Re: [PATCH] mtd: spi-nor: fix the wrong dummy value

[Huang Shijie <shijie8@gmail.com>]

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@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".

thanks
Huang Shijie