Messages in this thread | | | Date | Wed, 11 Mar 2009 09:48:23 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 02/18] Blackfin Serial Driver: use barrier instead of cpu_relax for Blackfin SMP like patch |
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On Wed, 11 Mar 2009 17:50:21 +0800 graff yang <graff.yang@gmail.com> wrote:
> 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.
Right. And we've demonstrated here that this was a bad idea.
So I suggest that the blackfin maintainers delete that code.
If it's really valuable (which I doubt) then it could be reimplemented via a statically-allocated array of longs and local_irq_save() protection. Or a statically allocated array of atomic_long_t's.
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