linux-kernel - Re: [PATCH v3 3/3] char drivers: ramoops debugfs entry

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Date:	Thu, 7 Jul 2011 16:33:35 -0700
From:	Greg KH <greg@...ah.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Sergiu Iordache <sergiu@...gle.com>,
	Marco Stornelli <marco.stornelli@...il.com>,
	"Ahmed S. Darwish" <darwish.07@...il.com>,
	Artem Bityutskiy <Artem.Bityutskiy@...ia.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] char drivers: ramoops debugfs entry

On Thu, Jul 07, 2011 at 04:27:27PM -0700, Andrew Morton wrote:
> On Thu, 7 Jul 2011 16:16:43 -0700
> Sergiu Iordache <sergiu@...gle.com> wrote:
> 
> > Ramoops currently dumps the log of a panic/oops in a memory area which
> > is known not to be overwritten on restart (for example 1MB starting at
> > 15MB). The way it works is by dividing the memory area in records of a
> > set size (fixed at 4K before my patches, configurable after) and by
> > dumping a record there for each oops/panic. The problem is that right
> > now you have to access that memory area through other means, such as
> > /dev/mem, which is not always possible.
> > 
> > What my patch did was to add a debugfs entry which returns a valid
> > record each time (a single dump done by ramoops). The first call
> > returns the first dump. The first call after the last valid dump
> > returns an empty buffer. .
> 
> Please fully describe this "record" in the v2 patch changelog.  We'll
> want to review it for endianness, 32/64-bit compat issues,
> maintainability, extensibility, etc.
> 
> > After it has returned nothing, the next
> > calls return records from the start again.
> 
> That sounds a bit weird.  One would expect it to keep returning zero,
> requiring userspace to lseek or close/open.
> 
> > The validity of a dump is
> > checked by looking after the header. Any comments on this approach are
> > welcome.
> > 
> > Changing the entry from debugfs to sysfs wouldn't be a problem. If
> > sysfs is a valid solution I'll come with a patch that updates the
> > documentation as well along with the sysfs entry.
> 
> sysfs sounds OK to me.  Then again, sysfs is supposed to be
> one-value-per-file, so using it would be naughty.
> 
> I dunno, I'd be inclined to abuse the sysfs rule and hope that nobody
> notices rather than create a fake char device.  But there's certainly
> plenty of precedent for the fake char driver.

No, please don't abuse sysfs that way.

Use debugfs or a char device node.

thanks,

greg k-h
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