powerpc/mm: Split hash page table sizing heuristic into a helper

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>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 21 insertions, 13 deletions

diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
index 335cd4115646..90dd9280894f 100644
--- a/arch/powerpc/mm/hash_utils_64.c
+++ b/arch/powerpc/mm/hash_utils_64.c
@@ -609,10 +609,28 @@ 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 of 128 bytes each, ie. 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
@@ -622,17 +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