lib_ppc: rework the flush_cache

The lib_ppc version basically is as below.
void flush_cache (ulong start_addr, ulong size)

{
	ulong addr, end_addr = start_addr + size;
	addr = start_addr & (CONFIG_SYS_CACHELINE_SIZE - 1);
	for (addr = start_addr; addr < end_addr; addr += CONFIG_SYS_CACHELINE_SIZE) {
		asm ("dcbst 0,%0": :"r" (addr));
	}
}

so, you are not completely right, the flush is from start_addr.

Quoted 4 lines

Review

Quoted 2 lines

(quoted diff) +2 -0

+	start = start_addr & ~(CONFIG_SYS_CACHELINE_SIZE - 1);
+	end = (start_addr + size) & ~(CONFIG_SYS_CACHELINE_SIZE - 1);

end = start_addr + size - 1;

Quoted 6 lines

The rounding is unnecessary for end, and without the - 1, if start_addr + size is on a cacheline boundary, you'll flush one cache line too many (which might not be mapped, or might cause end to wrap around to zero if flushing at the end of the address space).

I don't see what is the problem in my patch at here.

start_addr = 0
size = 0x1000

start will be 0
end will be 0x1000

The loop will flush the cache line at 0x1000, because it uses <= rather than <. That is outside the area that was requested to be

Maybe it's not mapped. Or, maybe start + size = 0 and > nothing gets