Re: [PATCH 15/16] mtd: mtdcore: fix initcall level

From: Alexander Holler
Date: Fri Sep 04 2015 - 00:00:33 EST

Am 02.09.2015 um 07:34 schrieb Alexander Holler:
Am 01.09.2015 um 23:19 schrieb Brian Norris:
Hi Alexander,

No judgment here for the rest of this series, but for this patch:

On Wed, Aug 26, 2015 at 02:28:27PM +0200, Alexander Holler wrote:
The mtd-core has to be initialized before other dependent mtd-drivers,
otherwise a crash might occur.

Currently mtd_init() is called in the initcall-level device, which is
same level where most mtd-drivers will end up. By luck this seemed to
been called most of the time before other mtd-drivers without having
explicitly enforced.

I can't really speak for the original authors, but it does not appear to
be entirely "by luck." Link order was one of the de facto ways to get
this ordering (though it's not really a great one), and mtdcore was
always linked first within the drivers/mtd/ directory structure.

But that's just background, I think this is worth fixing anyway. It
could, for instance, become a problem if drivers are located outside
drivers/mtd/; I see random board files in arch/ that register with MTD,
and I'm actually not sure how they have never tripped on this.

I've already found at least a half a dozen other drivers with the same
problem through my shuffling of the drivers which all end up in the
standard initcall level device. I'm aware that this is based on the link
order, which itself is based on linker behaviour (and maybe other things
like make or other build tools). I'm just calling it luck, because this
is nowhere explicitly stated, at least I've never seen such a statement,
neither in any source, nor somewhere in Documentation. So I've choosen
the term "by luck" in order to provoke a bit (or to stimulate a
discussion about that widespread problem).

A good example why "luck" might not be far away from the truth is what happens when a drivers moves e.g. from staging to it's real position. Also the position will stay inside the same initcall level, the move of the driver into another directory (maybe together with a rename) will likely change its position in the initcall-sequence, without anyone really expecting this.

But if mtd_init() is not called before a dependent
driver, a null-pointer exception might occur (e.g. because the mtd
class isn't registered).

To fix this, mtd-init() is moved to the initcall-level fs (right before
the standard initcall level device).

Is there a good reason we shouldn't just make this a subsys_initcall()?
That would match the naming better, and it mirrors what, e.g.,
block/genhd uses. It would also allow flexibility if there are other
current/future use cases that might need to sit between the subsystem
and the drivers.

No real reason here. The names for the initcall levels seem to be
outdated since a long time anyway, and I think just speaking about the
numbers 1-7 (or 0-14) would be better anyways. The only reason why I've
used the fs (sync) level is because I was too lazy to check out if
mtdcore might depend on something else in any other level. Therefor I've
used the one most close to were it was before.

Lazy was the wrong term. It is time consuming, cumbersome and therefor error-prone to check on what other stuff a driver depends. One reason why choosing the right place in the initcall sequence isn't that easy and why some automation make sense.

Also I've an idea about how to fix that in general for all drivers (by
using the same algorithm I've now introduced just for DT-based drivers
with a device description). Wouldn't be that hard to use that for all
drivers, but as I've written in a follow up to the introductory mail,
one step after another.


Alexander Holler

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