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SubjectRe: [PATCH] fat: don't use custom hex_to_bin()
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On Fri, 2011-09-23 at 14:05 +0200, Denys Vlasenko wrote: 
> On Fri, Sep 23, 2011 at 1:32 PM, Andy Shevchenko
> <andriy.shevchenko@linux.intel.com> wrote:
> > Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> > Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
> > ---
> > fs/fat/namei_vfat.c | 22 ++++++----------------
> > 1 files changed, 6 insertions(+), 16 deletions(-)
> >
> > diff --git a/fs/fat/namei_vfat.c b/fs/fat/namei_vfat.c
> > index bb3f29c..09cec4c 100644
> > --- a/fs/fat/namei_vfat.c
> > +++ b/fs/fat/namei_vfat.c
> > @@ -21,6 +21,8 @@
> > #include <linux/slab.h>
> > #include <linux/buffer_head.h>
> > #include <linux/namei.h>
> > +#include <linux/kernel.h>
> > +
> > #include "fat.h"
> >
> > /*
> > @@ -505,7 +507,6 @@ xlate_to_uni(const unsigned char *name, int len, unsigned char *outname,
> > struct nls_table *nls)
> > {
> > const unsigned char *ip;
> > - unsigned char nc;
> > unsigned char *op;
> > unsigned int ec;
> > int i, k, fill;
> > @@ -530,21 +531,10 @@ xlate_to_uni(const unsigned char *name, int len, unsigned char *outname,
> > return -EINVAL;
> > ec = 0;
> > for (k = 1; k < 5; k++) {
> > - nc = ip[k];
> > - ec <<= 4;
> > - if (nc >= '0' && nc <= '9') {
> > - ec |= nc - '0';
> > - continue;
> > - }
> > - if (nc >= 'a' && nc <= 'f') {
> > - ec |= nc - ('a' - 10);
> > - continue;
> > - }
> > - if (nc >= 'A' && nc <= 'F') {
> > - ec |= nc - ('A' - 10);
> > - continue;
> > - }
> > - return -EINVAL;
> > + int val = hex_to_bin(ip[k]);
> > + if (val < 0)
> > + return -EINVAL;
> > + ec = (ec << 4) | val;
> > }
> > *op++ = ec & 0xFF;
> > *op++ = ec >> 8;
>
> Function call per byte?
4 times in our case per 16 bit.

> That's expensive!
By the way, you can do those optimizations across the kernel. There are
dozens of places where one or few bytes are converted per half-byte.

> Can't we have a function which converts many sequential bytes?
We have one, but it doesn't return any error. We could wait until Mimi
integrates his patch series.


--
Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Intel Finland Oy


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