Subject: [RFC 3/5] acpi/serial: add DBG2 earlycon support

From: [email-masked] (Mark Rutland)
Date: Tue, 8 Sep 2015 14:09:51 +0100

On Tue, Sep 08, 2015 at 01:43:35PM +0100, Leif Lindholm wrote:
> The ACPI DBG2 table defines a debug console. Add support for parsing it
> and using it to select earlycon destination when no arguments provided.
> 
> Signed-off-by: Leif Lindholm <[email-masked]>

[...]

> diff --git a/drivers/acpi/console.c b/drivers/acpi/console.c
> new file mode 100644
> index 0000000..a985890
> --- /dev/null
> +++ b/drivers/acpi/console.c
> @@ -0,0 +1,103 @@
> +/*
> + * Copyright (c) 2012, Intel Corporation
> + * Copyright (c) 2015, Linaro Ltd.
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + *
> + */
> +
> +#define DEBUG

Why?

> +#define pr_fmt(fmt) "ACPI: " KBUILD_MODNAME ": " fmt
> +
> +#include <linux/acpi.h>
> +#include <linux/kernel.h>
> +#include <linux/serial_core.h>
> +
> +#define NUM_ELEMS(x) (sizeof(x) / sizeof(*x))

Use ARRAY_SIZE (from kernel.h).

> +
> +#ifdef CONFIG_SERIAL_EARLYCON
> +static int use_earlycon __initdata;
> +static int __init setup_acpi_earlycon(char *buf)
> +{
> +	if (!buf)
> +		use_earlycon = 1;
> +
> +	return 0;
> +}
> +early_param("earlycon", setup_acpi_earlycon);

It seems a shame to add this after folding the OF case into the earlycon
code. What necessitates this being a separate early_param? Why is it too
early to parse DBG2?

[...]

> diff --git a/drivers/tty/serial/earlycon.c b/drivers/tty/serial/earlycon.c
> index 2bda09a..c063cbb 100644
> --- a/drivers/tty/serial/earlycon.c
> +++ b/drivers/tty/serial/earlycon.c
> @@ -13,6 +13,7 @@
>  
>  #define pr_fmt(fmt)	KBUILD_MODNAME ": " fmt
>  
> +#include <linux/acpi.h>
>  #include <linux/console.h>
>  #include <linux/kernel.h>
>  #include <linux/init.h>
> @@ -184,12 +185,16 @@ static int __init param_setup_earlycon(char *buf)
>  	int err;
>  
>  	/*
> -	 * Just 'earlycon' is a valid param for devicetree earlycons;
> -	 * don't generate a warning from parse_early_params() in that case
> +	 * Just 'earlycon' is a valid param for devicetree or ACPI earlycons;
> +	 * ACPI cannot be parsed yet, so return without action if enabled.
> +	 * Otherwise, attempt initialization using DT.
>  	 */
> -	if (!buf || !buf[0])
> -		if (IS_ENABLED(CONFIG_OF_FLATTREE))
> +	if (!buf || !buf[0]) {
> +		if (!acpi_disabled)
> +			return 0;
> +		else if (IS_ENABLED(CONFIG_OF_FLATTREE))
>  			return early_init_dt_scan_chosen_serial();
> +	}

It would be much nicer if we could handle the ACPI earlycon parsing in
the same place. As above, I assume I'm missing something that prevents
that.

Mark.