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

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Message-ID: <CANGUGtBSsfP+syCGQ4EVN_GYhU+S+tBB=gDf-bxVD3RfXKsHrg@mail.gmail.com>
Date:	Tue, 12 Jul 2011 08:41:51 +0200
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Sergiu Iordache <sergiu@...omium.org>
Cc:	Greg KH <greg@...ah.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"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

2011/7/11 Sergiu Iordache <sergiu@...omium.org>:
> On Thu, Jul 7, 2011 at 11:43 PM, Marco Stornelli
> <marco.stornelli@...il.com> wrote:
>> 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
>
> Any consensus/decision on how to go on with this patch idea?
>
> The options that I see right now are:
> - keep access through /dev/mem only (but access to /dev/mem is
> sometimes restricted);
> - keep the debugfs entry as well(as in the patch);
> - remove the debugfs entry and add a char driver to access the memory
> using read and seek operations.
>
> + the rejected(?) options from before
>
> Sergiu
>

For me the best option it's to use a sysfs/proc entry to export
(read-only) the memory address, record size etc. At that point we can
use a generic script/program to access via /dev/mem. However I let
Andrew/Greg say the last word.

Marco
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