Re: [PATCH v4] tty: serial: don't do termios for BTIF

[Greg KH <gregkh@linuxfoundation.org>]

On Sat, Jun 20, 2020 at 03:59:14AM +0800, sean.wang@mediatek.com wrote:
> From: Sean Wang <sean.wang@mediatek.com>
> 
> Bluetooth Interface (BTIF) is designed dedicatedly for MediaTek SOC with
> BT in order to be instead of the UART interface between BT module and Host
> CPU, and not exported to user space to access.
> 
> As the UART design, BTIF will be an APB slave and can transmit or receive
> data by MCU access, but doesn't provide termios function like baudrate and
> flow control setup.
> 
> Even LCR on offset 0xC that is just a FAKELCR
> a. If FAKELCR[7] is equaled to 1, RBR(0x00), THR(0x00), IER(0x04)
>    will not be readable/writable.
> 
> b. If FAKELCR is equaled to 0xBF, RBR(0x00), THR(0x00), IER(0x04),
>    IIR(0x08), and LSR(0x14) will not be readable/writable.
> 
> So adding a new capability 'UART_CAP_NTIO' for the unusual unsupported
> case.
> 
> The bluetooth driver would use BTIF device as a serdev. So the termios
> still function would be called in kernelspace from ttyport_open in
> drivers/tty/serdev/serdev-ttyprt.c.
> 
> Fixes: 1c16ae65e250 ("serial: 8250: of: Add new port type for MediaTek BTIF controller on MT7622/23 SoC")
> Cc: Steven Liu <steven.liu@mediatek.com>
> Signed-off-by: Sean Wang <sean.wang@mediatek.com>
> Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
> 
> --
> v1->v2:
> no change on termios->c_cflag and refine commit message
> 
> v2->v3:
> change the naming from NMOD to NTIO as TIO is a well established prefix
> for termios IOCTLs.
> 
> v3->v4:
> 1. remove appropriate tag
> 2. add the explanation why the termios is required even when the connection
>    isn't exported to userspace.
> ---
>  drivers/tty/serial/8250/8250.h      | 1 +
>  drivers/tty/serial/8250/8250_port.c | 5 ++++-
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> index 52bb21205bb6..0d9d3bfe48af 100644
> --- a/drivers/tty/serial/8250/8250.h
> +++ b/drivers/tty/serial/8250/8250.h
> @@ -82,6 +82,7 @@ struct serial8250_config {
>  #define UART_CAP_MINI	(1 << 17)	/* Mini UART on BCM283X family lacks:
>  					 * STOP PARITY EPAR SPAR WLEN5 WLEN6
>  					 */
> +#define UART_CAP_NTIO	(1 << 18)	/* UART doesn't do termios */

Naming is hard.  I will never remember what "NTIO" is, how about we make
it explicit:
	define UART_CAP_IGNORE_TERMIOS

And the _CAP_ name is getting out of hand, this isn't a "capability",
it's a "quirk for this port" but that's a battle to worry about later...

thanks,

greg k-h