linux-kernel - Re: [PATCH 1/3] devmem: remove redundant test on len

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Date:	Sat, 12 Sep 2009 22:32:56 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"mtosatti@...hat.com" <mtosatti@...hat.com>,
	"gregkh@...e.de" <gregkh@...e.de>,
	"broonie@...nsource.wolfsonmicro.com" 
	<broonie@...nsource.wolfsonmicro.com>,
	"johannes@...solutions.net" <johannes@...solutions.net>,
	"avi@...ranet.com" <avi@...ranet.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] devmem: remove redundant test on len

On Sat, Sep 12, 2009 at 08:00:47AM +0800, Andrew Morton wrote:
> On Fri, 11 Sep 2009 10:23:34 +0800
> Wu Fengguang <fengguang.wu@...el.com> wrote:
> 
> > The len test in write_kmem() is always true, so can be reduced.
> > 
> > CC: Marcelo Tosatti <mtosatti@...hat.com>
> > CC: Greg Kroah-Hartman <gregkh@...e.de>
> > CC: Mark Brown <broonie@...nsource.wolfsonmicro.com>
> > CC: Johannes Berg <johannes@...solutions.net>
> > CC: Avi Kivity <avi@...ranet.com>
> > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > ---
> >  drivers/char/mem.c |   14 ++++++--------
> >  1 file changed, 6 insertions(+), 8 deletions(-)
> > 
> > --- linux.orig/drivers/char/mem.c
> > +++ linux/drivers/char/mem.c
> > @@ -580,18 +580,16 @@ static ssize_t write_kmem(struct file * 
> >  		while (count > 0) {
> >  			int len = count;
> >  
> >  			if (len > PAGE_SIZE)
> >  				len = PAGE_SIZE;
> > -			if (len) {
> > -				written = copy_from_user(kbuf, buf, len);
> > -				if (written) {
> > -					if (wrote + virtr)
> > -						break;
> > -					free_page((unsigned long)kbuf);
> > -					return -EFAULT;
> > -				}
> > +			written = copy_from_user(kbuf, buf, len);
> > +			if (written) {
> > +				if (wrote + virtr)
> > +					break;
> > +				free_page((unsigned long)kbuf);
> > +				return -EFAULT;
> >  			}
> >  			len = vwrite(kbuf, (char *)p, len);
> >  			count -= len;
> >  			buf += len;
> >  			virtr += len;
> 
> humpf.  But take a closer look at what remains.

Yeah, it asks for more cleanup patches..

> 
> Local var `written' is unneeded here.
> 
> 
> Which makes us look at what `written' _does_ do:
> 
> 		if (written != wrote)
> 			return written;
> 		wrote = written;
> 
> lolwhowrotethat?
> 
> local var `written' can at least be made local to the first loop.

Right, I'll make this straight.

> write_kmem() has a lot of typecasts which indicates that the choices of
> types were inappropriate.

It seems hard to improve because of a fundamental issue: the same
value is taken as both virtual and physical address, and also be
compared and calculated as numbers..

Thanks,
Fengguang

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