linux-kernel - Re: [PATCH 18/56] microblaze_v2: early

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Date:	Tue, 06 May 2008 10:14:32 +0200
From:	Michal Simek <monstr@...nam.cz>
To:	John Williams <john.williams@...alogix.com>
CC:	linux-kernel@...r.kernel.org, arnd@...db.de,
	linux-arch@...r.kernel.org, stephen.neuendorffer@...inx.com,
	John.Linn@...inx.com, matthew@....cx, will.newton@...il.com,
	drepper@...hat.com, microblaze-uclinux@...e.uq.edu.au,
	grant.likely@...retlab.ca, Michal Simek <monstr@...str.eu>
Subject: Re: [PATCH 18/56] microblaze_v2: early_printk support

Make sense.
I'll add it.

Thanks,
M
> On Sun, 2008-05-04 at 13:41 +0200, monstr@...nam.cz wrote:
> 
>> +++ b/arch/microblaze/kernel/early_printk.c
> 
>> +static void early_printk_putc(char c)
>> +{
>> +	while (ioread32(STATUS) & (1<<3));
>> +	iowrite32((c & 0xff), TX_FIFO);
>> +}
> 
> The while() loop needs a retry counter - if you configure
> EARLY_PRINTK_BASE_ADDRESS wrongly and it points to memory or anything
> that is not a uartlite, the loop spins forever and you get silent
> lockup.  Not nice behaviour from debug code :)
> 
> Here's my current implementation:
> 
> static void early_printk_putc(char c)
> {
>         /* Limit how many times we'll spin waiting for TX FIFO status.
>            This will prevent lockups if the base address is incorrectly
> set, or
>            any other issue on the UARTLITE.
> 
>            This limit is pretty arbitrary, unless we are at about 10
> baud 
>            we'll never timeout on a working UART. */
> 
>         unsigned retries=10000;
>         while (retries-- && (ioread32(STATUS) & (1<<3)))
>                 ;
> 
>         /* Only attempt the iowrite if we didn't timeout */
>         if(retries)
>                 iowrite32((c & 0xff), TX_FIFO);
> }
> 
> Cheers
> 
> John
> 
> 
> 
> 
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