linux-kernel - Re: [PATCH v7 3/5] misc: fuse: Add efuse driver for Tegra

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Date:	Wed, 11 Jun 2014 19:19:02 +0300
From:	Peter De Schrijver <pdeschrijver@...dia.com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	Mikko Perttunen <mperttunen@...dia.com>,
	Russell King <linux@....linux.org.uk>,
	Thierry Reding <thierry.reding@...il.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Wolfram Sang <wsa@...-dreams.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 3/5] misc: fuse: Add efuse driver for Tegra

On Wed, Jun 11, 2014 at 05:58:28PM +0200, Stephen Warren wrote:
> On 06/11/2014 09:25 AM, Peter De Schrijver wrote:
> > On Wed, Jun 11, 2014 at 02:47:31PM +0200, Mikko Perttunen wrote:
> >> On 05/06/14 16:09, Peter De Schrijver wrote:
> >> ...
> >>> +int tegra_fuse_readl(u32 offset, u32 *val)
> >>> +{
> >>> +       if (!fuse_readl)
> >>> +               return -ENXIO;
> >>> +
> >>> +       *val = fuse_readl(offset);
> >>> +
> >>> +       return 0;
> >>> +}
> >>> +
> >>
> >> -EPROBE_DEFER would be a better error value, so that drivers can work 
> > 
> > Ok.
> > 
> >> even if they are initially probed before the fuse driver. Of course, if 
> >> the fuse initialization is moved into machine init then this is a non-issue.
> > 
> > The exported function will always be initialized later because on Tegra20 it
> > requires APB DMA to be available. If you read the fuses directly, the system
> > sometimes hangs.
> 
> That's not true in the current code. IIRC, the bug was that *if* an APB
> DMA access to anything and a CPU access to the fuses happen at the same
> time, then there can be a hang. As such, the current fuse code accesses
> the fuses directly (without potential for a hang) if the APB DMA driver
> is not available, but once the driver becomes available, it reads the
> fuses through DMA instead. Does the new code not do that?
> 

I'm not so sure about that. I have seen the hang when dumping all fuses using
sysfs in an otherwise idle system booted from initrd. I don't think there
should be any APB DMA activity going on then?

Cheers,

Peter.
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