linux-kernel - Re: [PATCH 1/1] regmap: regcache-rbtree: Use GFP

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Message-ID: <8537578.XGSGY4If3j@ws-stein>
Date:	Tue, 21 Jul 2015 08:14:32 +0200
From:	Alexander Stein <alexander.stein@...tec-electronic.com>
To:	Mark Brown <broonie@...nel.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] regmap: regcache-rbtree: Use GFP_ATOMIC when using spinlocks

On Monday 20 July 2015 18:26:27, Mark Brown wrote:
> > AFAICS even a flat cache seems also only be usefull when providing
> > defaults, no? (Or having volatile registers).
> 
> Well, it's *better* to provide defaults since otherwise everything
> defaults to 0 but it does avoid the whole allocation during fast path
> issue since it allocates the cache on init and perhaps that's OK.

There is another reason for using REGCACHE_FLAT: Using regcache_cache_only 
(e.g. during suspend) which is not possible with REGCACHE_NONE.

> > So how to handle this properly? Bail out, if fast_io is available and
> > cache_type != (REGCACHE_NONE || REGCACHE_FLAT)?
> 
> Or perhaps just if we have to do an allocation?  I can see that someone
> might want to use an rbtree and would be careful enough to do the init,
> though I *am* a bit dubious about it.

I'm feeling uncomfortable this warning occured only when (at least) 
CONFIG_LOCKDEP is enabled. It warns right ahead but only if you begged for 
it...
Even if defaults are provided an extension to the register set (e.g. a more 
recent IP revision with more features) might not be synchronized with the 
defaults. Nobody might noticed until CONFIG_LOCKDEP is enabled and the 
register without defaults gets written.

Best regards,
Alexander
-- 
Dipl.-Inf. Alexander Stein
SYS TEC electronic GmbH
alexander.stein@...tec-electronic.com

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