Hi all,
While checking the openSUSE kernel configuration, I noticed a couple oddities regarding userptr support in the i915, radeon and amdgpu drm drivers. I'll like to discuss the current situation and come up with an agreement on how this could be cleaned up.
Firstly, i915. This driver has userptr code under #if defined(CONFIG_MMU_NOTIFIER), however it neither selects this option nor depends on it. Given that CONFIG_MMU_NOTIFIER is not a user-visible option (it can only be selected by other kernel configuration options), it means that you get full userptr support or not depending on other unrelated kernel options. This isn't good.
Secondly, radeon and amdgpu. They are slightly better in that they have Kconfig options selecting CONFIG_MMU_NOTIFIER (DRM_RADEON_USERPTR and DRM_AMDGPU_USERPTR respectively.) However I still find it confusing that full userptr support is under #if defined(CONFIG_MMU_NOTIFIER), and not under #if defined(CONFIG_RADEON_USERPTR), resp. #if defined(CONFIG_AMDGPU_USERPTR). It means that full userptr support may be included even if these options are disabled.
I am not too familiar with MMU and this userptr stuff, but from where I stand, only two approaches make sense:
* Either there is a good reason why people may want to disable full userptr support. In this case options CONFIG_RADEON_USERPTR and CONFIG_AMDGPU_USERPTR should really enable the code in question, it should not be built without these options. And a similar option should be introduced for the i915 driver to the same effect. Or alternatively a single option for the whole DRM subsystem may make more sense, as I doubt anyone would want to enable support in one driver and disable it in another.
* Or there is no specific reason why people would want to disable full userptr support, in which case options CONFIG_RADEON_USERPTR and CONFIG_AMDGPU_USERPTR should be removed and all 3 drivers should unconditionally select CONFIG_MMU_NOTIFIER.
If the sole purpose of these these settings is for development or debugging purposes, I'd go for option 1 with a run-time option to disable full userptr (drm.userptr=ro or some such.)
As a general rule, fewer configuration options is better.
Once a decision is made, I volunteer to write the patches.
Thanks,