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

[Greg Kroah-Hartman]

On Thu, Jul 02, 2020 at 01:37:13AM +0300, Serge Semin wrote:
> 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.

Sounds like a sane fix, thanks for looking into this.

greg k-h