nios2: ensure that memblock.current_limit is set when setting pfn limits

On nios2, with CONFIG_FLATMEM set, the kernel relies on
memblock_get_current_limit() to determine the limits of mem_map, in
particular for max_low_pfn.
Unfortunately, memblock.current_limit is only default initialized to
MEMBLOCK_ALLOC_ANYWHERE at this point of the bootup, potentially leading
to situations where max_low_pfn can erroneously exceed the value of
max_pfn and, thus, the valid range of available DRAM.

This can in turn cause kernel-level paging failures, e.g.:

[   76.900000] Unable to handle kernel paging request at virtual address 20303000
[   76.900000] ea = c0080890, ra = c000462c, cause = 14
[   76.900000] Kernel panic - not syncing: Oops
[   76.900000] ---[ end Kernel panic - not syncing: Oops ]---

This patch fixes this by pre-calculating memblock.current_limit
based on the upper limits of the available memory ranges via
adjust_lowmem_bounds, a simplified version of the equivalent
implementation within the arm architecture.

Signed-off-by: Simon Schuster <schuster.simon@siemens-energy.com>
Signed-off-by: Andreas Oetken <andreas.oetken@siemens-energy.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>

Author: Simon Schuster

arch/nios2/kernel/setup.c

@@ -142,6 +142,20 @@ static void __init find_limits(unsigned long *min, unsigned long *max_low,
 	*max_high = PFN_DOWN(memblock_end_of_DRAM());
 }
 
+static void __init adjust_lowmem_bounds(void)
+{
+	phys_addr_t block_start, block_end;
+	u64 i;
+	phys_addr_t memblock_limit = 0;
+
+	for_each_mem_range(i, &block_start, &block_end) {
+		if (block_end > memblock_limit)
+			memblock_limit = block_end;
+	}
+
+	memblock_set_current_limit(memblock_limit);
+}
+
 void __init setup_arch(char **cmdline_p)
 {
 	console_verbose();
@@ -157,6 +171,7 @@ void __init setup_arch(char **cmdline_p)
 	/* Keep a copy of command line */
 	*cmdline_p = boot_command_line;
 
+	adjust_lowmem_bounds();
 	find_limits(&min_low_pfn, &max_low_pfn, &max_pfn);
 
 	memblock_reserve(__pa_symbol(_stext), _end - _stext);