Re: How does the rate adaptive mask work on Realtek WiFi driver

[邱名碩 <ccchiu77@gmail.com>]

Pkshih <pkshih@realtek.com> 於 2021年5月14日 週五 下午2:08寫道：
>
>
> > -----Original Message-----
> > From: 邱名碩 [mailto:ccchiu77@gmail.com]
> > Sent: Monday, May 10, 2021 4:36 PM
> > To: Pkshih; Andy Huang; Larry.Finger@lwfinger.net; kuba@kernel.org; kvalo@codeaurora.org; Reto
> > Schneider; linux-wireless@vger.kernel.org; linux-kernel@vger.kernel.org
> > Subject: How does the rate adaptive mask work on Realtek WiFi driver
> >
> > Hi guys,
> >     I had a problem while verifying the ampdu tx throughput with the
> > rtl8xxxu driver on RTL8188CUS module. The throughput number is
> > relatively good, 39~42Mbps  TCP on 2.4GHz channel. However, the
> > retransmission rate is high, it's 15% ~ 21% with rtl8xxxu driver and
> > It's almost the same result with the rtl8192cu driver. I can get
> > averagely 7~10% retransmission rate in the same test bed with Realtek
> > vendor driver.
> >
> >     From the air capture, I can see the rtl8xxxu driver keep sending
> > the aggregated frames in MCS7 and doesn't even fall back to lower MCS
> > index in the subsequent retries. I can only see very few retried
> > packets been sent with MCS0 or 6Mbps grate. On the vendor driver, I'll
> > see the retried ampdu packets with MCS4 after 3 retries w/o ack from
> > the receiver.
> >
> >     From the rate mask command issued by the h2c command, I force both
> > the rtl8xxxu driver and vendor driver to use the same ratemask 0xfffff
> > (MCS 0-7 and b/g rate included) and leave the arg0 as-is (mostly 0xa0)
> > and I expect both drivers can do the rate adaptive thing in the same
> > way, but it seems to make no difference. The rtl8xxxu driver still
> > sends the packets with highest MCS.
> >
> >     Can anyone tell me what should I expect the rate adaptive to work
> > with the rate mask 0xfffff and 0xf0000? Does the 0xf0000 means that it
> > will pick up a tx rate only between nrate MCS4 to MCS7? I need a base
> > line so that I can judge it's simply a rate mask problem or maybe the
> > h2c command is not written correctly. Please kindly suggest what I
> > should do next. Thanks
> >
>
> The rate mask indicates which rates will be used by rate adaptive mechanism.
> I'm not sure the exact bit allocation for CCK/OFDM/MCS, maybe
> 0x0000f/0x00ff0/0xff0000 for CCK/OFDM/MCS respectively, but you can trace
> vendor driver to know the detail.
>
> I suggest you can try to send only OFDM rate mask, and expect to see OFDM
> rate only by your sniffer. If it's still keep on MCS7, rate adaptive may
> not work properly.
>
Thanks. That's my expectation and I'll try to verify it on vendor
driver and upstream rtl8192cu driver.

> Also, you can compare the content of rate adaptive H2C with vendor driver to
> see if the format is correct.
>
> Another thing is to try 'fix_rate' in tx_desc. Check the vendor driver to
> know the use_rate/rate/bw fields of tx_desc. Then, try to fix the rate you
> want.
>
If I set the fix_rate in tx_desc, will the rate mask in H2C command be
ignored? Or the underlying firmware will do the tx rate fallback for
the retry packets?

Chris

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> Ping-Ke
>