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

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Date:	Tue, 16 Feb 2016 16:00:20 +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 3:38 PM, Vladimir Murzin
<vladimir.murzin@....com> wrote:
> On 16/02/16 13:13, Andy Shevchenko wrote:
>> 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:

>>>>> +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.
>
> I thought about counter too, like some of drivers do, but had no idea
> what to do next after counter reaches zero. In case of TX overrun we
> could get it either because of broken hw or because we entered Linux
> with that state already set. In both cases, it looks handy to me to
> attach a debugger and see that we are spinning in this putchar loop.
>
> If you do insist to have a counter I'll add one, but if you are not
> strong about it I'd prefer to leave it as is.

I would wait for Peter Hurley, Alan, or other experienced guys to talk.

>> You may try to recover by flushing TX queue in HW (fifo and / or Tx
>> shift register) if HW supports it.
>>
>
> I'm afraid this is not supported.

Maybe soft reset? Anyway, see above.

-- 
With Best Regards,
Andy Shevchenko