linux-kernel - Re: [PATCH 04/11] hwmon/lm63,lm90: take msecs_to_jiffies

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Message-ID: <20130513142300.3a44ebe1@endymion.delvare>
Date:	Mon, 13 May 2013 14:23:00 +0200
From:	Jean Delvare <khali@...ux-fr.org>
To:	imre.deak@...el.com
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Guenter Roeck <linux@...ck-us.net>, lm-sensors@...sensors.org
Subject: Re: [PATCH 04/11] hwmon/lm63,lm90: take msecs_to_jiffies_min into 
 use

On Mon, 13 May 2013 14:56:33 +0300, Imre Deak wrote:
> On Mon, 2013-05-13 at 09:47 +0200, Jean Delvare wrote:
> > Hi Imre,
> > 
> > On Fri, 10 May 2013 15:13:22 +0300, Imre Deak wrote:
> > > Use msecs_to_jiffies_min instead of open-coding the same.
> > > 
> > > Signed-off-by: Imre Deak <imre.deak@...el.com>
> > > ---
> > >  drivers/hwmon/lm63.c |    2 +-
> > >  drivers/hwmon/lm90.c |    2 +-
> > >  2 files changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/hwmon/lm63.c b/drivers/hwmon/lm63.c
> > > index f644a2e..db44bcb 100644
> > > --- a/drivers/hwmon/lm63.c
> > > +++ b/drivers/hwmon/lm63.c
> > > @@ -248,7 +248,7 @@ static struct lm63_data *lm63_update_device(struct device *dev)
> > >  	mutex_lock(&data->update_lock);
> > >  
> > >  	next_update = data->last_updated
> > > -	  + msecs_to_jiffies(data->update_interval) + 1;
> > > +	  + msecs_to_jiffies_min(data->update_interval);
> > >  
> > >  	if (time_after(jiffies, next_update) || !data->valid) {
> > >  		if (data->config & 0x04) { /* tachometer enabled  */
> > > diff --git a/drivers/hwmon/lm90.c b/drivers/hwmon/lm90.c
> > > index 8eeb141..314f9d3 100644
> > > --- a/drivers/hwmon/lm90.c
> > > +++ b/drivers/hwmon/lm90.c
> > > @@ -471,7 +471,7 @@ static struct lm90_data *lm90_update_device(struct device *dev)
> > >  	mutex_lock(&data->update_lock);
> > >  
> > >  	next_update = data->last_updated
> > > -	  + msecs_to_jiffies(data->update_interval) + 1;
> > > +	  + msecs_to_jiffies_min(data->update_interval);
> > >  	if (time_after(jiffies, next_update) || !data->valid) {
> > >  		u8 h, l;
> > >  		u8 alarms;
> > 
> > I don't like this. The + 1 aren't there because msecs_to_jiffies() may
> > return less than the required time, as you claim. I don't think this is
> > the case (see the doubts expressed in my reply to patch 1/11.)
> > 
> > These + 1 are there because the chip may need exactly
> > data->update_interval for the data sampling and the datasheet isn't
> > completely clear about what would happen if the host polls for results
> > at exactly the same frequency the chip is sampling.
> > 
> > Most likely it is just paranoia from me and the + 1 can be removed.
> > Especially when time_after (as opposed to time_after_eq) already adds
> > some margin - probably exactly what we need here. I have a few chips
> > here I could test this on, BTW (ADM1032 and LM64.)
> 
> Yes I realize now that time_after() vs. time_after_eq() guarantees
> already that we wait at least data->update_interval. Moreover +1 was
> there to wait more than this, so clearly my change here was incorrect.
> 
> > And even if this was actually needed, then it is written the wrong way.
> > We don't need one more jiffy, the SMBus slave doesn't even know what a
> > jiffy is. We need an arbitrary additional amount of time, expressed in
> > a standard time unit like ms. So
> >   msecs_to_jiffies(data->update_interval + 1)
> > would be the right way to write it.
> 
> Agreed, this describes the intention better.

I'll look into it and send a patch.

> > On top of that, your proposed change will make the resulting binary
> > larger, the compilation longer,
> 
> True. This could be solved by turning the macros into uninlined
> functions.
> 
> > the future code reviews harder,
> 
> Not sure what you mean by this. Having a properly named helper instead
> of just writing +1 makes it more obvious what the intention was and
> easier to spot places that are doing the wrong thing.

I mean that this is one more thing to check when reviewing a new
driver: did the author use msecs_to_jiffies or msecs_to_jiffies_min.
But that wasn't a good point from me, I'll admit, as we already have to
check if the timeout value is set properly.

> > and backporting these drivers harder.
> 
> This would be the case for places where the adjustment is already there
> and we would just make this more explicit with the new helper. But for
> places we actually fix in the current code backporting the fix would be
> a benefit.

My point is that a backported driver can't use a function which was
added in a later kernel version than the backport target. Either the
missing function must be backported as well (but most often this isn't
an option) or some compatibility code must be thrown in. Even though I
agree that backporting isn't a primary concern and should never prevent
good changes from happening upstream, I think it is a gentle reminder
that changes should only be done when we're sure they are worth the
effort. I am maintaining two dozens of standalone hwmon drivers to be
used with older kernels, and the less work I have to do on these, the
more time I can devote to more interesting tasks.

> (...)
> Thanks a lot for the review and thoughts,

You're welcome.

-- 
Jean Delvare
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