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

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 7 Jul 2011 16:16:43 -0700
Sergiu Iordache <sergiu@google.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.