[U-Boot] [PATCH 2/4] net: round up before calling flush_cache

Mike Frysinger
2012-04-02T03:06:48Z

On Sunday 01 April 2012 21:44:39 Marek Vasut wrote:
> Dear Mike Frysinger,
> > On Sunday 01 April 2012 17:00:56 Marek Vasut wrote:
> > > Dear Mike Frysinger,
> > > > On Sunday 01 April 2012 09:22:59 Stefano Babic wrote:
> > > > > If the range passed to flush_cache is not multiple
> > > > > of ARCH_DMA_MINALIGN, a warning due to mislaignment
> > > > > is printed.
> > > > > Detected with fec_mxc, mx35 boards:
> > > > >
> > > > > CACHE: Misaligned operation at range [80800000, 8083c310]
> > > >
> > > > warning on flushing is broken. the arch/arm/cpu/arm926ejs/cache.c
> > > > code should probably be fixed instead.
> > >
> > > Why exactly?
> >
> > the flush isn't harmful (ignoring the fact that a few extra bytes might
> > get written back to external memory), and the data isn't evicted from
> > cache. after all, we aren't talking about invalidate here, we're talking
> > about flush.
>
> Right ... and can you be sure nothing important is overwritten in RAM?

except i'd bet money you're already running dcache in writethrough mode, so
the flush is largely irrelevant to this line of reasoning

> > plus, no other arch (linux or u-boot) does this.
> >
> > so the better question is, why exactly should you be warning ? you
> > should provide justification when doing something unusual ...
>
> Because you can destroy data in DRAM that arrived there by DMA transfer for
> example?

that isn't the problem of the flush functions. there would have been an
invalidate call at some point with misaligned addresses, iff it actually
mattered. you could argue for invalidation triggering a warning, but that
isn't what we're talking about. and still, linux doesn't trigger warnings,
and i think only one other arm soc does atm in u-boot.

as already shown here, the flush call was perfectly fine, and adding roundup to
that call site is a waste of code space to "fix" something that isn't a
problem.
-mike
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