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From:   Tony Chuang <yhchuang@realtek.com>
To:     Kalle Valo <kvalo@codeaurora.org>
CC:     "linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>,
        "briannorris@chromium.org" <briannorris@chromium.org>
Subject: RE: [PATCH] rtw88: add debugfs to fix tx rate
Thread-Topic: [PATCH] rtw88: add debugfs to fix tx rate
Thread-Index: AQHV+QPS9h9ZLy9E5U2M/GVMmM07xahGU7JUgAQrRBA=
Date:   Mon, 16 Mar 2020 02:28:05 +0000
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 <87eetwo87q.fsf@kamboji.qca.qualcomm.com>
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Kalle Valo <kvalo@codeaurora.org> writes:

> <yhchuang@realtek.com> writes:
> 
> > From: Yan-Hsuan Chuang <yhchuang@realtek.com>
> >
> > It is useful to fix the bit rate of TX packets. For example, if
> > someone is measuring the TX power, or debugging with the issues
> > of the TX throughput on the field.
> >
> > To set the value of fixed rate, one should input corresponding
> > desc rate index (ex, 0x0b for DESC_RATE54M to fix at 54 Mbps).
> > Set a value larger than DESC_RATE_MAX will disable fix rate, so
> > the rate adaptive mechanism can resume to work.
> >
> > Example,
> >   To fix rate at MCS 1:
> >   echo 0x0d > /sys/kernel/debug/ieee80211/phy0/rtw88/fix_rate
> >
> >   To not to fix rate:
> >   echo 0xff > /sys/kernel/debug/ieee80211/phy0/rtw88/fix_rate
> >
> >   To know which rate was fixed at:
> >   cat /sys/kernel/debug/ieee80211/phy0/rtw88/fix_rate
> >
> > Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
> 
> No way, debugfs is not a method for working around nl80211 and doing
> whatever idea you come up with. The goal is that we have a generic
> nl80211 command for all generic actions, like this one. And I think we
> already have an nl80211 command for fixing the tx rate, right?
> 

No, as I can see, there's no suitable nl80211 command that can achieve
what I want. If you are saying about NL80211_CMD_SET_TX_BITRATE_MASK,
it's used to allow some rates. But actually the firmware has its own rate
adaptive mechanism, so mask out the other rates does not mean the rate
left will be chosen. Moreover, the hardware will choose a lower bit rate
when retry, then the TX rate is not fixed at all. So the debugfs can disable
the firmware's RA mechanism, also disable the TX rate fall back when retry.
Both of them cannot be done by setting TX bitrate mask.

I am sorry I need to add another debugfs for it, but to actually fix the TX
bitrate, we really need another debugfs or module parameter. Because
according to the design of the device there is not a good enough general
command I can use to fix the TX rate. If there is a command that can fix
the TX bitrate for me, please let me know, I can switch to it.

Thanks,
Yen-Hsuan