drm/nouveau: add kconfig option to turn off nouveau legacy contexts. (v3)

There was a nouveau DDX that relied on legacy context ioctls to work,
but we fixed it years ago, give distros that have a modern DDX the
option to break the uAPI and close the mess of holes that legacy
context support is.

Full context of the story:

commit 0e97598
Author: Peter Antoine <[email protected]>
Date:   Tue Jun 23 08:18:49 2015 +0100

    drm: Turn off Legacy Context Functions

    The context functions are not used by the i915 driver and should not
    be used by modeset drivers. These driver functions contain several bugs
    and security holes. This change makes these functions optional can be
    turned on by a setting, they are turned off by default for modeset
    driver with the exception of the nouvea driver that may require them with
    an old version of libdrm.

    The previous attempt was

    commit 7c51013
    Author: Daniel Vetter <[email protected]>
    Date:   Thu Aug 8 15:41:21 2013 +0200

        drm: mark context support as a legacy subsystem

    but this had to be reverted

    commit c21eb21
    Author: Dave Airlie <[email protected]>
    Date:   Fri Sep 20 08:32:59 2013 +1000

        Revert "drm: mark context support as a legacy subsystem"

    v2: remove returns from void function, and formatting (Daniel Vetter)

    v3:
    - s/Nova/nouveau/ in the commit message, and add references to the
      previous attempts
    - drop the part touching the drm hw lock, that should be a separate
      patch.

    Signed-off-by: Peter Antoine <[email protected]> (v2)
    Cc: Peter Antoine <[email protected]> (v2)
    Reviewed-by: Peter Antoine <[email protected]>
    Signed-off-by: Daniel Vetter <[email protected]>

v2: move DRM_VM dependency into legacy config.
v3: fix missing dep (kbuild robot)

Cc: [email protected]
Reviewed-by: Daniel Vetter <[email protected]>
Signed-off-by: Dave Airlie <[email protected]>

Author: airlied

drivers/gpu/drm/nouveau/Kconfig

@@ -17,10 +17,21 @@ config DRM_NOUVEAU
 	select INPUT if ACPI && X86
 	select THERMAL if ACPI && X86
 	select ACPI_VIDEO if ACPI && X86
-	select DRM_VM
 	help
 	  Choose this option for open-source NVIDIA support.
 
+config NOUVEAU_LEGACY_CTX_SUPPORT
+	bool "Nouveau legacy context support"
+	depends on DRM_NOUVEAU
+	select DRM_VM
+	default y
+	help
+	  There was a version of the nouveau DDX that relied on legacy
+	  ctx ioctls not erroring out. But that was back in time a long
+	  ways, so offer a way to disable it now. For uapi compat with
+	  old nouveau ddx this should be on by default, but modern distros
+	  should consider turning it off.
+
 config NOUVEAU_PLATFORM_DRIVER
 	bool "Nouveau (NVIDIA) SoC GPUs"
 	depends on DRM_NOUVEAU && ARCH_TEGRA

drivers/gpu/drm/nouveau/nouveau_drm.c

@@ -1094,8 +1094,11 @@ nouveau_driver_fops = {
 static struct drm_driver
 driver_stub = {
 	.driver_features =
-		DRIVER_GEM | DRIVER_MODESET | DRIVER_PRIME | DRIVER_RENDER |
-		DRIVER_KMS_LEGACY_CONTEXT,
+		DRIVER_GEM | DRIVER_MODESET | DRIVER_PRIME | DRIVER_RENDER
+#if defined(CONFIG_NOUVEAU_LEGACY_CTX_SUPPORT)
+		| DRIVER_KMS_LEGACY_CONTEXT
+#endif
+		,
 
 	.open = nouveau_drm_open,
 	.postclose = nouveau_drm_postclose,