linux-kernel - Re: earlycon: no match?

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Message-ID: <55477BC1.6000805@hurleysoftware.com>
Date:	Mon, 04 May 2015 10:01:37 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Robert Schwebel <r.schwebel@...gutronix.de>
CC:	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel@...gutronix.de
Subject: Re: earlycon: no match?

Hi Robert,

On 05/03/2015 05:10 PM, Robert Schwebel wrote:
> Hi Peter,
> 
> with 4.1-rc1, my boxes with early console enabled show something like
> this (the example is vexpress, but it for example also happens on an
> AM335x board):
> 
>   earlycon: no match for ttyAMA0,38400n8

This shouldn't impact any previous earlycon setup. Are you saying
you're seeing a regression?

How do you have early console enabled, via the command line or via DT?


> The box was booted with "console=ttyAMA0,38400n8" on the commandline.
> If I understand this right, the code in drivers/tty/serial/earlycon.c
> calls setup_earlycon() with the string above ("ttyAMA0,38400n8") and
> fails to find that string in the "names" part of the __earlycon_table,
> because for the pl011 component on vexpress, the early console was
> registered in drivers/tty/serial/amba-pl011.c with:
> 
> OF_EARLYCON_DECLARE(pl011, "arm,pl011", pl011_early_console_setup);
>                     ^^^^^ name
> 
> So isn't that trying to match "ttyAMA0" against "arm,pl011"? I have the
> feeling that I didn't understand the logic behind that.
> 
> Can you elaborate about how this is supposed to work correctly?

Yeah, I've been meaning to write about this but simply haven't had the
time yet; apologies for that.

The facility is hopefully best explained by the existing 8250 exemplar.
Normally, an 8250 early console is started via command line with a
command line parameter like:

	earlycon=uart,io,0x2f8,115200n8

Since 2007, an 8250 early console can also be started via command line
using console= instead, like:

	console=uart,io,0x2f8,115200n8

In this alternate form, this early console will go on to become the
corresponding ttyS console.

However, that functionality was exclusive to 8250 console/earlycon.
To get this same behavior for the amba-pl011 console would look
something like:

/* drivers/tty/serial/amba-pl011.c */

/* returns 0 if the console matches; otherwise, non-zero to use default matching */
static int pl011_console_match(struct console *co, char *name, int idx, char *options)
{
	unsigned char iotype;
	unsigned long addr;

	if (strncmp(name, "pl" 2) != 0 || idx != 11)
		return -ENODEV;

	if (uart_parse_earlycon(options, &iotype, &addr, &options))
		return -ENODEV;

	/* find the port from the addr */
	for (i = 0; i < ARRAY_SIZE(amba_ports); i++) {
		if (amba_ports[i] == NULL)
			continue;
		if (port->mapbase != addr)
			continue;

		co->index = i;
		return pl011_console_setup(co, options);
	}

	return -ENODEV;
}

...

static struct console amba_console = {
	...
	.match		= pl011_console_match,
	...
};


Regards,
Peter Hurley
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