linux-kernel - Re: [PATCH] mtd: fsl_ifc_nand: use msecs_to

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Date:	Wed, 1 Apr 2015 10:08:24 -0500 (CDT)
From:	Aaron Sierra <asierra@...-inc.com>
To:	Nicholas Mc Guire <der.herr@...r.at>
Cc:	Nicholas Mc Guire <hofrat@...dl.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Brian Norris <computersforpeace@...il.com>,
	Joe Schultz <jschultz@...-inc.com>,
	Wolfram Sang <wsa@...-dreams.de>,
	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mtd: fsl_ifc_nand: use msecs_to_jiffies for time
	conversion

----- Original Message -----
> From: "Nicholas Mc Guire" <der.herr@...r.at>
> Sent: Wednesday, April 1, 2015 9:24:44 AM
> 
> On Wed, 01 Apr 2015, Aaron Sierra wrote:
> 
> > ----- Original Message -----
> > > From: "Nicholas Mc Guire" <hofrat@...dl.org>
> > > Sent: Friday, March 13, 2015 6:23:47 AM
> > > 
> > > This is only an API consolidation and should make things more readable
> > > it replaces var * HZ / 1000 by msecs_to_jiffies(var) which helps
> > > readability
> > > and also handles all corner-cases properly.
> > > 
> > > Signed-off-by: Nicholas Mc Guire <hofrat@...dl.org>
> > > ---
> > > 
> > > Patch was compile tested with corenet64_smp_defconfig
> > > (implies CONFIG_MTD_NAND_FSL_IFC=y)
> > > 
> > > Patch is against 4.0-rc3 (localversion-next is -next-20150313
> > > 
> > >  drivers/mtd/nand/fsl_ifc_nand.c |    4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/mtd/nand/fsl_ifc_nand.c
> > > b/drivers/mtd/nand/fsl_ifc_nand.c
> > > index 4c05f4f..51394e5 100644
> > > --- a/drivers/mtd/nand/fsl_ifc_nand.c
> > > +++ b/drivers/mtd/nand/fsl_ifc_nand.c
> > > @@ -317,7 +317,7 @@ static void fsl_ifc_run_command(struct mtd_info *mtd)
> > >  
> > >  	/* wait for command complete flag or timeout */
> > >  	wait_event_timeout(ctrl->nand_wait, ctrl->nand_stat,
> > > -			   IFC_TIMEOUT_MSECS * HZ/1000);
> > > +			   msecs_to_jiffies(IFC_TIMEOUT_MSECS));
> > 
> > Nicholas,
> > Your patch makes me think that this timeout value should be calculated
> > once during initialization and stored in a new member of
> > struct fsl_ifc_mtd. That would improve readability AND have some
> > positive impact on performance.
> >
> 
> is it not a bit wasteful to put it into struct fsl_ifc_mtd
> which is used quite often in here ?

Storing the timeout in the private structure would use 4 or 8 bytes,
but the structure is passed around as pointer reference, so there is
no added overhead there.
 
> #define IFC_TIMEOUT msecs_to_jiffies(500)
>
> sould do - its only referenced twice and would achieve what you
> suggested - initializing it in one location and improved readability
> 
> 	wait_event_timeout(ctrl->nand_wait, ctrl->nand_stat, IFC_TIMEOUT);
> 
> would there be any disadvantage of this solution?

I don't have an issue with the compactness of your code. The issue
to me is that msecs_to_jiffies() is a real function call, not a macro.
That means that there would be a branch to that function followed by a
non-trivial calculation *every* time fsl_ifc_run_command() is called.

If the timeout value were calculated during init, then the cost of
accessing it would simply a pointer dereference.

-Aaron S.
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