[PATCH 2/2] drm/amdgpu: Add modeset module option

[Daniel Vetter]

On Sun, Apr 01, 2018 at 10:12:10PM +0200, Christian König wrote:
> Am 01.04.2018 um 20:21 schrieb Takashi Iwai:
> > On Sun, 01 Apr 2018 19:58:11 +0200,
> > Christian K6nig wrote:
> > > Am 01.04.2018 um 19:45 schrieb Ilia Mirkin:
> > > > On Sun, Apr 1, 2018 at 1:39 PM, Christian König
> > > > <christian.koenig at amd.com> wrote:
> > > > > Am 30.03.2018 um 22:45 schrieb Takashi Iwai:
> > > > > > amdgpu driver lacks of modeset module option other drm drivers provide
> > > > > > for enforcing or disabling the driver load.  Interestingly, the
> > > > > > amdgpu_mode variable declaration is already found in the header file,
> > > > > > but the actual implementation seems to have been forgotten.
> > > > > > 
> > > > > > This patch adds the missing piece.
> > > > > NAK, modesetting is mandatory for amdgpu and we should probably remove the
> > > > > option to disable it from other DRM drivers without UMS support as well
> > > > > (pretty much all of them now).
> > > > > 
> > > > > If you want to prevent a driver from loading I think the correct way to do
> > > > > so is to give modprobe.blacklist=amdgpu on the kernel commandline.
> > > > > 
> > > > > That would remove the possibility to prevent the driver from loading when it
> > > > > is compiled in, but I don't see much of a problem with that.
> > > > Having a way to kill the graphics driver is a very useful debugging
> > > > tool, and also a quick and easy way to get out of an unpleasant
> > > > situation where graphics are messed up / system hangs / etc. The
> > > > modprobe blacklist kernel arg only works in certain environments (and
> > > > only if it's a module).
> > > > 
> > > > Every other DRM driver has this and this is a well-documented
> > > > workaround for "graphics are messed up when I install linux", why not
> > > > allow a uniform experience for the end users who are just trying to
> > > > get their systems up and running?
> > > Because it is not well documented and repeated over and over again in
> > > drivers.
> > > 
> > > The problem is that people don't realized that the driver isn't loaded
> > > at all without the modeset=0 module option and demand fixing the
> > > resulting bad performance without modesetting.
> > Hm, I don't get it.  What this options has to do with performance for
> > a KMS-only driver...?
> 
> Well exactly that's the point, nothing.
> 
> The problem is that the option name is confusing to the end user because the
> expectation is that "nomodeset" just means that they only can't change the
> display mode.
> 
> Some (unfortunately quite a lot) people don't realize that for KMS drivers
> this means that the driver isn't even loaded and they also don't get any
> acceleration.
> 
> We had to explain that numerous times now. I think it would be best to give
> the option a more meaningful name.

Yeah, agreed with Christian. If we want a generic "pls disable all gfx
accel" knob then probably best to put that into the drm core. And just
outright fail loading the drm core if that happens, which will prevent all
gfx drivers from loading.

That likely means a hole bunch of stuff won't work (usually sound keels
over too), but that's what you get for this. Only disabling modesetting
without disabling the entire driver doesn't work (never has, except for
this UMS+GEM combo only i915 support, and not for long).

And once we have that knob, probably best to phase out all the per-driver
options.
-Daniel

> 
> Christian.
> 
> > > I can't count how often I had to explain that this approach won't
> > > work. We could rename it to something like disable_gfx_accel or
> > > something like that to make this clear.
> > Maybe it's a different view from a different perspective.
> >   From the distro side, nomodeset and the force reload via modeset
> > option has been the most reliable way to achieve the purpose, so far.
> > It's easier especially when the system has a hybrid graphics.
> > 
> > It might be not same for developers, though.
> > 
> > 
> > thanks,
> > 
> > Takashi
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch