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

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Message-ID: <20150402002020.GA30284@opentech.at>
Date:	Thu, 2 Apr 2015 02:20:20 +0200
From:	Nicholas Mc Guire <der.herr@...r.at>
To:	Joe Perches <joe@...ches.com>
Cc:	Aaron Sierra <asierra@...-inc.com>,
	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

On Wed, 01 Apr 2015, Joe Perches wrote:

> On Wed, 2015-04-01 at 10:08 -0500, Aaron Sierra wrote:
> > ----- 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 ?cho
> > 
> > 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.
> 
> Nicholas, any progress on that builtin_const_p variant
>
did not get to it yet - but thats the next thing
on my fun list.

thx!
hofrat
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