On Sunday 02 May 2010 13:14:59 David H. Lynch Jr. wrote:
> On 05/02/2010 03:47 AM, Benoit Papillault wrote:
> > Le 01/05/2010 22:45, Christian Lamparter a écrit :
> >> On Saturday 01 May 2010 20:23:26 David H. Lynch Jr. wrote:
> >>> I think I can tell you what I am supposed add - I need to be able
> >>> to provide userspace apps with precise timing information for each
> >>> packet.
> >>> Since i am working on GPL'd code and the results are going to be
> >>> provided to third parties whatever I do is GPL'd too.
> >>
> >> if by precise timing you mean "exact mac time in TU/usecs when frame
> >> was received at the radio", then you might have a _problem_.
> >> You see, the firmware already receives fully packed frames from
> >> the MAC processor and only _forwards_ them all [as is] in
> >> one big DMA to the USB subsystem.
> >> (this is done in src/wlan.c handle_rx())
> >>
> >> So, unless the HW has a _magic_ flag to enable this capability...
> >> you are sort of screwed :-/.
> >
> > I would love this feature as well. I have a device to test if that can
> > help. I share the feeling of Christian however... but maybe your
> > documentation says something about a special flag.
> I am interested in round trip time as measured from some fixed point
> in the sending process to some fixed point in the packet acknowledgement.
> The more accurate the better. Preferably measured by events at the
> radio rather than on the linux side.
> I am interested in tx packets rather than rx packets.
ahh, well... there goes' benoit interest ;).
> If necessary I can measure the times myself as delta's from one
> event to another withing the SH2.
>
> I have not digested the docs I have thoroughly yet but a cursory
> review suggests a less than trivial project.
> I have not yet found a good high resolution clock inside the ar9170
> there are alot of clocks but they all seem to be 16bit. Probably that
> will make things harder.
TSF timer could be used for this. (see 802.11-2007, Section 11 et seq.)
It's a 2^64 bit timer with a 1us resolution and a accuracy _better_
or equal to +/- 0.01%.
The register address are at:
- 0x1c3514 (low, AR9170_MAC_REG_TSF_L, read only)
- 0x1c3518 (high, AR9170_MAC_REG_TSF_H, read only)
But it comes at a small price: this timer is sometimes
update & synchronized (802.11 11.1.3.4 and 11.1.4) in
station or ad-hoc mode. The exact details are hidden
inside MAC chip, but it should be possible to disable
both by selecting the monitor mode.
> I was expecting to have to make changes to the ar9170 firmware. I
> was expecting to have to devise some means of passing that information
> to the Linux driver and to the userspace application.
Well, then you have two more good reasons why to use carl9170:
* the ar9170usb driver and ar9170 firmware don't track tx frames.
carl9170 on the other hand does (every frame has a 8 bit "cookie").
This feature was necessary to generate an accurate tx transmission
feedback report for every individual frame for the driver.
* carl9170 has the ability to store additional per-frame data.
In fact, if you don't need to have a different retry rates
you could realloc the 3 * 32 bit "rr" (as in retry rate)
array in the carl9170_tx_superdesc and _carl9170_tx_superdesc
struct (see wlan.h) for your purposes (storage for your time values).
And if you fetched all the data, everything will be sent
with an ordinary tx status feedback report to the application
(add the timer fields into carl9170_tx_status and _carl9170_tx_status
struct - see fwcmd.h)
(* talked about this earlier, but you never know...
carlu _tool_ already provides a low-level HW driver for userspace.
This has the obvious advantage that you won't need to mess with
the kernel driver and network stacks.
The only work you'll have to do is: get the kernel code for
the MAC & PHY initialization and put it into carlu.
But the framework is already there so it's mostly copy + paste )
> I would be happy to do that in some fashion that conformed to an existing
> or future standard, but I was not anticipating a broad desire for what I am
> doing. Variable latencies are highly undesirable in this application,
> but the userspace application will be aggregating large amounts of
> information, if latencies in what is measured are constrained and the
> unit of time measurement is small enough everything will work.
> If it comes to that we switch to different hardware, but my project
> is bringing a concept that was demonstrated with an expensive SDR to an
> ar9170.
It's always nice to have some "added value" for cheap and generic devices.
e.g.: Atheros AR92xx chips can be used as among other stuff as a
full range spectrum analyzer.
Regards,
Chr
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