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

[Marco Stornelli]

Il 29/07/2011 02:15, Sergiu Iordache ha scritto:
> On Mon, Jul 11, 2011 at 11:41 PM, Marco Stornelli
> <marco.stornelli@xxxxxxxxx>  wrote:
> > 2011/7/11 Sergiu Iordache<sergiu@xxxxxxxxxxxx>:
> > > On Thu, Jul 7, 2011 at 11:43 PM, Marco Stornelli
> > > <marco.stornelli@xxxxxxxxx>  wrote:
> > > > 2011/7/8 Greg KH<greg@xxxxxxxxx>:
> > > > > 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@xxxxxxxxxx>  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.
>
> Well, since the only method to read the dump data is /dev/mem,
> exporting the record size/address/etc is needed in order to parse it
> properly. But as far as I can see the data is already exported through
> sysfs in /sys/module/ramoops/parameters/.
> The current module still needs a patch to write the variables of the
> module parameters from the platform data in case that is used, but is
> there any reason why we would need other sysfs entries except these?

I'd say no. I think it's sufficient.

Marco
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