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

Michel Dänzer michel at daenzer.net
Tue Apr 3 13:32:19 UTC 2018

On 2018-04-03 03:26 PM, Ilia Mirkin wrote:
> On Tue, Apr 3, 2018 at 5:29 AM, Daniel Vetter <daniel at 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 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.
> 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.


works as well as the modeset parameter for this.

> The names are horrid. I think everyone agrees. In fact, when the
> driver is disabled by nomodeset or modeset=0, I think they should
> print like "driver completely disabled by modeset option" to avoid
> some of the user confusion Christian is referring to (which I've run
> into on a number of occasions as well). And/or maybe just refuse the
> driver load entirely rather than load-but-do-nothing. However being
> able to help users debug things seems more important than users being
> occasionally confused by options they added to kernel cmdline.

FWIW, if a new parameter or other mechanism is added for this, ideally
it should allow preventing initialization on a per-GPU basis, instead of
only per-driver, for systems with multiple GPUs supported by the same

Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer

More information about the amd-gfx mailing list