[PATCH v2 0/1] Revert "serial: 8250: Fix max baud limit in generic 8250 port"

[Serge Semin]

On Wed, Jul 01, 2020 at 02:13:36PM -0700, Daniel Winkler wrote:
> 
> This change regresses the QCA6174A-3 bluetooth chip, preventing
> firmware from being properly loaded. Without this change, the
> chip works as intended.
> 
> The device is the Kukui Chromebook using the Mediatek chipset
> and the 8250_mtk uart. Initial controller baudrate is 115200
> and operating speed is 3000000. Our entire suite of bluetooth
> tests now fail on this platform due to an apparent failure to
> sync its firmware on initialization.

Ok. It's mediatek 8250 driver, which is responsible for the failure.
Then we'll have two options:

1) Add a new capability like UART_CAP_NO16DIV and take it into account
   in the serial8250_get_baud_rate() method.
 
I don't have a documentation for the Mediatek UART port, but it seems to me
that that controller calculates the baud rate differently from the standard
8250 port. A standard 8250 port does that by the next formulae:
  baud = uartclk / (16 * divisor).
While it seems to me that the Mediatek port uses the formulae like:
  baud = uartclk / divisor. (Please, correct me if I'm wrong)
If so, then we could introduce a new capability like UART_CAP_NO16DIV. The
8250_mtk driver will add it to the 8250-port capabilities field. The
serial8250_get_baud_rate() method should be altered in a way so one would check
whether the UART_CAP_NO16DIV flag is set and if it is then the
uart_get_baud_rate() function will be called without uartclk normalized by the
factor of 16.

2) Manually call serial8250_do_set_divisor() in the custom set_termios()
   callback.

Just add the uart_update_timeout() and serial8250_do_set_divisor() methods
invocation into the mtk8250_set_termios() function, which the original commit
81bb549fdf14 ("serial: 8250_mtk: support big baud rate") author should have
done in the first place.

-Sergey

> 
> The driver is in the cros tree at drivers/bluetooth/hci_qca.c
> and uses the serdev interface. Specifically, this is the
> QCA_ROME chipset.
> 
> 
> Daniel Winkler (1):
>   Revert "serial: 8250: Fix max baud limit in generic 8250 port"
> 
>  drivers/tty/serial/8250/8250_port.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> -- 
> 2.27.0.212.ge8ba1cc988-goog
>