On Mon, 31 May 2021 15:41:36 +0100
"Russell King (Oracle)" [off-list ref] wrote:

Hi Greg,

On Mon, May 31, 2021 at 03:21:54PM +0200, Greg Kroah-Hartman wrote:

quoted

On Sun, May 30, 2021 at 05:34:54PM +0100, Andre Przywara wrote:  

quoted

An obvious easy hack-fix is to just define
CONFIG_SERIAL_8250_16550A_VARIANTS, which brings the delays back and
seems to avoid the problem for me.
Another hack which seems to mitigate the problem is to avoid switching
the baudrate to something faster than 115200.

I observed this on a BananaPi-M64 (Allwinner A64 SoC with AP6212 WiFi/BT
chip), but others reported the same issue on a NanoPi Air (Allwinner H3
with 6212), but also other SoCs and devices (at least one AP6210).

Obviously those workarounds are not real solutions, and I was
wondering if anybody has an idea how to properly fix this?
What puzzles me is that the delay is happening during the *UART*
probe, so before we even start dealing with the Bluetooth device.  

What type of bluetooth device is this, and what does it have to do with
the serial port?  Is the SoC device using the same IP blocks for both?  

Many bluetooth "devices" (I mean the interface from the local machine
to the BT world, not as in remote devices) are connected through a
standard UART. Pictorially, it's:

  CPU <---> UART <---> BT chip <---> Bluetooth RF world

The reporter seems to be saying is that a change to the UART driver now
means that the bluetooth chip wired to that UART no longer functions due
to slightly different initialisation timings of the host UART.

Yes, exactly, thanks Russell for clarifying this.
How this works (when it does) is that the UART driver probes, then we
look at the children of the UART devicetree node, to probe for those, by
virtue of the serdev bus.

My question was about if this rings a bell with someone, because I have
a hard time piecing together how a delay in the *UART probe* could
affect devices depending on it. And how to fix this ...

Cheers,
Andre


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