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

["Ville Syrjälä" <ville.syrjala-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Tue, Apr 03, 2018 at 03:47:57PM +0200, Michel Dänzer wrote:
> On 2018-04-03 03:39 PM, Ilia Mirkin wrote:
> > On Tue, Apr 3, 2018 at 9:32 AM, Michel Dänzer <michel@daenzer.net> wrote:
> >> On 2018-04-03 03:26 PM, Ilia Mirkin wrote:
> >>> On Tue, Apr 3, 2018 at 5:29 AM, Daniel Vetter <daniel@ffwll.ch> wrote:
> >>>> 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@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.
> >>>
> >>> Another use-case that the per-driver disables enable is "i915 works
> >>> but nouveau is broken due to crazy ACPI / PCIe PM / whatever". It
> >>> seems likely this could happen with amdgpu as well.
> >>
> >> modprobe.blacklist=amdgpu
> >>
> >> works as well as the modeset parameter for this.
> > 
> > People who build their own kernels run into trouble too.
> 
> There have always been various more or less serious issues with building
> amdgpu (and radeon) into the kernel. People who do so get to keep all
> pieces when it breaks.
> 
> 
> > Also does this work uniformly across all systems where it is a module?
> 
> AFAIK yes.

Sadly modprobe.blacklist doesn't prevent X/whatever from loading the
module anyway.

I use modprobe.blacklist myself all the time for i915 development,
but I also have all GUI junk disabled as well so that I can load the
module myself when I'm actually ready for it (typically after I've
enabled netconsole).

-- 
Ville Syrjälä
Intel OTC
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