[RFC] drm: Allow DRM_IOCTL_MODE_MAP_DUMB for render nodes

[Robert Foss]

From: Tomasz Figa <tfiga at chromium.org>

There is no particular reason to prevent userspace for using this IOCTL,
considering that it already has access to other, platform-specific
IOCTLs. This patch makes is possible to have the same userspace code
work regardless on the underlying platform, which significantly
simplifies the stack.

Signed-off-by: Tomasz Figa <tfiga at chromium.org>
Reviewed-by: Zach Reizner <zachr at chromium.org>
Signed-off-by: Nicolas Norvez <norvez at chromium.org>
Reviewed-by: Tomasz Figa <tfiga at chromium.org>
Signed-off-by: Robert Foss <robert.foss at collabora.com>
---

I've been looking into enabling a kms_swrast based driver for mesa on
the Android platform[1].

But have come up against the issue of dumb buffer related ioctls below 
not being flagged with DRM_RENDER_ALLOW.

DRM_IOCTL_MODE_CREATE_DUMB
DRM_IOCTL_MODE_MAP_DUMB

To be more precise, I've been seeing a failure due to DRM_IOCTL_MODE_MAP_DUMB
not being allowed for /dev/dri/renderD* nodes, and used in mesa
kms_sw_displaytarget_map().


As I understand it the DRM_RENDER_ALLOW flag being unset is a very intentional
restriction placed on dumb buffers in order to minimize its use.
But as far as alternative solutions for software renderers there seems to only
be VGEM and mmap()-ing DMABUFs.

While it would be convenient from the point of view of software renderers if
dumb buffers had more promiscuous permissions, it may be a hard sell upstream.

If dumb buffers aren't the way forward, what is? VGEM? Or are there any other
preferable ways?


[1] https://patchwork.freedesktop.org/series/45966/

 drivers/gpu/drm/drm_ioctl.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 0d4cfb232576..ef716246baf6 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -642,8 +642,8 @@ static const struct drm_ioctl_desc drm_ioctls[] = {
 	DRM_IOCTL_DEF(DRM_IOCTL_MODE_RMFB, drm_mode_rmfb, DRM_UNLOCKED),
 	DRM_IOCTL_DEF(DRM_IOCTL_MODE_PAGE_FLIP, drm_mode_page_flip_ioctl, DRM_MASTER|DRM_UNLOCKED),
 	DRM_IOCTL_DEF(DRM_IOCTL_MODE_DIRTYFB, drm_mode_dirtyfb_ioctl, DRM_MASTER|DRM_UNLOCKED),
-	DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_DUMB, drm_mode_create_dumb_ioctl, DRM_UNLOCKED),
-	DRM_IOCTL_DEF(DRM_IOCTL_MODE_MAP_DUMB, drm_mode_mmap_dumb_ioctl, DRM_UNLOCKED),
+	DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_DUMB, drm_mode_create_dumb_ioctl, DRM_UNLOCKED|DRM_RENDER_ALLOW),
+	DRM_IOCTL_DEF(DRM_IOCTL_MODE_MAP_DUMB, drm_mode_mmap_dumb_ioctl, DRM_UNLOCKED|DRM_RENDER_ALLOW),
 	DRM_IOCTL_DEF(DRM_IOCTL_MODE_DESTROY_DUMB, drm_mode_destroy_dumb_ioctl, DRM_UNLOCKED),
 	DRM_IOCTL_DEF(DRM_IOCTL_MODE_OBJ_GETPROPERTIES, drm_mode_obj_get_properties_ioctl, DRM_UNLOCKED),
 	DRM_IOCTL_DEF(DRM_IOCTL_MODE_OBJ_SETPROPERTY, drm_mode_obj_set_property_ioctl, DRM_MASTER|DRM_UNLOCKED),
-- 
2.17.1