On Mon, Sep 21, 2009 at 9:04 PM, Christian Lamparter
<chunkeey@xxxxxxxxxxxxxx> wrote:
> On Monday 21 September 2009 20:19:35 Andre K wrote:
>> Hi all,
>>
>> I've been using the new p54spi driver on a couple of n800s. Some specifics:
>> Kernel : 2.6.29-omap1
>> Wireless-compat: compat-wireless-2009-08-22
>> OS: Maemo
>> Options: disabled CONFIG_RFKILL_BACKPORT_LEDS because it didn't compile
>>
>> The driver loads and functions normally. However, the values for the
>> dbm_antsignal (from RadioTap header), are they RSSI or dbm? Initially
>> I thought they were dbm since tcpdump showed negative values, which
>> makes sense:
>>
>> Tcpdump:
>> 14:08:25.312988 4469382526us tsft 2.0 Mb/s 2437 MHz (0x00a0) -49dB
>> signal -41dB noise antenna 0 [0x0000000e] Acknowledgment RA:00:1d:e0:3d:5a:53 (oui Unknown)
> heh, noise is higher than the signal.
>
>> 14:08:25.313903 4469383345us tsft 1.0 Mb/s 2437 MHz (0x00a0) 4dB
>> signal -41dB noise antenna 0 [0x0000000e] 04:c2:08:00:07:07 (oui
>> Unknown) Unknown SSAP 0x0a >
>>
>> Which shows up the first positive dbm value. Next I set up another
>> n800 to broadcast ping packets (using hping3, since busybox's ping
>> doesn't allow broadcast).
>>
>> 14:12:22.066192 13296010370us tsft 1.0 Mb/s 2437 MHz (0x00a0) 87dB
>> signal -62dB noise antenna 0 [0x0000000e] IP 192.168.10.11 >
>> 192.168.10.255: ICMP echo request, id 6662, seq 40704, length 8
>> 14:12:22.092376 13296036864us tsft 18.0 Mb/s 2437 MHz (0x00c0) 82dB
>> signal -62dB noise antenna 0 [0x0000000e] IP 192.168.10.11 >
>> 192.168.10.255: ICMP echo request, id 6662, seq 40960, length 8
>> 14:12:22.094787 13296037678us tsft 1.0 Mb/s 2437 MHz (0x00a0) 112dB
>> signal -62dB noise antenna 0 [0x0000000e] IP 192.168.10.11 >
>> 192.168.10.255: ICMP echo request, id 6662, seq 40960, length 8
>>
>> I get strange dbm values, in the range of 50 - 100dbm, which doesn't
>> make much sense.
>>
>> Does anyone have an idea what value is being returned and if there is
>> an error in the dbm calculation?
>>
>
> no, this has been on the TODO list for some time now.
>
> (drivers/net/wireless/p54/txrx.c)
> static int p54_rssi_to_dbm(struct p54_common *priv, int rssi)
> {
> int band = priv->hw->conf.channel->band;
>
> if (priv->rxhw != 5)
> return ((rssi * priv->rssical_db[band].mul) / 64 +
> priv->rssical_db[band].add) / 4;
> else
> /*
> * TODO: find the correct formula
> */
> return ((rssi * priv->rssical_db[band].mul) / 64 +
> priv->rssical_db[band].add) / 4;
> }
>
> the .mul & .add values can be retrieved from the p54spi_eeprom.h blob.
>
> 0x09, 0x00, 0xad, 0xde, /* PDR_RSSI_LINEAR_APPROXIMATION_CUSTOM */
> 0x0a, 0x01, 0x72, 0xfe, 0x1a, 0x00, 0x00, 0x00,
> 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
>
> => translates into: (best viewed with a fixed-width font)
> .mul = 0x010a = 416 (base 10)
> .add = 0xfe72 = -398
> .longbow_unkn1 = 0x001a = 26
> .longbow_unkn2 = 0x0000 = 0
>
> ---
>
> stlc45xx has a completely different phy/rf (longbow - rxhw = 5) and
> the specs are not a part of the freely available fw iface documentation.
>
> here's the relevant quote from a Nokia official: "
>> If this is so, what's so secret about the values that it needs a
>> binary-only tool and hidden storage area? Is there legal issues with
>> it (related to the radio device regulations)?
>
> Yes, this is because regulatory requirements. So thank FCC and ETSI
> for all this. This causes much grief for all wireless vendors, for
> example Intel had to do major changes in iwl3945 firmware for their
> mac80211 driver. "
> (ref: http://www.mail-archive.com/stlc45xx-devel@xxxxxxxxxxxxxxxx/msg00021.html )
I've actually read somewhere (AFAIK on this list, but I am not sure)
that this is actually a misinterpretation of the FCC requirements...
>
> ---
>
> if you need real dBm reading for signal and noise, you have
> to some "reverse engineering" on your own or find someone else
> with the hardware and some spare time to burn.
>
> Regards,
> Chr
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