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

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Date:	Fri, 8 Jul 2011 08:43:50 +0200
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Greg KH <greg@...ah.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Sergiu Iordache <sergiu@...gle.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

2011/7/8 Greg KH <greg@...ah.com>:
> 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
>

I agree with Greg. I asked to not break the existent way to read data
via /dev/mem because for me it's the right way to do this thing.
However to do an easy *debug* a debugfs entry can be useful. IMHO, a
"production" script/application that use debugfs instead of /dev/mem
in this case is simply broken because the debugfs can't be like a
system call or other kernel interaction mechanism. Debugfs should be
used only for debug.

Marco
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