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

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Date:	Tue, 16 Feb 2016 15:13:50 +0200
From:	Andy Shevchenko <andy.shevchenko@...il.com>
To:	Vladimir Murzin <vladimir.murzin@....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 Tue, Feb 16, 2016 at 3:09 PM, Vladimir Murzin
<vladimir.murzin@....com> wrote:
> 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

>>> +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?

Set sane amount of loops like

int count = 100;
…
while (… && --count)

Though I have no idea what to do if count == 0. Perhaps it's a TX overrun state.

You may try to recover by flushing TX queue in HW (fifo and / or Tx
shift register) if HW supports it.

-- 
With Best Regards,
Andy Shevchenko