On Mon, Jun 23, 2014 at 11:43 AM, Barry Song <21cn...@gmail.com> wrote:
> From: Yibo Cai <yibo....@csr.com>
>
> Get square root of a 64-bit digit on 32bit platforms. CSR SiRFSoC
> touchscreen driver needs it.
>
> Signed-off-by: Yibo Cai <yibo....@csr.com>
> Signed-off-by: Barry Song <baohua.s...@csr.com>
> ---
> include/linux/kernel.h | 1 +
> lib/int_sqrt.c | 27 +++++++++++++++++++++++++++
> 2 files changed, 28 insertions(+)
>
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 4c52907..6e63081 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -418,6 +418,7 @@ struct pid;
> extern struct pid *session_of_pgrp(struct pid *pgrp);
>
> unsigned long int_sqrt(unsigned long);
> +unsigned long long int64_sqrt(unsigned long long);
>
> extern void bust_spinlocks(int yes);
> extern int oops_in_progress; /* If set, an oops, panic(), BUG() or
> die() is in progress */
> diff --git a/lib/int_sqrt.c b/lib/int_sqrt.c
> index 1ef4cc3..2aa9fcc 100644
> --- a/lib/int_sqrt.c
> +++ b/lib/int_sqrt.c
> @@ -36,3 +36,30 @@ unsigned long int_sqrt(unsigned long x)
> return y;
> }
> EXPORT_SYMBOL(int_sqrt);
> +
> +/*
> + * Square root of a 64-bit digit.
> + * Same as int_sqrt on 64-bit platforms where "long" equals "long long"
> + */
> +unsigned long long int64_sqrt(unsigned long long x)
> +{
> + unsigned long long m = 0, y = 0, b = 0;
> +
> + if (x <= 1)
> + return x;
> +
> + m = 1ULL << (BITS_PER_LONG_LONG - 2);
> + while (m != 0) {
> + b = y + m;
> + y >>= 1;
> +
> + if (x >= b) {
> + x -= b;
> + y += m;
> + }
> + m >>= 2;
> + }
> +
> + return y;
> +}
> +EXPORT_SYMBOL(int64_sqrt);
>
Maybe it's a stupid question (I'm new to kernel programming),
but why don't you use a for-loop instead of a while-loop?
(apart from following the already-existing code of int_sqrt.c)
E.g.:
for (m = 1ULL << (BITS_PER_LONG_LONG - 2); m != 0; m >>= 2) {
b = y + m;
y >>= 1;
if (x >= b) {
x -= b;
y += m;
}
}
Instead of:
m = 1ULL << (BITS_PER_LONG_LONG - 2);
while (m != 0) {
b = y + m;
y >>= 1;
if (x >= b) {
x -= b;
y += m;
}
m >>= 2;
}
Thanks,
Elias
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