[Intel-gfx] [PATCH v5 10/12] drm/i915: Defer probe if gmux is present but its driver isn't

[Daniel Vetter]

On Thu, Feb 18, 2016 at 9:34 PM, Lukas Wunner <lukas at wunner.de> wrote:
>
>> Ok, makes sense. I still think adding the check to the client_register
>> function would be good, just as a safety measure.
>
> Hm, the idea of calling vga_switcheroo_client_probe_defer() twice
> causes me pain in the stomach. It's surprising for drivers which
> just don't need it at the moment (amdgpu and snd_hda_intel) and
> it feels like overengineering and pampering driver developers
> beyond reasonable measure. Also while the single existing check is
> cheap, we might later on add checks that take more time and slow
> things down.

It's motivated by Rusty's API Manifesto:

http://sweng.the-davies.net/Home/rustys-api-design-manifesto

With the mandatory check in _register we reach level 5 - it'll blow up
at runtime when we try to register it. Without that the failure is
completely silent, and you need to read the right mailing list thread
(level 1), but at least the kerneldocs lift it up to level 3.

For more context: We have tons of fun with EPROBE_DEFER handling
between i915 and snd-hda, and I'm looking into all possible means to
make any api/subsystem using deferred probing as robust as possible by
default. One of the ideas is to inject deferred probe failures at
runtime, but that's kinda hard to do in a generic way. At least making
it as close to impossible to abuse as feasible is the next best
option.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch