Re: [PATCH] rtl8187b: do not do per packet TX AGC

[Larry Finger <Larry.Finger@lwfinger.net>]

On 08/30/2010 09:12 AM, John W. Linville wrote:
> On Sat, Aug 28, 2010 at 04:32:48PM -0500, Larry Finger wrote:
>> On 08/28/2010 12:54 AM, Thadeu Lima de Souza Cascardo wrote:
>>> The code for rtl8187 does not do per packet TX AGC. Resetting the per
>>> packet TX AGC for rtl8187b appears to increase its overall TX power.
>>> This allows the device to associate and a connection be established
>>> using APs a little further away.
>>>
>>> Tested-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
>>> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
>>> Cc: linux-wireless@vger.kernel.org
>>> Cc: Larry Finger <Larry.Finger@lwfinger.net>
>>> Cc: Rogerio Luz Coelho <rogluz.news@gmail.com>
>>> Cc: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
>>> Cc: Hin-Tak Leung <hintak.leung@gmail.com>
>>> ---
>>
>> I'm a little confused here. The subject says not to set the per packet TX AGC,
>> while the submission text says that setting it helps. I don't have access to any
>> documents that describe this register, but I expect that clearing
>> RTL818X_TX_AGC_CTL_PERPACKET_GAIN_SHIFT would disable that feature. As a result
>> the code change seems to match the subject.
>>
>> I have not yet tested this code, but I do note that all the Realtek drivers for
>> the RTL8187B set this bit.
>>
>> Until testing, I withhold judgment on the actual code change, but NACK for the
>> inconsistency.
> 
> Perhaps the word "clearing" rather than "resetting" would be less confusing?

I see the source of my confusion. I am in the habit of using lower case to
indicate the driver and upper case for the device. I would rewrite the commit
code to say "The code for the RTL8187L does not do per packet TX AGC. Clearing
the per packet TX AGC for RTL8187B ..."

I have now tested the patch. For my RTL8187B device, I get the following:

Distance from AP      Indicated rate       Signal       Measured TX rate
      2m                   18 Mb/s         -17 dBm      10 Mb/s
     10m(original)         11 Mb/s         -48 dBm      2.6 Mb/s
     10m(with patch)       11 Mb/s         -43 dBm      4.1 Mb/s

The measured TX rates were obtained using tcpperf. At a distance of 2 m, the
results were the same with and without the patch. At 10 m, the patch went
through 2 exterior walls with drywall, insulation, and stucco. I'm not sure that
the increase from 2.6 to 4.1 Mb/s is reproducible. About all I can say is that
the change did not hurt the performance of the device, which is already pretty bad.

If the OP can post numbers documenting the improvement, I have no objection to
this patch, even though every Realtek driver that I have seen turns "per packet
TX AGC" on for the RTL8187B and turns it off for the RTL8187L.

Larry