[PATCH] mtd: spi-nor: fix the wrong dummy value
Marek Vasut
marex at denx.de
Thu Apr 17 07:15:12 PDT 2014
On Thursday, April 17, 2014 at 03:41:29 PM, Huang Shijie wrote:
> On Wed, Apr 16, 2014 at 10:08:49PM +0200, Gerhard Sittig wrote:
> > On Wed, 2014-04-16 at 16:18 +0800, Huang Shijie wrote:
> > > The dummy cycles is actually 8 for SPI fast/dual/quad read.
> > >
> > > This patch fixes the wrong dummy value for both the spi-nor.c and
> > > m25p80.c.
> > >
> > > Signed-off-by: Huang Shijie <b32955 at freescale.com>
> > > ---
> > >
> > > drivers/mtd/devices/m25p80.c | 3 +++
> > > drivers/mtd/spi-nor/spi-nor.c | 2 +-
> > > 2 files changed, 4 insertions(+), 1 deletions(-)
> > >
> > > diff --git a/drivers/mtd/devices/m25p80.c
> > > b/drivers/mtd/devices/m25p80.c index 1557d8f..112ca8b 100644
> > > --- a/drivers/mtd/devices/m25p80.c
> > > +++ b/drivers/mtd/devices/m25p80.c
> > > @@ -131,6 +131,9 @@ static int m25p80_read(struct spi_nor *nor, loff_t
> > > from, size_t len,
> > >
> > > int dummy = nor->read_dummy;
> > > int ret;
> > >
> > > + /* convert the dummy cycles to the number of byte */
> > > + dummy >>= 3;
> > > +
> >
> > "dummy /= 8" to match the comment / commit message, and for
> > better mental association with a byte's width and the below
> > return value?
>
> The disassemble code for "int dummy = 8; dummy /= 8;" is:
> --------------------------------------------------
> 83a6: 2308 movs r3, #8
> 83a8: 607b str r3, [r7, #4]
> 83aa: 687b ldr r3, [r7, #4]
> 83ac: 1dda adds r2, r3, #7
> 83ae: 2b00 cmp r3, #0
> 83b0: bfb4 ite lt
> 83b2: 4613 movlt r3, r2
> 83b4: 461b movge r3, r3
> 83b6: 10db asrs r3, r3, #3
> 83b8: 607b str r3, [r7, #4]
> --------------------------------------------------
>
> The disassemble code for "int dummy = 8; dummy >>= 3;" is:
> --------------------------------------------------
> 83a6: 2308 movs r3, #8
> 83a8: 607b str r3, [r7, #4]
> 83aa: 687b ldr r3, [r7, #4]
> 83ac: 10db asrs r3, r3, #3
> 83ae: 607b str r3, [r7, #4]
> --------------------------------------------------
>
> Obviously, the "dummy >>= 3" is faster then "dummy /= 8".
Which GCC compiler is that please ? I'm surprised GCC doesn't optimize it.
Best regards,
Marek Vasut
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