linux-kernel - Re: [PATCH] staging: nvec: fixed few coding style warnings

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Date:	Thu, 15 Oct 2015 11:15:07 +0200
From:	Marc Dietrich <marvin24@....de>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	gregkh@...uxfoundation.org, linux-tegra@...r.kernel.org,
	Sakshi Bansal <sakshi.april5@...il.com>,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: nvec: fixed few coding style warnings

Am Donnerstag, 15. Oktober 2015, 11:48:12 schrieb Dan Carpenter:
> On Thu, Oct 15, 2015 at 10:39:02AM +0200, Marc Dietrich wrote:
> > > > -			if (unlikely(nvec->rx == NULL)) {
> > > > +			if (!unlikely(nvec->rx)) {
> > > 
> > > This isn't right.  You intented to say:
> > > 	if (unlikely(!nvec->rx)) {
> > > 
> > > But even better to just remove the unlikely entirely.
> > > 
> > > 	if (!nvec->rx) {
> > 
> > why? the "unlikely" is there to optimize a critical interrupt path.
> 
> The rule is that drivers should not use likely/unlikely() unless there
> is a difference in benchmark numbers. 

well, we know that additional cpu cycles in this path break transfer for 
unknown reasons. However, the unlikely here may be overkill. On the other 
hand, I prefer not to change something here until these timing effects are 
better understood.

> How critical can it be when it's
> always followed by a udelay(100)???

yes, this delay shouldn't be there at all. This is one of the timing mysteries 
we still have to resolve.

> There are more important optimizations needed here.

sure. We are currently trying to move all this out into the tegra-i2c driver, 
so this code block will get a major review/rewrite in the near future anyway.

Marc

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