[PATCH 0/3] ARM: mvebu: disable I/O coherency on !SMP

[Russell King - ARM Linux]

On Thu, Jul 17, 2014 at 12:04:26PM -0400, Santosh Shilimkar wrote:
> Thanks for spotting it RMK. Will have a loot at it.

I already have this for it - which includes a lot more comments on the
code (some in anticipation of the reply from the architecture people.)
The first hunk alone should fix the spotted problem.

diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
index ab14b79b03f0..917aabd6b2dc 100644
--- a/arch/arm/mm/mmu.c
+++ b/arch/arm/mm/mmu.c
@@ -1406,8 +1406,8 @@ void __init early_paging_init(const struct machine_desc *mdesc,
 		return;
 
 	/* remap kernel code and data */
-	map_start = init_mm.start_code;
-	map_end   = init_mm.brk;
+	map_start = init_mm.start_code & PMD_MASK;
+	map_end   = ALIGN(init_mm.brk, PMD_SIZE);
 
 	/* get a handle on things... */
 	pgd0 = pgd_offset_k(0);
@@ -1434,23 +1434,60 @@ void __init early_paging_init(const struct machine_desc *mdesc,
 	dsb(ishst);
 	isb();
 
-	/* remap level 1 table */
+	/*
+	 * WARNING: This code is not architecturally compliant: we modify
+	 * the mappings in-place, indeed while they are in use by this
+	 * very same code.
+	 *
+	 * Even modifying the mappings in a separate page table does
+	 * not resolve this.
+	 *
+	 * The architecture strongly recommends that when a mapping is
+	 * changed, that it is changed by first going via an invalid
+	 * mapping and back to the new mapping.  This is to ensure
+	 * that no TLB conflicts (caused by the TLB having more than
+	 * one TLB entry match a translation) can occur.
+	 */
+
+	/*
+	 * Remap level 1 table.  This changes the physical addresses
+	 * used to refer to the level 2 page tables to the high
+	 * physical address alias, leaving everything else the same.
+	 */
 	for (i = 0; i < PTRS_PER_PGD; pud0++, i++) {
 		set_pud(pud0,
 			__pud(__pa(pmd0) | PMD_TYPE_TABLE | L_PGD_SWAPPER));
 		pmd0 += PTRS_PER_PMD;
 	}
 
-	/* remap pmds for kernel mapping */
-	phys = __pa(map_start) & PMD_MASK;
+	/*
+	 * Remap the level 2 table, pointing the mappings at the high
+	 * physical address alias of these pages.
+	 */
+	phys = __pa(map_start);
 	do {
 		*pmdk++ = __pmd(phys | pmdprot);
 		phys += PMD_SIZE;
 	} while (phys < map_end);
 
+	/*
+	 * Ensure that the above updates are flushed out of the cache.
+	 * This is not strictly correct; on a system where the caches
+	 * are coherent with each other, but the MMU page table walks
+	 * may not be coherent, flush_cache_all() may be a no-op, and
+	 * this will fail.
+	 */
 	flush_cache_all();
+
+	/*
+	 * Re-write the TTBR values to point them at the high physical
+	 * alias of the page tables.  We expect __va() will work on
+	 * cpu_get_pgd(), which returns the value of TTBR0.
+	 */
 	cpu_switch_mm(pgd0, &init_mm);
 	cpu_set_ttbr(1, __pa(pgd0) + TTBR1_OFFSET);
+
+	/* Finally flush any stale TLB values. */
 	local_flush_bp_all();
 	local_flush_tlb_all();
 }


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