[U-Boot] [RFC] command/cache: Add flush_cache command

[Albert ARIBAUD]

Hi Scott,

On Tue, 19 Mar 2013 18:32:39 -0500, Scott Wood
<scottwood at freescale.com> wrote:

> On 03/19/2013 05:07:33 PM, York Sun wrote:
> > On 03/19/2013 03:01 PM, Albert ARIBAUD wrote:
> > > What's the point of this command exactly? I can see the point of  
> > range
> > > flushes (and invalidates) in the code for drivers that use DMA, but  
> > as a
> > > shell command, I fail to see the interest of it.
> > >
> > 
> > I am actually expecting this question. We have a situation that an
> > application is copied by u-boot to its destination in memory. The code
> > needs to be "seen" in the main memory. Without flushing cache, it is
> > only visible to cores.
> 
> It's the same purpose as the cache flushing that happens in bootm,  
> except for code loading that happens outside bootm.
> 
> -Scott

I do understand what it does, but I still don't get why it should be
done, since precisely payload control transfer happens through bootm and
the like which already properly flush cache.

Also, AFAIK U-Boot on multiple cores runs on a single core (possibly
started from a smaller adjunct core) and will load and execute its
payload on that same single core; the payload might enable and run
additional cores if it so decides, but I don't know that U-Boot would
start another main core. Still, I may have missed something.

Anyway:

Is there an ARM multi-core target in U-Boot where U-Boot runs on
one core but its payload shall be started on another, "un-booted",
core, and which experiences issues due to the first core not flushing
cache? If no existing target needs this, then this patch is useless. If
there exists such a target and issue, then the right fix is not a shell
command, it is a programmatic flush before the other core is enabled,
so that it always sees correct RAM.

Maybe this is some code that might come in handy for some future target
not in U-Boot yes which will have the first core start a payload on
another core? Then the previous argument applies (the fix should not be
a shell command, it should be in source code), plus, the patch is dead
code until and unless said target is also added in a single series.

Amicalement,
-- 
Albert.