Messages in this thread | | | Date | Wed, 11 Mar 2009 17:50:21 +0800 | Subject | Re: [PATCH 02/18] Blackfin Serial Driver: use barrier instead of cpu_relax for Blackfin SMP like patch | From | graff yang <> |
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On Wed, Mar 11, 2009 at 12:48 AM, Andrew Morton <akpm@linux-foundation.org> wrote: > > On Tue, 10 Mar 2009 06:25:08 -0400 Mike Frysinger <vapier.adi@gmail.com> wrote: > > > On Tue, Mar 10, 2009 at 06:07, gyang wrote: > > > On Fri, 2009-03-06 at 14:37 -0800, Andrew Morton wrote: > > >> On Fri, 6 Mar 2009 14:42:44 +0800 > > >> Bryan Wu <cooloney@kernel.org> wrote: > > >> > > >> > From: Graf Yang <graf.yang@analog.com> > > >> > > > >> > We are making a SMP like patch to blackfin, cpu_relax() is replaced by a > > >> > data cache flush function which will count it to a per-cpu counter. > > >> > If this serial function is called too early, the per-cpu data area have > > >> > not been initialized, this call will cause crash. > > >> > > >> That's a bug in blackfin architecture support. The kernel should be > > >> able to call cpu_relax() at any time, surely. It's a very low-level > > >> and simple thing. > > >> > > >> > So we'd like to use barrier() instead of cpu_relax(). > > >> > > > >> > > >> barrier() is purely a compiler concept. We might as well just remove > > >> the cpu_relax() altogether. > > > > > > Do you mean remove cpu_relax(), and either not add barrier() here? > > > > afaik, early printk all runs before SMP is setup, so having it be a > > 100% busy wait is fine > > No, blackfin is busted, please fix this bug in blackfin core. > > What happens if core kernel code decides to run cpu_relax() prior to > initialising per-cpu data?
cpu_relax() will call smp_mb(), and it need to invalidate data-cache in blackfin smp-like kernel, the cache flush number is increased and recorded into a per-cpu data.
When cpu_relax() is called from early-printk functions, the per-cpu data areas have not been initialized.
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