linux-kernel - Re: [PATCH 10/11] serial: make uart_console

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Date:   Wed, 26 Jan 2022 17:57:13 +0000 (GMT)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     Jiri Slaby <jslaby@...e.cz>
cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        johan@...nel.org, Paul Cercueil <paul@...pouillou.net>,
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        Russell King <linux@...linux.org.uk>,
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        Michal Simek <michal.simek@...inx.com>
Subject: Re: [PATCH 10/11] serial: make uart_console_write->putchar()'s
 character a char

On Mon, 24 Jan 2022, Jiri Slaby wrote:

> diff --git a/drivers/tty/serial/dz.c b/drivers/tty/serial/dz.c
> index e9edabc5a211..3493e201d67f 100644
> --- a/drivers/tty/serial/dz.c
> +++ b/drivers/tty/serial/dz.c
> @@ -802,7 +802,7 @@ static void __init dz_init_ports(void)
>   * restored.  Welcome to the world of PDP-11!
>   * -------------------------------------------------------------------
>   */
> -static void dz_console_putchar(struct uart_port *uport, int ch)
> +static void dz_console_putchar(struct uart_port *uport, char ch)
>  {
>  	struct dz_port *dport = to_dport(uport);
>  	unsigned long flags;

 Hmm, this is unsafe, because on the MIPS target the lone `char' type is 
signed and therefore a call to `->putchar' will see `ch' sign-extended 
from bit #7 to the width of the argument register used.  Which means that 
if a character is sent to the console that has its bit #7 set, then the 
call to:

		dz_out(dport, DZ_TDR, ch);

i.e.:

static void dz_out(struct dz_port *dport, unsigned offset, u16 value)

will send a value to DZ_TDR with bits #15:8 set to all-ones.  And bits 
#11:8 there are the BREAK control bits, active high, for serial lines #3:0 
respectively.

 We could handle this with a preparatory change by calling:

		dz_out(dport, DZ_TDR, ch & 0xffu);

instead, but perhaps `->putchar' should simply take `unsigned char' or 
maybe even `u8' as its third argument?

> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index c58cc142d23f..68e62703eaa6 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -399,7 +399,7 @@ int uart_set_options(struct uart_port *port, struct console
> *co, int baud,
>  struct tty_driver *uart_console_device(struct console *co, int *index);
>  void uart_console_write(struct uart_port *port, const char *s,
>  			unsigned int count,
> -			void (*putchar)(struct uart_port *, int));
> +			void (*putchar)(struct uart_port *, char));
>  
>  /*
>   * Port/driver registration/removal

 I.e.:

			void (*putchar)(struct uart_port *, unsigned char));

I can see we get it right already with:

	unsigned char		x_char;			/* xon/xoff char */

and for `dz_transmit_chars' we have:

	unsigned char tmp;
[...]
	tmp = xmit->buf[xmit->tail];
	xmit->tail = (xmit->tail + 1) & (DZ_XMIT_SIZE - 1);
	dz_out(dport, DZ_TDR, tmp);

(because `struct circ_buf' is generic and not limited to unsigned buffer 
contents interpretation; it's not clear to me if that has been intended 
though).

  Maciej