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

[Tom Rini <trini@ti.com>]

On Wed, Mar 20, 2013 at 02:36:05PM -0500, Scott Wood wrote:
> On 03/20/2013 02:15:19 PM, Tom Rini wrote:
> >On Wed, Mar 20, 2013 at 11:43:15AM -0500, Scott Wood wrote:
> >> On 03/20/2013 09:58:36 AM, Wolfgang Denk wrote:
> >> >Dear Albert,
> >> >
> >> >In message <20130320145927.2031b913@lilith> you wrote:
> >> >>
> >> >> 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.
> >>
> >> It doesn't always happen through bootm.  Standalone apps use the
> >> "go" command.
> >
> >So, to try and be a bit more verbose about this, for U-Boot
> >applications
> >which use 'go', we still need to ensure cache coherence, which is why
> >bootm does a cache flush, we need some way to flush in this case.
> 
> It's also an issue with using the "cpu <n> release" command.

Hadn't seen that command before, where is it?

> >And in this case you aren't better served by say bootelf ?
> 
> That wouldn't handle the "cpu release" case.  In any case, "go"
> exists and is currently the recommended way to launch a standalone
> application in doc/README.standalone.
> 
> >> It's a user command!  How can it be dead code?  I don't know of a
> >> way to include a human user in a patchset...
> >
> >Can you hightlight what exactly causes the world today to go off and
> >fail?  Is the hello_world example app sufficient in this case or do we
> >need something much larger?
> 
> A user inside Freescale is running standalone performance test apps,
> using both "go" and "cpu <n> release" (since the test needs to run
> on all CPUs).  They are seeing cache problems running on a T4240 if
> they don't have this flush.  This flush is architecturally required
> between modifying/loading code and running it.

OK, so this does sound like a real need / use for it, and if we added
the granularity of CONFIG_CMD_CACHE_FLUSH or similar, it would be
reasonable to turn it on to a large number of boards for a small space
savings (so lets not).  My next concern is that this needs build testing
(and some inspection) on say ARM where we have a weak flush_cache
already.  But perhaps the right answer is to say it doesn't make sense
to add CONFIG_CMD_CACHE on an architecture which doesn't already provide
flush_cache, so drop the weak one from this patch.

-- 
Tom
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