On 14/10/15 18:07, Mark Rutland wrote:
On Wed, Oct 14, 2015 at 12:20:26PM +0100, Suzuki K. Poulose wrote:
+ * Number of page-table levels required to address 'va_bits' wide
+ * address, without section mapping. We resolve the top (va_bits - PAGE_SHIFT)
+ * bits with (PAGE_SHIFT - 3) bits at each page table level. Hence:
+ *
+ * levels = DIV_ROUND_UP((va_bits - PAGE_SHIFT), (PAGE_SHIFT - 3))
+ *
+ * We cannot include linux/kernel.h which defines DIV_ROUND_UP here
+ * due to build issues. So we use the following magic formula.
+ */
+#define ARM64_HW_PGTABLE_LEVELS(va_bits) (((va_bits) - 4) / (PAGE_SHIFT - 3))
I think I failed the interview question [1]. :(
I read the comment to mean this was a brand-new piece of magic, as
opposed to a constant-folded copy of DIV_ROUND_UP. So it seems there's
still some scope for confusion, even if that only includes me.
Wouldn't it be better to modify the comment to say, we open coded the
DIV_ROUND_UP ?
We could potentially end up in a conflict if somebody else does __DIV_ROUND_UP.
I have seen similar issues with the CPU feature series, where if I include one
particular header file in another, kernel build breaks without giving you a
clue,
what caused the error. Usually due to the multiple definitions (e.g
NSEC_PER_SEC)
and other conflicts. Given that this header file gets included with asm/page.h
and
hence would be used included for people outside arch/arm64, I would prefer, not
to
head there, instead update the comment, something like this :
/*
* Number of page-table levels required to address 'va_bits' wide
* address, without section mapping. We resolve the top (va_bits - PAGE_SHIFT)
* bits with (PAGE_SHIFT - 3) bits at each page table level. Hence:
*
* levels = DIV_ROUND_UP((va_bits - PAGE_SHIFT), (PAGE_SHIFT - 3))
*
* where DIV_ROUND_UP (n, d) = > ((n) + (d) - 1) / (d)
*
* We cannot include linux/kernel.h which defines DIV_ROUND_UP here
* due to build issues. So we open code the DIV_ROUND_UP and hence
* we get :
* ((va_bits - PAGE_SHIFT) + (PAGE_SHIFT - 3) -1) / (PAGE_SHIFT - 3)
*
* which gets simplified as :
* (((va_bits) - 4) / (PAGE_SHIFT - 3))
*
*/
Let me know if you are happy with that ?
Thanks
Suzuki
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