Re: [PATCH v1 2/2] Bluetooth: btnxpuart: Fix nxp_setup

[Neeraj Sanjay Kale]

Hi Marcel,

> Hi Neeraj
> 
> On Wed, 2023-10-18 at 15:28 +0000, Neeraj sanjay kale wrote:
> > Hi Marcel,
> >
> > Thank you for your patch. This change looks good to me.
> >
> > I think the scenario you are testing/resolving is:
> > 1) Load btnxpuart.ko first (which "may" load BT-only FW if chip is
> > powered on)
> > 2) Power cycle or power on chip
> > 3) Load WLAN driver with combo FW
> > Right?
> 
> Yes, kinda, but there are really a slew of issues with this driver or the
> combination of it with mwifiex_sdio:
> 
> 1. Shared power-down pin (PD#) between Bluetooth and Wi-Fi
> Issue: There is currently no concept in the Linux kernel to achieve this. Last
> failed attempt [1].
> Workaround: Instead of using mmc-pwrseq tied to the Wi-Fi driver
> (mwifiex_sdio) this could be hogged at boot.
> However, then it may no longer easily be controlled e.g. for a (manual)
> power-cycle.
> A novel approach might be using a regulator which could be shared, however,
> this would require us implementing regulator support in btnxpuart. Not sure
> whether you would actually approve us doing so.
> 
> 2. Bluetooth driver (btnxpuart) vs. Wi-Fi driver (mwifiex) load order
> Issue: By default, the Bluetooth driver (btnxpuart) loads before the Wi-Fi
> driver (mwifiex) and using regular mmc-pwrseq for mwifiex_sdio will only
> power-on the module late.
> Workaround: The install directive in modprobe.conf could be (ab-)used to
> enforce mwifiex/mwifiex_sdio to be loaded first. However, doing so adds an
> unnecessary dependency with user space and is in general discouraged.
> 
> 3. Distinguish powered-on vs. powered-off state
> Issue: Without that knowledge the driver has a hard time figuring out
> whether or not it needs to load the firmware as only if it is powered-on
> (and/or without firmware) will the Bluetooth part of the module send its
> boot signature. So the absence of such boot signature may mean either
> firmware is already loaded or module powered-off.
> Workaround: The Bluetooth UART RTS pin seems to activate an internal pull-
> up upon the module being powered on.
> However, programmatically it is rather hard to determine this as the regular
> UART driver (now driving RTS) usually gets loaded.
> 
> 4. Determine whether or not to load the firmware
> Issue: If it is without firmware (and powered-on) the boot loader will send its
> boot signature. If there is no boot signature it could as well also still be
> powered-off.
> Workaround: Also check CTS as it goes up if the firmware is loaded.
> Unfortunately, integrating this in btnxpuart is not so trivial as serdev
> introduces its own asynchronous concurrency which is already used in
> existing checks.
> 
> 5. Deploy separate firmware
> Issue: Does not really solve anything resp. as the power-down pin is shared
> this seems kinda pointless.
> 
> Your input on any of those topics is much appreciated.

I have sent a new patch with subject: "[RFC PATCH] Bluetooth: btnxpuart: Fix nxp_setup in case chip is powered on late"

I would very much appreciate if you could have a look at the patch and let me know if it solves the above issues you mentioned in a neat way.

As per the new change, if no bootloader signatures are read, and CTS is high, the setup function returns an error, preventing HCI intializations.
After every 5 seconds, we attempt a power-on, which results in calling the setup function again to check for bootloader signatures and CTS line.
This "polling" sequence will continue as long as the module stays inserted, or until the btnxpuart driver downloads BT-only FW or the mwifiex downloads the combo FW (or both modules download BT-only/Wi-Fi only respective FWs).

This handling is supposed to come into picture only when mwifiex pwrseq controls the shared PDn pin of the chip, while the normal setup sequence should work as it did earlier for others.

Thanks,
Neeraj