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

[Lukas Wunner]

Hi,

On Thu, Feb 18, 2016 at 10:39:05PM +0100, Daniel Vetter wrote:
> 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

Interesting, thank you.


> With the mandatory check in _register we reach level 5 - it'll blow up
> at runtime when we try to register it.

The manifesto says "5. Do it right or it will always break at runtime".

However even if we add a WARN_ON(vga_switcheroo_client_probe_defer(pdev))
to register_client(), it will not *always* spew a stacktrace but only on
the machines this concerns (MacBook Pros). Since failure to defer breaks
GPU switching, level 5 is already reached. Chances are this won't go
unnoticed by the user.


> For more context: We have tons of fun with EPROBE_DEFER handling
> between i915 and snd-hda

I don't understand, there is currently not a single occurrence of
EPROBE_DEFER in i915, apart from the one I added.

In sound/ there are 88 occurrences of EPROBE_DEFER in soc/, plus 1 in
ppc/ and that's it. So not a single one in pci/hda/ where hda_intel.c
resides.

Is the fun with EPROBE_DEFER handling caused by the lack thereof?


Best regards,

Lukas