[PATCH RFC 2/2] ARM: add soc memory barrier extension

[Russell King - ARM Linux]

On Wed, Jun 03, 2015 at 02:15:25PM +0100, Catalin Marinas wrote:
> On Wed, Jun 03, 2015 at 01:35:20PM +0100, Russell King wrote:
> > Add an extension to the heavy barrier code to allow a SoC specific
> > memory barrier function to be provided.  This is needed for platforms
> > where the interconnect has weak ordering, and thus needs assistance
> > to ensure that memory writes are properly visible in the correct order
> > to other parts of the system.
> 
> Do you have an example of where this is needed? Were they previously
> handled by hijacking outer_cache.sync?

Look for omap_bus_sync() - you need to go back through the kernel history
as it has been half-heartedly removed (OMAP's mach/barriers.h still exists
but it isn't used by anything, and omap_bus_sync() is now just an assembly
stub.)

Basically, OMAP used to set CONFIG_ARCH_HAS_BARRIERS, which caused OMAPs
mach/barriers.h to be picked up.  This contained:

#define rmb()		dsb()
#define wmb()		do { dsb(); outer_sync(); omap_bus_sync(); } while (0)
#define mb()		wmb()

and omap_bus_sync() was implemented arch/arm/mach-omap2/omap4-common.c
as:

void __iomem *dram_sync, *sram_sync;

void omap_bus_sync(void)
{
	if (dram_sync && sram_sync) {
		writel_relaxed(readl_relaxed(dram_sync), dram_sync);
		writel_relaxed(readl_relaxed(sram_sync), sram_sync);
		isb();
	}
}
EXPORT_SYMBOL(omap_bus_sync);

with the initialisation:

	dram_io_desc[0].virtual = OMAP4_DRAM_BARRIER_VA;
	dram_io_desc[0].pfn = __phys_to_pfn(paddr);
	dram_io_desc[0].length = size;
	dram_io_desc[0].type = MT_MEMORY_RW_SO;
	iotable_init(dram_io_desc, ARRAY_SIZE(dram_io_desc));
	dram_sync = (void __iomem *) dram_io_desc[0].virtual;

and:

		sram_sync = (void *)gen_pool_alloc(sram_pool, PAGE_SIZE);


which has the effect of issuing a strongly ordered access out to memory,
which cause the OMAP interconnects to be flushed.

The message I'm hearing from TI is that the removal of this from mainline
is a mistake (it sounds to me like it's caused a regression), and it needs
to be restored, otherwise data accesses via various paths to DRAM can end
up being seen out of order.

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