[PATCH 07/16] ARM: bL_platsmp.c: close the kernel entry gate before hot-unplugging a CPU

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Mon, Jan 14, 2013 at 12:15:07PM -0500, Nicolas Pitre wrote:
> The same could be said about the outer cache ops.  If a DSB is needed 
> for their intent to be valid, then why isn't this DSB always implied by 
> the corresponding cache op calls?

Hmm, just been thinking about this.

The L2x0 calls do contain a DSB but it's not obvious.  They hold a
raw spinlock, and when that spinlock is dropped, we issue a dsb and
sev instruction.

Whether the other L2 implementations do this or not I'm not sure -
but the above is a requirement of the spinlock implementation, and
it just happens to provide the right behaviour for L2x0.

But... we _probably_ don't want to impose that down at the L2 cache
level of things - at least not for DMA ops, particular for the sanity
of the scatter-list operating operations.  We really want to avoid
doing one DSB per scatterlist entry, doing one DSB per scatterlist
operation instead.

That does affect how the L2 cache API gets used - maybe we want to
separate out the DMA stuff from the other users so that we can have
dsbs in that path for non-DMA users.

Thoughts?