linux-kernel - Re: [PATCH v3 05/10] serial: mps2-uart: add support for early console

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Message-ID: <56C31F9F.10000@arm.com>
Date:	Tue, 16 Feb 2016 13:09:51 +0000
From:	Vladimir Murzin <vladimir.murzin@....com>
To:	Andy Shevchenko <andy.shevchenko@...il.com>
CC:	Arnd Bergmann <arnd@...db.de>,
	Russell King <linux@....linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	ijc+devicetree <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>, Jiri Slaby <jslaby@...e.cz>,
	Rob Herring <robh+dt@...nel.org>,
	devicetree <devicetree@...r.kernel.org>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 05/10] serial: mps2-uart: add support for early console

On 16/02/16 10:36, Andy Shevchenko wrote:
> On Tue, Feb 16, 2016 at 12:08 PM, Vladimir Murzin
> <vladimir.murzin@....com> wrote:
>> This adds support early console for MPS2 UART which can be enabled via
>> earlycon=mps2,0x40004000
> 
> 
>> --- a/drivers/tty/serial/mps2-uart.c
>> +++ b/drivers/tty/serial/mps2-uart.c
>> @@ -435,6 +435,36 @@ static struct console mps2_uart_console = {
>>
>>  #define MPS2_SERIAL_CONSOLE (&mps2_uart_console)
>>
>> +static void mps2_early_putchar(struct uart_port *port, int ch)
>> +{
>> +
>> +       while (readb(port->membase + UARTn_STATE) & UARTn_STATE_TX_FULL)
>> +               cpu_relax();
> 
> Infinite busy loop?

In case of broken hw, yes, but I'm quite unsure how we can help to fix
it. Do you have something in mind?

> 
>> +
>> +       writeb((unsigned char)ch, port->membase + UARTn_DATA);
>> +}
>> +
>> +
>> +static void mps2_early_write(struct console *con, const char *s, unsigned n)
>> +{
>> +       struct earlycon_device *dev = con->data;
>> +
>> +       uart_console_write(&dev->port, s, n, mps2_early_putchar);
>> +}
>> +
>> +static int __init mps2_early_console_setup(struct earlycon_device *device,
>> +                                          const char *opt)
>> +{
>> +       if (!device->port.membase)
>> +               return -ENODEV;
>> +
>> +       device->con->write = mps2_early_write;
>> +
>> +       return 0;
>> +}
>> +EARLYCON_DECLARE(mps2, mps2_early_console_setup);
>> +OF_EARLYCON_DECLARE(mps2, "arm,mps2-uart", mps2_early_console_setup);
> 
> IIRC Peter Hurley mentioned you don't need to put both anymore, OF_
> one is enough.
> 

I've just tried with OF_ only and seems it works fine. Thanks for
pointing at it!

Cheers
Vladimir