Re: [RFC] fs: add userspace critical mounts event support

From: Herbert, Marc
Date: Fri Sep 23 2016 - 22:51:56 EST

On 06/09/2016 16:04, Luis R. Rodriguez wrote:
> They claim that without it there is the race between /lib/firmware
> being ready and driver asking for the firmware.

Hope it's understood by now.

> I was told there were quite a bit of out-of-tree hacks to address
> this without using the usermode helper,

There are:
wait until SYSTEM_RUNNING before loading DMC firmware.

> the goal of this patch was to create the discussion needed to a
> proper resolution to this.

Sincere thanks.

>>> On Tue 06 Sep 11:32 PDT 2016, Linus Torvalds wrote:
>>>> On Tue, Sep 6, 2016 at 10:46 AM, Bjorn Andersson
>>>> Nobody has actually answered the "why don't we just tie the
>>>> firmware and module together" question.
>>> The answer to this depends on the details of the suggestion; but
>>> generally there's a much stronger bond between the kernel and the
>>> driver than between the driver and the firmware in my cases.


The i915 DMC firmware is an interesting example. First of all it’s
_optional_! It’s critical for battery-powered systems but the i915
driver works without it.

Dan wrote:
> Plus all gpu drivers which need firmware. And yes we must load them
> at probe because people are generally pissed when they boot their
> machine and the screen goes black. On top of that a lot of people
> want their gpu drivers to be built-in, but can't ship the firmware
> blobs in the kernel image because gpl. Yep, there's a bit a
> contradiction there ...

Eppur si muove:
1) As Dan just wrote, users expect the screen to light up as soon as they
press the power button so the i915 driver is built-in
2) ... yet they’ll never notice the nanojoules of battery loss caused
by the DMC firmware being on a filesystem and loaded a tiny bit later.

SoCs and platforms have become some new kind of distributed systems
where other processors run their own, specific software/OS/firmware.
>From this perspective the kernel plays a role similar to a boot server;
and choke point. Granted: booting various and heterogeneous
distributed systems doesn’t look like a simple problem to solve
generically. Yet at the moment the kernel doesn’t help by not
even supporting something as basic as being told when the files it’s
(unfortunately) in charge to deploy to other nodes become available and
ready to deploy.

It can’t be assumed that the driver and the firmware are two parts of
the same software piece whereas they actually run on two different
processors, are most likely developed and validated by completely
different teams and released on different lifecycles. Especially in
the Linux case.

I hope this distributed systems analogy captures the essence of the
examples and rationales detailed elsewhere in this thread.