[U-Boot] [RFC PATCH] Pre-console buffer for ARM

[Mike Frysinger <vapier@gentoo.org>]

On Monday, August 29, 2011 19:00:46 Graeme Russ wrote:
> On Tue, Aug 30, 2011 at 6:10 AM, Mike Frysinger wrote:
> > On Monday, August 29, 2011 15:42:23 Simon Glass wrote:
> >> On Mon, Aug 29, 2011 at 12:20 PM, Mike Frysinger wrote:
> >> > On Monday, August 29, 2011 13:21:57 Simon Glass wrote:
> >> >> +     if (gd->con_buf_idx < CONFIG_SYS_TMP_CON_BUF_SZ)
> >> >> +             buffer[gd->con_buf_idx++] = c;
> >> > 
> >> > seems like a circular buffer would make more sense ... usually the
> >> > part of the log you want is the last chunk and not the first
> 
> Well you would need an 'overflow' flag and then based on that you would
> need to do two printf()'s when dumping the buffer - One from the 'index to
> end' which is the 'top' of the buffer and one from 'start to index' which
> is the bottom.

you wouldnt need an overflow flag unless you wrote out more than 2^32 bytes.  
look at the logic again ... it isnt masking the write, it's masking the read.  
so by virtual of the con_buf_idx being larger than the max, you know you've 
wrapped around.

otherwise yes, you'd need to split up the writes.  not that big of a deal i 
dont think ...

> >> Yes I agree, although if you have more than 1KB of data it might be a
> >> bug.
> 
> I personally don't see the need - I expect the amount of pre-console output
> would be faily limited considering that the board should do everything it
> can to initialise console as early as possible.

until people hit an early fail and add a lot of debug printf's and then the 
output gets silently clipped.  it's confusing imo, and i say this having 
implemented early debug output in other systems (including linux) and seeing 
how people used/reacted to it.

> > give people a foot and they'll take 1MiB :p
> > 
> > it's fairly easy as well:
> > #define CIRC_BUF_IDX(idx) ((idx) & (CONFIG_SYS_TMP_CON_BUF_SZ-1))
> > buffer[CIRC_BUF_IDX(gd->conf_buf_idx++)] = c;
> 
> But does that work for non power-of-two buffer sizes...

no, but not that big of a deal.  so you get limited to the last 1KiB, 4KiB, 
8KiB, 16KiB, etc... amount of data.
-mike
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