linux-kernel - Re: [PATCH 0/2] ARM: omap2+: omap_hwmod: Fix false lockdep warning

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Date:	Mon, 9 Feb 2015 18:55:49 +0000 (UTC)
From:	Paul Walmsley <paul@...an.com>
To:	Tony Lindgren <tony@...mide.com>
cc:	Peter Ujfalusi <peter.ujfalusi@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-omap@...r.kernel.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org, balbi@...com,
	linux-arm-kernel@...ts.infradead.org, Tero Kristo <t-kristo@...com>
Subject: Re: [PATCH 0/2] ARM: omap2+: omap_hwmod: Fix false lockdep warning

On Mon, 9 Feb 2015, Tony Lindgren wrote:

> * Paul Walmsley <paul@...an.com> [150209 08:04]:
> > On Mon, 9 Feb 2015, Peter Ujfalusi wrote:
> > 
> > > On 02/06/2015 09:26 PM, Peter Ujfalusi wrote:
> > > >> Yeah, I've never really bothered with data too much, its a debug
> > > >> feature. So lock_class_key is 8 bytes, and strictly speaking you could
> > > >> union them over other fields, all we really need is unique addresses, we
> > > >> don't actually use the storage.
> > > > 
> > > > True. our omap2plus defconfig does not have LOCKDEP enabled so it should not
> > > > add anything to the data when running default kernel.
> > > > I'll test the lockdep_set_class() method you suggested on Monday (not
> > > > tomorrow), but still as first thing.
> > > > If it is working as expected I'll send a patch with you as author.
> > > 
> > > With omap2plus_defconfig my build produces (vmlinux size):
> > > Base: 				99905522
> > > with my series:			99908385 (base + 2863)
> > > with Peter Zijlstra's patch:	99910625 (base + 5103)
> > > 
> > > The reason for this is that we will only have
> > > struct lock_class_key { };
> > > in case of !CONFIG_LOCKDEP. On ARM however CONFIG_LOCKDEP is enabled by
> > > default, while the CONFIG_DEBUG_LOCKDEP is disabled.
> > > 
> > > So it does add more data to our default omap2plus config.
> > > 
> > > Tony: do you have preference on the way we fix this issue?
> > > 
> > > As I recall there is a plan to remove the hwmod static database and move it or
> > > generate it from DT? Not sure when and how this will be done, but will it
> > > affect the lockdep_set_class() way?
> > 
> > Well I guess we could see what Tony says, but you do realize that the 
> > difference in sizes that you posted above is about .003% of the total 
> > binary size, right?  
> > 
> > If there's one thing we can say about the last few years of ARM kernel 
> > development, it's that those kind of size increases are utterly dwarfed by 
> > other changes in the kernel.  So I'd say, post a patch based on PeterZ's 
> > fix and be done with it...
> 
> Well the thing to consider here is what Peter U is saying about
> having struct omap_hwmod allocated based on the data from .dts
> files. If the fix makes the dynamic allocation harder to do later on,
> we should probably avoid it. If it's relatively easy to do later on,
> then I don't have a problem with it.

The future destination for that code that makes the most sense to me is 
for it to become integrated with the OMAP Sonics & Arteris bus drivers and 
DT data.  So I wouldn't worry too much about it; I don't think the 
lockdep fix will affect that at all.


- Paul
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