linux-kernel - Re: early_printk accessing __log

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Date:	Wed, 18 Jul 2007 15:16:17 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Robin Getz <rgetz@...ckfin.uclinux.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: early_printk accessing __log_buf

On Wed, 18 Jul 2007 17:56:43 -0400
Robin Getz <rgetz@...ckfin.uclinux.org> wrote:

> Quick question:
> 
> I am currently setting up an early_printk for the Blackfin - & I'm populating
> an early fault interrupt handler at the same time. This allows things like
> catching interrupts, or faults before the kernel has had a chance to
> enable it's standard interrupt/fault handlers which happen midway 
> through the boot process.
> 
> I would like to be able to print out the standard __log_buf from my early
> fault handler - so everyone can see where things are when they died. In
> most situations - printk has dumped some info to the log_buf, and it is
> normally handy to see figure out what is going on.
> 
> However - __log_buf & associated pointers are declared as static in
> kernel/printk.c - so they can't be accessed by external files.
> 
> I would normally use do_syslog([23],...) to read from the buffer - 
> but these crashes happens before the kernel is fully initialized,
> so spin_lock_irq, cond_resched, or wait_event_interruptible
> will cause problems...
> 
> Although there are other solutions - I would prefer to just don't declare them
> as static when EARLY_PRINTK is defined. This way if something bad happens - 
> everyone can dump out the standard buffers to assist in debugging...

Being able to get at the log buffer from external debug-style code is a
fairly common requirement.

> ===================================================================
> --- linux-2.6.x/kernel/printk.c (revision 3400)
> +++ linux-2.6.x/kernel/printk.c (working copy)
> @@ -117,7 +117,12 @@
> 
>  #ifdef CONFIG_PRINTK
> 
> +#ifdef CONFIG_EARLY_PRINTK
> +char __log_buf[__LOG_BUF_LEN];
> +#else
>  static char __log_buf[__LOG_BUF_LEN];
> +#endif
> +
>  static char *log_buf = __log_buf;
>  static int log_buf_len = __LOG_BUF_LEN;
>  static unsigned long logged_chars; /* Number of chars produced since last read+clear operation */
> 

erk.  There isn't much point in making something externally accessible
without also declaring it in a header file.  And callers will need to be
able to get at log_buf_len too, I guess.

I'd suggest that any interface into here should be via function calls, not
via direct access to printk internals: think up some nice
copy_me_some_of_the_log_buffer() interface.
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