[ath9k-devel] [PATCH v2] ath9k: interpret requested txpower in EIRP domain

[Zefir Kurtisi <zefir.kurtisi@neratec.com>]

On 05/14/2016 02:50 PM, Felix Fietkau wrote:
> On 2016-04-01 11:37, Zefir Kurtisi wrote:
>> Tx power limitations at upper layers are interpreted in
>> the EIRP domain. When the user requests a given maximum
>> txpower, e.g. with: 'iw phy0 set txpower fixed 1500',
>> he expects the EIRP to be at or below 15dBm.
>>
>> In ath9k_hw_apply_txpower(), the interpretation is
>> different: the antenna-gain is capped against the
>> current txpower limit in the regulatory, but not
>> against the user set value. It ensures that the
>> resulting EIRP is below the limit defined by the
>> active countrycode, but not below the value the
>> user requested.
>>
>> In a scenario like e.g.
>>  a) antenna_gain=6
>>  b) countrycode limits to eirp=18
>>  c) user set txpower=15
>> this will cause a setting for AR_PHY_POWER_TX_RATE
>> regs resulting in an EIRP > 15.
>>
>> This patch ensures that antenna-gain is considered
>> whenever the txpower limit is adjusted and with that
>> the user set limits are kept.
>>
>> Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
> I just noticed this change and I believe it should be reverted. In many
> cases the EEPROM antenna gain value does not accurately reflect the real
> antenna gain and is used more as a worst case value to prevent exceeding
> regulatory limits.
> 
> I believe using this to limit the user specified tx power values will
> not only make this inconsistent with other drivers, but it will also
> confuse users by using significantly lower tx power than they wanted.
> 
> The EEPROM antenna gain value is already causing more tx power reduction
> than necessary, because AFAIK at least the FCC regulatory rules allow an
> antenna gain of 3 dB while at the power limit, yet this is not
> subtracted from the EEPROM antenna gain value when considering the limit.
> 
> - Felix
> 
Two things to be considered before reverting that commit:
1) the change affects only setups where a valid antenna gain value is set
I checked some consumer ath9k cards I collected over time from (rather old) WiFi
routers - none of them have an antenna gain set in EEPROM. Therefore, none of them
would be affected by the change (alas I have no current ath9k NICs at hand to
check if this is still valid). I'd argue that valid antenna gain values are set by
manufacturers or professional integrators who took time to calibrate and measure
the antennas' characteristics accurately.

2) EIRP is what matters
As explained in the commit, every layer above the driver is interpreting txpower
in the EIRP domain. When you visit a certification lab and the engineer sets a max
txpower of 15dBm but measures an EIRP of 18 (as in the example above), the device
won't pass the test.


I think the latter point is a strong argument to leave the change intact.



Cheers,
Zefir