ath/carl9170/tx.c
> +++ b/drivers/net/wireless/ath/carl9170/tx.c
> @@ -719,6 +719,8 @@ static void carl9170_tx_rate_tpc_chains(struct ar9170 *ar,
> ? ? ? ? ? ? ? ?else
> ? ? ? ? ? ? ? ? ? ? ? ?*chains = AR9170_TX_PHY_TXCHAIN_2;
> ? ? ? ?}
> +
> + ? ? ? *tpc = min_t(unsigned int, *tpc, ar->hw->conf.power_level * 2);
> ?}
>
> ?static __le32 carl9170_tx_physet(struct ar9170 *ar,

Christian,

Thank you very much for this patch. I can now use 'iw' to set the
transmit power.

root@OpenWrt:/# iw wlan0 set txpower fixed 1000 	- 	sets it to 10dBm

root@OpenWrt:/# iw wlan0 set txpower fixed 100 	- 	sets it to 1 dBm

And I can see the difference in the transmitted packets.

Do you have a suggestion on how to implement this in an older version
of compat-wireless (specifically
compat-wireless-2011-04-19)?

This is because your beacon IE reordering suggestion from a few months
back (http://marc.info/?l=linux-wireless&m=131643758607833&w=2 )  was
critical in having a usable network.

The tx.c file has changed considerably since then and I can't find the
right place to apply tx power change.

How do I change the carl9170/tx.c file from compat-wireless-2011-04-19
to support tx power control? It doesn't have the
carl9170_tx_rate_tpc_chains() function. But it has a
carl9170_tx_physet() function. Can I do something like

        power = min_t(unsigned int, *tpc, ar->hw->conf.power_level * 2);

in this function after
        power <<= AR9170_TX_PHY_TX_PWR_S;
        power &= AR9170_TX_PHY_TX_PWR;
but before
        tmp |= cpu_to_le32(power);


Alternatively, can you suggest where do I apply your beacon IE
reordering suggestion in a recent (e.g. from 2011-11-15)
compat-wireless release? You had said:

in net/mac80211/tx.c look for the NL80211_IFTYPE_AP case
and move:

	if (beacon->tail)
		memcpy(skb_put(skb, beacon->tail_len),
				beacon->tail, beacon->tail_len);

in front of
	if (local->tim_in_locked_section) {


Thanks again for all your help.

- Harshal