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

From: Alexander Holler
Date: Wed Sep 02 2015 - 01:35:09 EST

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 the
same level where most mtd-drivers will end up. By luck this seemed to have
been called most of the time before other mtd-drivers without having been
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).

But if mtd_init() is not called before a dependent
driver, a null-pointer exception might occur (e.g. because the mtd device
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 The names for the initcall levels seem to be outdated since a long time anyway, and I think just speaking about the numbers 1-7 (or 0-14) would be better anyways.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.

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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