[GIT PULL v3] updates to qbman (soc drivers) to support arm/arm64

[Russell King - ARM Linux]

On Fri, Jun 23, 2017 at 06:58:17PM +0000, Roy Pledge wrote:
> On 6/23/2017 11:23 AM, Mark Rutland wrote:
> > On Fri, Jun 23, 2017 at 04:56:10PM +0200, Arnd Bergmann wrote:
> >> static inline void dpaa_flush(void *p)
> >> {
> >> #ifdef CONFIG_PPC
> >>         flush_dcache_range((unsigned long)p, (unsigned long)p+64);
> >> #elif defined(CONFIG_ARM32)
> >>         __cpuc_flush_dcache_area(p, 64);
> >> #elif defined(CONFIG_ARM64)
> >>         __flush_dcache_area(p, 64);
> >> #endif
> >> }
> > Assuming this is memory, why can't the driver use the DMA APIs to handle
> > this without reaching into arch-internal APIs?
> I agree this isn't pretty - I think we could use
> dma_sync_single_for_device() here but I am concerned it will be
> expensive and hurt performance significantly. The DMA APIs have a lot of
> branches. At some point we were doing 'dc cvac' here and even switching
> to the above calls caused a measurable drop in throughput at high frame
> rates.

Well...

__cpuc_flush_dcache_area() is used to implement flush_dcache_page() and
flush_anon_page().  The former is about ensuring coherency between
multiple mappings of a kernel page cache page and userspace.  The latter
is about ensuring coherency between an anonymous page and userspace.

Currently, on ARMv7, this is implemented using "mcr p15, 0, r0, c7, c14, 1"
but we _could_ decide that is too heavy for these interfaces, and instead
switch to a lighter cache flush if one were available in a future
architecture revision (since the use case of this only requires it to
flush to the point of unification, not to the point of coherence.)

The overall effect is that changing the behaviour of this would introduce
a regression into your driver, which would have to be reverted - and that
makes my job as architecture maintainer difficult.

It may make sense to explicitly introduce a "flush data cache to point
of coherence" function - we already have gicv3 and kvm wanting this, and
doing it the right way, via:

#define gic_flush_dcache_to_poc(a,l)    __cpuc_flush_dcache_area((a), (l))
#define kvm_flush_dcache_to_poc(a,l)     __cpuc_flush_dcache_area((a), (l))

So, how about we introduce something like:

	void flush_dcache_to_poc(void *addr, size_t size)

which is defined to ensure that data is visible to the point of coherence
on all architectures?

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