linux-kernel - Re: [PATCH 05/25] ubifs: implement ubifs_lpt_lookup using ubifs_pnode

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Date:   Mon, 13 Aug 2018 10:12:38 +0200
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Richard Weinberger <richard@....at>
Cc:     linux-mtd@...ts.infradead.org, David Gstir <david@...ma-star.at>,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATCH 05/25] ubifs: implement ubifs_lpt_lookup using
 ubifs_pnode_lookup

On Mon, Aug 13, 2018 at 08:34:00AM +0200, Richard Weinberger wrote:
> Am Montag, 13. August 2018, 08:31:27 CEST schrieb Sascha Hauer:
> > On Wed, Jul 04, 2018 at 02:41:17PM +0200, Sascha Hauer wrote:
> > > ubifs_lpt_lookup() starts by looking up the nth pnode in the LPT. We
> > > already have this functionality in ubifs_pnode_lookup(). Use this
> > > function rather than open coding its functionality.
> > > 
> > > Signed-off-by: Sascha Hauer <s.hauer@...gutronix.de>
> > > ---
> > >  fs/ubifs/lpt.c | 20 ++------------------
> > >  1 file changed, 2 insertions(+), 18 deletions(-)
> > > 
> > > diff --git a/fs/ubifs/lpt.c b/fs/ubifs/lpt.c
> > > index 6cd6f23d4512..cde7b9484157 100644
> > > --- a/fs/ubifs/lpt.c
> > > +++ b/fs/ubifs/lpt.c
> > > @@ -1478,27 +1478,11 @@ struct ubifs_pnode *ubifs_pnode_lookup(struct ubifs_info *c, int i)
> > >   */
> > >  struct ubifs_lprops *ubifs_lpt_lookup(struct ubifs_info *c, int lnum)
> > >  {
> > > -	int err, i, h, iip, shft;
> > > -	struct ubifs_nnode *nnode;
> > > +	int i, iip;
> > >  	struct ubifs_pnode *pnode;
> > >  
> > > -	if (!c->nroot) {
> > > -		err = ubifs_read_nnode(c, NULL, 0);
> > > -		if (err)
> > > -			return ERR_PTR(err);
> > > -	}
> > > -	nnode = c->nroot;
> > >  	i = lnum - c->main_first;
> > > -	shft = c->lpt_hght * UBIFS_LPT_FANOUT_SHIFT;
> > > -	for (h = 1; h < c->lpt_hght; h++) {
> > > -		iip = ((i >> shft) & (UBIFS_LPT_FANOUT - 1));
> > > -		shft -= UBIFS_LPT_FANOUT_SHIFT;
> > > -		nnode = ubifs_get_nnode(c, nnode, iip);
> > > -		if (IS_ERR(nnode))
> > > -			return ERR_CAST(nnode);
> > > -	}
> > > -	iip = ((i >> shft) & (UBIFS_LPT_FANOUT - 1));
> > > -	pnode = ubifs_get_pnode(c, nnode, iip);
> > > +	pnode = ubifs_pnode_lookup(c, i);
> > 
> > This should be
> > 
> > 	ubifs_pnode_lookup(c, i >> UBIFS_LPT_FANOUT_SHIFT);
> 
> Can you please add a helper function for that? These shift games are
> always confusing and not easy to spot.

Do you have a suggestion for a helper function? lpt.c is full of rather
non obvious arithmetic operations and I couldn't find a helper function
that could be reused at least once in the code.

I tried with:

static int lnum_to_pnode_num(struct ubifs_info *c, int lnum)
{
	return (lnum - c->main_first) >> UBIFS_LPT_FANOUT_SHIFT;
}

But I am not sure this really improves things

Sascha

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