Re: [PATCH 9/9] powerpc/pm: support deep sleep feature on T1040

[Scott Wood]

On Fri, 2014-03-21 at 09:21 +0000, David Laight wrote:
> From: Scott Wood [mailto:scottwood@xxxxxxxxxxxxx]
> > On Thu, 2014-03-20 at 11:59 +0000, David Laight wrote:
> > > I tried to work out what the 'twi, isync' instructions were for (in in_le32()).
> > > The best I could come up with was to ensure a synchronous bus-fault.
> > > But bus faults are probably only expected during device probing - not
> > > normal operation, and the instructions will have a significant cost.
> > >
> > > Additionally in_le32() and out_le32() both start with a 'sync' instruction.
> > > In many cases that isn't needed either - an explicit iosync() can be
> > > used after groups of instructions.
> > 
> > The idea is that it's better to be maximally safe by default, and let
> > performance critical sections be optimized using raw accessors and
> > explicit synchronization if needed, than to have hard-to-debug bugs due
> > to missing/wrong sync.  A lot of I/O is slow enough that the performance
> > impact doesn't really matter, but the brain-time cost of getting the
> > sync right is still there.
> 
> Hmmm....
> 
> That might be an excuse for the 'sync', but not the twi and isync.

That might be true if I/O is always cache inhibited and guarded, in
which case I think we can rely on that to ensure that the load has
completed before we do things like wrtee or rfi.  In any case, I'd want
to hear Ben's explanation.

> I was setting up a dma request (for the ppc 83xx PCIe bridge) and
> was doing back to back little-endian writes into memory.
> I had difficulty finding and including header files containing
> the definitions for byteswapped accesses I needed.
> arch/powerpc/include/asm/swab.h contains some - but I couldn't
> work out how to get it included (apart from giving the full path).
> 
> In any case you need to understand when synchronisation is
> required - otherwise you will get it wrong.
> Especially since non-byteswapped accesses are done by direct
> access.

Yes, it's bad that rawness combines the lack of byteswapping with the
lack of synchronization.  Ideally the raw accessors would also come in
big and little endian form, plus a native endian form if it's really
needed.

-Scott


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