linux-kernel - Re: [PATCH 1/2] hvc_xen: add earlycon support

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Date:	Wed, 24 Feb 2016 09:43:14 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	xen-devel@...ts.xensource.com
Cc:	linux-kernel@...r.kernel.org, david.vrabel@...rix.com,
	konrad.wilk@...cle.com
Subject: Re: [PATCH 1/2] hvc_xen: add earlycon support

On 02/24/2016 07:23 AM, Stefano Stabellini wrote:
> Introduce EARLYCON support in hvc_xen, useful for early debugging on arm
> and arm64, where xen early_printk is not available. Differently from
> xenboot_write_console on x86, we won't just return if !xen_pv_domain(),
> because arm and arm64 guests are actually xen_hvm_domain() from linux
> pov, and also because we want to capture all possible writes, including
> the ones earlier than xen_early_init().
>
> Signed-off-by: Stefano Stabellini<stefano.stabellini@...citrix.com>
> ---
>   drivers/tty/hvc/hvc_xen.c |   48 ++++++++++++++++++++++++++++++++++-----------
>   1 file changed, 37 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
> index fa816b7..34e8e9f 100644
> --- a/drivers/tty/hvc/hvc_xen.c
> +++ b/drivers/tty/hvc/hvc_xen.c
> @@ -25,6 +25,7 @@
>   #include <linux/init.h>
>   #include <linux/types.h>
>   #include <linux/list.h>
> +#include <linux/serial_core.h>
>   
>   #include <asm/io.h>
>   #include <asm/xen/hypervisor.h>
> @@ -597,21 +598,11 @@ static int xen_cons_init(void)
>   }
>   console_initcall(xen_cons_init);
>   
> -#ifdef CONFIG_EARLY_PRINTK
> -static void xenboot_write_console(struct console *console, const char *string,
> -				  unsigned len)
> +static void domU_write_early_console(const char *string, unsigned len)
>   {
>   	unsigned int linelen, off = 0;
>   	const char *pos;
>   
> -	if (!xen_pv_domain())
> -		return;
> -
> -	dom0_write_console(0, string, len);
> -
> -	if (xen_initial_domain())
> -		return;
> -
>   	domU_write_console(0, "(early) ", 8);
>   	while (off < len && NULL != (pos = strchr(string+off, '\n'))) {
>   		linelen = pos-string+off;
> @@ -625,6 +616,21 @@ static void xenboot_write_console(struct console *console, const char *string,
>   		domU_write_console(0, string+off, len-off);
>   }
>   
> +#ifdef CONFIG_EARLY_PRINTK
> +static void xenboot_write_console(struct console *console, const char *string,
> +				  unsigned len)
> +{
> +	if (!xen_pv_domain())
> +		return;

This, BTW, will break PVHv2 code (which is not there yet). Your patch is 
not making the code any different from what it is now but I wonder 
whether xenboot_write_console() can be made to work for HVM guests at 
this time since you making changes there anyway.

Or we can delay it until HVMlite/PVHv2 patches make it in.

> +
> +	dom0_write_console(0, string, len);
> +
> +	if (xen_initial_domain())
> +		return;
> +
> +	domU_write_early_console(string, len);
> +}
> +
>   struct console xenboot_console = {
>   	.name		= "xenboot",
>   	.write		= xenboot_write_console,
> @@ -664,3 +670,23 @@ void xen_raw_printk(const char *fmt, ...)
>   
>   	xen_raw_console_write(buf);
>   }
> +
> +static void xenboot_earlycon_write(struct console *console,
> +				  const char *string,
> +				  unsigned len)
> +{
> +	dom0_write_console(0, string, len);
> +
> +	if (xen_initial_domain())
> +		return;
> +
> +	domU_write_early_console(string, len);
> +}

xenboot_earlycon_write() and xenboot_write_console() share most of the 
code, can it be factored out?

-boris

> +
> +static int __init xenboot_earlycon_setup(struct earlycon_device *device,
> +					    const char *opt)
> +{
> +	device->con->write = xenboot_earlycon_write;
> +	return 0;
> +}
> +EARLYCON_DECLARE(xenboot, xenboot_earlycon_setup);