[RFC 04/18] powerpc/mm: Split hash page table sizing heuristic into a helper

[David Gibson <david@gibson.dropbear.id.au>]

htab_get_table_size() either retrieve the size of the hash page table (HPT)
from the device tree - if the HPT size is determined by firmware - or
uses a heuristic to determine a good size based on RAM size if the kernel
is responsible for allocating the HPT.

To support a PAPR extension allowing resizing of the HPT, we're going to
want the memory size -> HPT size logic elsewhere, so split it out into a
helper function.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 arch/powerpc/include/asm/mmu-hash64.h |  3 +++
 arch/powerpc/mm/hash_utils_64.c       | 32 +++++++++++++++++++-------------
 2 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/arch/powerpc/include/asm/mmu-hash64.h b/arch/powerpc/include/asm/mmu-hash64.h
index 7352d3f..cf070fd 100644
--- a/arch/powerpc/include/asm/mmu-hash64.h
+++ b/arch/powerpc/include/asm/mmu-hash64.h
@@ -607,6 +607,9 @@ static inline unsigned long get_kernel_vsid(unsigned long ea, int ssize)
 	context = (MAX_USER_CONTEXT) + ((ea >> 60) - 0xc) + 1;
 	return get_vsid(context, ea, ssize);
 }
+
+unsigned htab_shift_for_mem_size(unsigned long mem_size);
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* _ASM_POWERPC_MMU_HASH64_H_ */
diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
index fdcf9d1..da5d279 100644
--- a/arch/powerpc/mm/hash_utils_64.c
+++ b/arch/powerpc/mm/hash_utils_64.c
@@ -611,10 +611,26 @@ static int __init htab_dt_scan_pftsize(unsigned long node,
 	return 0;
 }
 
-static unsigned long __init htab_get_table_size(void)
+unsigned htab_shift_for_mem_size(unsigned long mem_size)
 {
-	unsigned long mem_size, rnd_mem_size, pteg_count, psize;
+	unsigned memshift = __ilog2(mem_size);
+	unsigned pshift = mmu_psize_defs[mmu_virtual_psize].shift;
+	unsigned pteg_shift;
+
+	/* round mem_size up to next power of 2 */
+	if ((1UL << memshift) < mem_size)
+		memshift += 1;
+
+	/* aim for 2 pages / pteg */
+	pteg_shift = memshift - (pshift + 1);
+
+	/* 2^11 PTEGS / 2^18 bytes is the minimum htab size permitted
+	 * by the architecture */
+	return max(pteg_shift + 7, 18U);
+}
 
+static unsigned long __init htab_get_table_size(void)
+{
 	/* If hash size isn't already provided by the platform, we try to
 	 * retrieve it from the device-tree. If it's not there neither, we
 	 * calculate it now based on the total RAM size
@@ -624,17 +640,7 @@ static unsigned long __init htab_get_table_size(void)
 	if (ppc64_pft_size)
 		return 1UL << ppc64_pft_size;
 
-	/* round mem_size up to next power of 2 */
-	mem_size = memblock_phys_mem_size();
-	rnd_mem_size = 1UL << __ilog2(mem_size);
-	if (rnd_mem_size < mem_size)
-		rnd_mem_size <<= 1;
-
-	/* # pages / 2 */
-	psize = mmu_psize_defs[mmu_virtual_psize].shift;
-	pteg_count = max(rnd_mem_size >> (psize + 1), 1UL << 11);
-
-	return pteg_count << 7;
+	return 1UL << htab_shift_for_mem_size(memblock_phys_mem_size());
 }
 
 #ifdef CONFIG_MEMORY_HOTPLUG
-- 
2.5.0